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Mountain Treelines: A Roadmap for Research Orientation

Mountain Treelines: A Roadmap for Research Orientation Lynn M. Resler{" For over 100 years, mountain treelines have been the subject of varied research endeavors and remain a strong area of investigation. The purpose of this paper is to Maaike Y. Bader{ examine aspects of the epistemology of mountain treeline research—that is, to Friedrich-Karl Holtmeier1 investigate how knowledge on treelines has been acquired and the changes in David R. Butler# knowledge acquisition over time, through a review of fundamental questions and approaches. The questions treeline researchers have raised and continue to raise have Daniel J. Weiss@ undoubtedly directed the current state of knowledge. A continuing, fundamental Lori D. Daniels$ and emphasis has centered on seeking the general cause of mountain treelines, thus Daniel B. Fagre& seeking an answer to the question, ‘‘What causes treeline?’’ with a primary emphasis on searching for ecophysiological mechanisms of low-temperature limitation for tree *Department of Geography, University growth and regeneration. However, treeline research today also includes a rich of Iowa, Iowa City, Iowa 52242, U.S.A. {Department of Geography, Virginia literature that seeks local, landscape-scale causes of treelines and reasons why Tech, Blacksburg, Virginia 24061, treelines vary so widely in three-dimensional patterns from one location to the next, U.S.A. and this approach and some of its consequences are elaborated here. In recent years, {Functional Ecology of Plants, both lines of research have been motivated greatly by global climate change. Given Department of Biology and the current state of knowledge, we propose that future research directions focused on Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, a spatial approach should specifically address cross-scale hypotheses using statistics Germany and simulations designed for nested hierarchies; these analyses will benefit from 1Institute for Landscape Ecology, geographic extension of treeline research. Westfa ¨ lische Wilhelms-Universita ¨t, D-48149 Mu ¨ nster, Germany #Department of Geography, Texas State University–San Marcos, San Marcos, Texas 78666-4616, U.S.A. @Yellowstone Ecological Research Center, 2048 Analysis Drive, Bozeman, Montana 59718, U.S.A. $Department of Geography, University of British Columbia, Vancouver, British Columbia V6T 1Z2, Canada &U.S. Geological Survey, Northern Rocky Mountain Science Center, Glacier National Park, West Glacier, Montana 59936, U.S.A. "Corresponding author: resler@vt.edu DOI: 10.1657/1938-4246-43.2.167 ecological processes and relations) will depend on how they are Introduction approached and thus what we believe we know about this topic. Mountain treeline ecotones represent the spatial transition Our purpose here is not to review what is known, or thought from forested to treeless mountain landscapes and are, therefore, to be known, about the geography, ecology, and plant physiology considered among the most prominent features of mountain of mountain treeline ecotones; Holtmeier (2009) has already environments. Mountain treeline ecotones also represent the upper covered this ground from a worldwide perspective. Instead, our physiological limits of tree species and a lower boundary (though aim is to examine the nature of knowledge, through a review of the not a physiological limit) for alpine herbaceous species. The paths treeline researchers have followed to reach the current state importance of this boundary for nutrient fluxes and biodiversity, of knowledge about alpine treeline ecotones, and also how the the variety and complexity of spatial patterns found at treeline, goals for knowledge acquisition differ. For simplicity, we refer to and the intriguing and still enigmatic life-form limit it represents, these as studies of mountain ‘‘treeline,’’ and distinguish a has given rise to numerous and diverse research endeavors from a conceptual line versus ecotone only where necessary. broad disciplinary base for over 100 years. An important motivation for many recent studies of treeline is ongoing global climate change due to their hypothesized use as an indicator of APPROACHES climate change and to potential loss of biodiversity and ecosystem function as tundra is replaced by woody species. The usefulness of From the variety of research conducted at mountain treelines treeline dynamics as indicators of climate change (or of general worldwide, two major research approaches emerge: searches for 2011 Regents of the University of Colorado G. P. MALANSON ET AL. / 167 1523-0430/11 $7.00 global-scale and landscape-scale causes, wherein the latter are ‘‘descriptio montis fracti’’ (1555) that the altitudinal position of within and modify the limits of the former (Troll, 1973; Wardle, the vegetation belts is related to the decrease of temperature and 1974, 1993; Holtmeier, 2009). Ko ¨ rner (2003) identified these as length of growing season with altitude (Holtmeier, 1965). It is fundamental/global and modulative/regional, and we also see likely that Alexander von Humboldt (von Humboldt and Bonp- these approaches as inquiry at different spatial scales within a land, 1805) was among the first researchers to mention the geographic hierarchy but with the modulation being more local existence of alpine treelines in a manner that connected than regional. Studies exploring the broad causes of treeline observation with causality. His foundational work in physical attempt to answer the fundamental question, ‘‘What causes geography and meteorology was the first to document isotherms treelines?’’ Such studies attempt to understand the factors that and their relationship with elevation and plant geography of cause treelines to exist on a global level, and we refer to it here as mountain slopes (Marek, 1910; Troll, 1962). Additional work the global approach. Furthermore, these studies focus primarily included the early identification of heat (Da ¨ niker, 1923) and tree on limitations to tree growth from an ecophysiological perspective physiology (e.g., Michaelis, 1934; Steiner, 1935; Schmidt, 1936; Pisek because ecophysiology is replicated globally, but ecophysiological and Cartellieri, 1939), and more recent summaries combine the two questions are also examined locally. Studies examining the fine- (Tranquillini, 1979; Wieser and Tausz, 2007). Holtmeier (2009) scale (or local) causes of treelines attempt to answer the question, provides a more complete treatment of more recent research. ‘‘How and why do treelines differ across locations?’’ This Here, we devote most of our analysis to the fundamental approach focuses on landscape patterns, especially the effects of questions and themes in the landscape approach. We conclude by topography and treeline history (such as climatic changes, human outlining needs for future research on mountain treelines impact, and wildfire), because these factors differ within regions, conducted with a spatial perspective. In this paper, our emphasis and we refer to it as the landscape approach. It is not a coincidence on the landscape perspective in no way discredits the excellent that these two approaches have different foci in terms of treeline contributions to understanding treelines offered by the global studies, with the former most concerned with the upper elevational approach, and reflects, rather, the collective training and expertise limit of ‘trees,’ i.e., full upright stems (or at least 2 m; e.g., Wieser of the authors. and Tausz, 2007), and less with tree seedlings, while the latter is most concerned with the three-dimensional gradient or pattern across the ecotone with seedlings at the fore (e.g., Johnson et al., LANDSCAPE FOUNDATIONS 2004). The landscape approach, our main focus, is first and foremost Global treeline research is currently dominated by the search a part of landscape ecology. Much of the early impetus for work in for mechanisms of low-temperature limitations on tree growth the Alps was from the perspective of landscape level management (e.g., Ko ¨ rner, 1998a). Here the discussion centers on the role of problems (Holtmeier, 2009), the root of European landscape carbon acquisition versus use (i.e. source versus sink limitation) at ecology. Much of the pattern-process paradigm in landscape low temperatures (particularly root zone temperature) and/or ecology derived from island biogeography, and so it blossomed in limitations related to the utilization of carbon within trees. In spite the 1970s and boomed in the 1980s (Turner, 1989). This latter of abundant sampling in recent years (Li et al., 2002; Hoch and history is mirrored in the increasing interest in three-dimensional Ko ¨ rner, 2003; Piper et al., 2006; Bansal and Germino, 2008; Shi et pattern in treeline studies (e.g., Humphries et al., 2008; see Fig. 1). al., 2008), there is generally no evidence for a poor carbon status in However, landscape ecology has been affected by ideas formed in treeline trees in the long term, although it may influence local mountain environments, perhaps beginning in the mid-20th pattern (Cairns and Malanson, 1997; Cairns, 2005). Therefore, the century with Troll’s (1971) emphasis on ‘geo-ecology.’ attention is now turned to mechanisms of direct growth The landscape approach, second, includes hierarchy theory in limitations, with an emphasis on understanding how a mean ecology, which developed in the 1980s (e.g., Allen and Starr, 1982) growing season temperature can best correlate with treeline and has been applied to treeline research since the early 1990s. The positions worldwide (Ko ¨ rner and Paulsen, 2004), in spite of this core idea here is that processes and patterns develop at multiple mean being composed of widely differing temperature regimes spatial and temporal scales. Together they are comprised of (Hoch and Ko ¨ rner, 2009). interactions of processes at finer scales, while constrained by Investigations following the landscape approach have typi- patterns at coarser scales (O’Neill et al., 1989). For example, the cally studied treelines from the perspective of spatial pattern and establishment of a forest patch in the treeline ecotone would process (Walsh et al., 1997). Landscape change, linkages among depend on fine-scale processes such as seedling establishment but biotic and physical elements of the environment, and human/ is also constrained by coarse-scale patterns such as mountain environment interactions are often studied (Holtmeier, 1974; Broll topography (slope aspect, exposure to wind). Also, the general et al., 2007; Holtmeier, 2009). Landscape studies may also look at temperature control on treelines can be seen as ‘merely’ a coarser specific ecophysiological causes of treeline patterns, e.g., studies scale constraint on the focal three-dimensional pattern dynamics that address establishment limitations rather than growth in adult driven by population level processes (e.g., Brown et al., 1994; trees (e.g., Germino et al., 2002; Piper et al., 2006; Johnson and Walsh et al., 1997). Smith, 2007; Bader et al., 2007a; Bansal and Germino, 2008; Smith Third, complexity science has, explicitly or implicitly, become et al., 2009). Such studies often find that landscape position and part of many landscape-scale treeline studies. Complexity science facilitation by neighbors are very important. is a body of concepts that examines how higher order pattern or Though a divergence in knowledge ‘camps’ currently exists, structure in systems is produced by few, simple, but nonlinear present-day research has evolved from a similar line of inquiry— interactions at a lower level, and thus includes hierarchy (e.g., Sole one that originated and continues in the consideration of global- and Bascompte, 2006). The higher order patterns could not be scale causes of treelines. A search for broad-scale causes of treelines evolved from the work of great observationists. After predicted from the properties of the lower order units, but instead Leonardo da Vinci (1452–1519) had noticed the altitudinal belts the ‘emergence’ of structure must be determined by observing the and their specific organisms on Monte Rosa (Italian Alps), evolution of the system. Thus, a standard reductionist approach is Conrad Gesner (1516–1565) from Zurich mentioned in his overturned. Such systems can be said to be self-organized in that 168 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH FIGURE 1. Hedges, illustrat- ing positive spatial association, and individual seedlings, illustrat- ing association with microrelief, extend into tundra above the contiguous forest in the treeline ecotone at Lee Ridge, Glacier National Park, Montana, U.S.A. Photo: G. P. Malanson. the higher order structure can be reduced to fewer, but still coarser levels constrain the finer scale dynamics that create and endogenous, nonlinear dimensions or drivers. Complexity science reproduce them (e.g., Malanson et al., 2007). We further see the was developed and distinctly promoted by physicists (Gell-Mann, importance of dynamics as a conceptual framework, with both 1994; Bak, 1996), but quickly spread to other disciplines. straightforward and higher order patterns of change as objects of Malanson (1999) discussed its applicability to treeline studies, study and both endogenous feedbacks and exogenous drivers as and the work by Rietkerk et al. (2002) in spatial ecology hypothetical processes. influenced later advances by Zeng and Malanson (2006) and These epistemological foundations were enabled, if not Bader et al. (2008). Advances in all three domains developed driven, by methodological developments. Common to landscape simultaneously and were clearly being used together by landscape ecology in general, the landscape approach in treeline studies was ecologists. The work of O’Neill (O’Neill et al., 1982, 1986, 1989, advanced by remote sensing—aerial photography at first (e.g., 1992, inter alia) alone illustrates this synergy, and work in Troll, 1939), followed by satellite-based imagery (e.g., Baker and landscape ecology (e.g., Li, 2002) led to the initiation of a new Weisberg, 1995; Allen and Walsh, 1996; Walsh et al., 2003) and journal, Ecological Complexity, in 2004. geographic information systems (GIS) analysis (e.g., Brown, 1994; One can see why linkages among these three areas (landscape Ho ¨ rsch, 2003). Quantification of pattern was an essential ecology, hierarchy theory, and complexity) have been identified contributor to the concepts, and these technologies greatly and are relevant to treeline research. They link spatial heteroge- increased the available data on spatial pattern and simplified its neity and feedbacks (Troll, 1971), scale dependence in a analysis. Computer power itself enabled the development of geographic hierarchy (Pattee, ed., 1973), and self-organization hierarchy theory and complexity science in general (Pagels, 1988). (Haken, 1975). The relations between processes and patterns, in For example, cellular automata, or cellular models with stochas- this case between advances or retreats of treelines on mountain ticity added to neighborhood effects, were core tools in the slopes and the spatial structure of patches and edges (Fig. 1), can development of the complexity approach at ecotones in general be seen to have characteristics in common with other spatial (e.g., Loehle et al., 1996; Li, 2002) and treelines in particular phenomena at higher orders (e.g., fractal patterns or power-law (Alftine and Malanson, 2004; Malanson and Zeng, 2004; Zeng and distributions). Ecologists have found such higher order phenom- Malanson, 2006; Wiegand et al., 2006; Zeng et al., 2007; Bader et ena may explain important characteristics such as metabolic al., 2008). capacity (West et al., 1999) for which evolutionary processes may be revealed. For biogeography (e.g., Deng et al., 2006), including ecotones (Milne et al., 1996), the existence of these higher order Understanding Landscape-Scale Relations patterns may indicate constraints on the variability and explana- In landscape-scale treeline studies, an understanding of the tions possible (i.e., self-organization; Zeng and Malanson, 2006). treeline phenomenon and its local causes is obtained by focusing We might hope that these linkages would provide predictive power on variations of treeline spatial and temporal patterns (e.g., for the response of treelines to climate change, but prediction may Blu ¨ thgen, 1942; Griggs, 1946; Holtmeier, 1965; Smith et al., 2003; be limited. While self-organization, and thus fractal patterns, hold Broll et al., 2007; Malanson et al., 2009; Butler et al., 2009a; see under strong exogenous forcing from either climate change or Holtmeier, 2009, for further references). Like broad-scale ap- underlying geomorphic patterns (Zeng, 2010), the constraint that proaches to treeline, treeline dynamics under global climate patterns will remain fractal does not provide the kind of prediction change scenarios have been a major theme of current research that ecologists or landscape managers desire. from a landscape perspective. However, for landscape-scale Nevertheless, from these three perspectives we have learned to treeline scientists (who acknowledge that heat deficiency is the look at treelines at multiple scales, seeing organisms within ultimate limit on tree growth at high elevation), the problem is patches within landscapes in three dimensions, and how the G. P. MALANSON ET AL. / 169 alpine treeline. Cronartium ribicola, the introduced pathogen that that treeline ecotones present complex patterns in three dimen- causes blister rust in five-needled white pines, has caused sions that are beyond the explanatory power of temperature alone. widespread mortality in whitebark pine (Pinus albicaulis), a The ribbons, hedges, and other patchy patterns (see Holtmeier, foundation and keystone species of northern Rocky Mountain 1982, and Fig. 1), while constrained by temperature limits, require subalpine and treeline ecosystems (Resler and Tomback, 2008), explanation. The landscape approach to treeline is motivated by which has potentially serious and cascading consequences (Tom- interest in these three-dimensional patterns, and processes are seen back and Resler, 2007). Mass outbreaks of the autumnal moth as nested at multiple scales even within the landscape-scale domain (Epirrita autumnata) have repeatedly destroyed vast areas of (e.g., Elliott and Kipfmueller, 2010). mountain birch (Betula pubescens ssp. czerepanovii) forests up to the treeline in Finland, which has been followed by severe soil LOCAL DRIVERS erosion on sandy substrates that now impedes the re-establishment of this species (Holtmeier, 2002; Holtmeier et al., 2003; Broll et al., The primary fine-scale modulators of treeline patterns may be 2007). Mycorrhizae, which increase nutrient availability, may be those that affect heat deficiency, but fine-scale modulators have important to successful seedling establishment, tree growth, and effects on water, nutrients, and disturbance. Temperature defi- afforestation at and above the treeline (Hasselquist et al., 2005; ciency is not only a broad-scale cause of treelines, but can also be a Germino et al., 2006). Birds affect trees in the treeline ecotone by landscape-scale modulator of treeline pattern. At a landscape scale consuming seeds and buds, and dispersing and caching seeds (e.g., it is affected by local modulators such as topography, and it can the nutcrackers [Nucifraga caryocatatces and subspecies in also affect other landscape-scale modulators, such as distribution, Eurasia, Nucifraga columbiana in North America]; Holtmeier, depth, and duration of the winter snowpack. Factors that directly 1966, 2002; Tomback, 1977; Mattes, 1978, 1982, 1985). The reduce temperatures at local scale are wind, cold air drainage, impacts of burrowing animals can be positive because they expose snow cover in summer, and shade. Temperature affects snow cover the mineral soil and thus create open patches that may facilitate directly, in terms of determining the mix of rain vs. snow and the the establishment of seedlings (e.g., Butler et al., 2009b; Butler and rate of melting and, indirectly, by determining snow density and Butler, 2009); conversely they can destroy seedlings and saplings thus snow removal and redeposition by wind. by girdling and by pushing seedlings out of the ground (Holtmeier, Landscape-scale modulators not directly related to heat are 1987, 2002). At the other end of the size scale, grizzly bears tear quite diverse, but the one most important in ecophysiology is swaths of tundra with similar mixed effects (Butler 1992, 1995). water (e.g., Brodersen et al., 2006). Treeline areas vary in the soil Also, the activities of wild-living ungulates are detrimental to moisture available for plant growth at landscape scales, and much treelines (Holtmeier, 2002, 2009; Cairns and Moen, 2004; Cairns et depends on the removal and redeposition of snow by wind. Some al., 2007). These phenomena are spatially variable and their areas are scoured of snow and so experience drought conditions relations with temperature are not well known. much beyond what the regional precipitation pattern would Additionally, competition with alpine herbaceous species can indicate, whereas other areas have multiples of the regional limit or facilitate seedling establishment. Seedlings are facilitated precipitation due to snow collection and eventual melt (e.g., Walsh by a moderate amount of herb cover and limited by too little or et al., 1994; Hiemstra et al., 2002, 2006; Geddes et al., 2005; too much (Germino et al., 2002). These relations can affect pattern Holtmeier, 2005). Other effects of excessive snow cover, which development (Malanson and Butler, 1994). Moreover, allelopathic may impede or even prevent successful seedling establishment, effects of the associated vegetation (e.g., some lichen species or include shortened growing season, physical damage through dwarf shrubs), may impair germination, mycorrhiza development creeping and settling snow, and snow fungi, which may affect and seedling development (Holtmeier, 2009, further references seedlings and saplings of evergreen conifers (e.g., Cunningham et therein). al., 2006). Geomorphology is also a landscape-scale modulator of treeline patterns and dynamics, exerting its influence through SPATIAL DYNAMICS more direct causes like snow, soil, and disturbance (e.g., Kullman, Change through time is integral to explaining three-dimen- 1997; Walsh et al., 2003; Holtmeier and Broll, 2005; Butler et al., sional treeline patterns in a spatial hierarchy (Armand, 1992; 2007; Zeng et al., 2007; Humphries et al., 2008; Butler et al., 2009a, Wiegand et al.. 2006). The feedbacks of growing tree populations 2009b; Bekker and Malanson, 2009; Munier et al., 2010). Local on their neighborhood become increasingly important as the size topography may provide shelter from the wind, as on the lee sides of individuals and patches increases (e.g., Holtmeier, 1999, 2005, of small ridges, solifluction terraces, rocky outcrops and other 2009), but fade as these patches merge into contiguous forest convex sites (e.g., Resler et al., 2005, in the Rocky Mountains; because of reduced edge (Zeng and Malanson, 2006). These Holtmeier et al., 2003; Anschlag et al., 2008, in Finland; see feedbacks imply a strong effect of existing patterns on dynamics Fig. 1). Depressions may be covered too long with snow, however, and indicate self-organization, and more generally, an important and waterlogging can be an additional adverse factor in such role for landscape history. poorly drained places. On wind-exposed convex topography with little or no winter snowpack, wind actions (physiologically, Patterns and Feedbacks mechanically) may cause serious damage to seedlings and saplings (Cairns and Malanson, 1998). Temperatures—their pattern as well Treelines frequently exhibit very discrete patterns, consisting, as limits—also affect local geomorphic processes such as for instance, of abrupt forest edges or distinct dwarf tree or solifluction and frost heaving that may affect seedling establish- krummholz patches (e.g., Walsh et al., 1992; Humphries et al., ment (Butler et al., 2004, 2009b). 2008). The variety of three-dimensional patterns observed are Further local causes of treeline not directly related to heat are unlikely to emerge on environmental gradients of temperature due due to biotic factors such as pathogens, insects, mycorrhizae, to lapse rates unless positive feedback processes amplify initial birds, and mammals. Pathogen outbreaks can cause regional environmental differences (i.e., a positive feedback switch sensu mortality of trees that could ultimately influence spatial pattern at Wilson and Agnew, 1992) or growth responses are nonlinear 170 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH (Batllori et al., 2009). The most notable driver in the pattern- process feedback is wind (e.g., Marr, 1977; Akhalkatsi et al., 2006; Holtmeier and Broll, 2010); shade and protection from sky exposure is probably next (e.g., Orlander, 1993; Germino and Smith, 2000; Germino et al., 2002; Smith et al., 2003; Slot et al., 2005); followed by nutrient enrichment (Holtmeier and Broll, 1992; Seastedt and Adams, 2001; Shiels and Sanford, 2001; Liptzin and Seastedt, 2009) and albedo. The negative feedback (shading and cooler soils) identified by Ko ¨ rner (1998a, 1998b) acts in the opposite direction. However, tree seedlings at treeline are often found preferentially beneath tree canopies (e.g., Griggs, 1946; Ball et al., 1991; Germino and Smith, 1999; Resler and Fonstad, 2009), indicating that the positive feedback through shelter and protection from radiative stress is more important for treeline spatial dynamics than the negative feedback of lower soil temperatures. Several treeline studies have investigated the role of landscape position, including effects of neighboring plants, on demographic processes (e.g., Daly and Shankman, 1985; Bekker, 2005; Resler et al., 2005; Maher and Germino, 2006; Batllori et al., 2009, 2010; Hughes et al., 2009). However, few of these studies have included indices or direct measurements of ecophysiological parameters like FIGURE 2. Emergence of abrupt treeline transitions in time and photosynthetic capacity (Germino and Smith, 1999; Maher et al., space. (a) Tree growth is determined by the microclimate, which is determined by the external macroclimate [e] and modifications 2005) or pigmentation (Akhalkatsi et al., 2006), thus indicating the caused by the vegetation itself [i]. Depending on the strength of i mechanisms by which spatial associations have probably arisen relative to e, the vegetation will be more (large e) or less (large i) (e.g., protection from cold-induced photoinhibition). Combina- coupled to the external climate and hence more or less sensitive to tions of such detailed mechanistic knowledge could strongly climatic changes. (b) Hysteresis fold demonstrating the alternative improve predictions of response to climate change. Another stable states of forest and alpine vegetation existing under the same interesting issue is the effect of landscape position on the growth external environmental conditions (gray area; at treelines the stress and general performance of adult trees, bringing local detail to the factor can be e.g., freezing temperatures, wind, or high solar general ecophysiological principles that may (or may not) be radiation). Abrupt (‘catastrophic’) transitions from forest to alpine applicable at treelines globally (Li and Yang, 2004; Wilmking et vegetation or vice versa can occur at threshold conditions or due to al., 2004). disturbances. (c) Changes in microclimatic conditions as a result of vegetation cover. Some stress factors can be aggravated just outside Given the difficulty of capturing feedbacks in space and time tree stands (gray graph) due to redirection of wind and snow. with field data, several authors have used simulations to try to Adapted from Bader (2007). understand the effects of feedbacks between spatial patterns and dynamics (e.g., Malanson, 1997; Malanson et al., 2001; Alftine and Veblen, 2003; Stueve et al., 2009; Colombaroli et al., 2010). and Malanson, 2004; Malanson and Zeng, 2004; Wiegand et al., For example, the destruction of the soil organic layer by severe 2006; Bader et al., 2008; Elliott, 2009; Diaz-Varela et al., 2010). fires can result in an almost complete loss of nutrient supply, These computer simulations predict that alpine treelines exhibit reduced water-holding capacity of the soils, and consequent unusual dynamics. Zeng and Malanson (2006) found that a model increased surface runoff and soil erosion (e.g., Holtmeier, 2009; that included both positive and negative feedback could generate Holtmeier and Broll, 2005; Holtmeier et al., 2003; Broll et al., many observed patterns (notably fractals, cf. Allen and Walsh, 2007). Thus, the legacy of specific patterns in specific situations 1996) in a single long-term realization driven only by the becomes a dominant local control. At a broader temporal scale, endogenous feedback. Zeng et al. (2007) further found that such many regions of the Rocky Mountains possess ‘relict treelines’ self-organization maintained higher order pattern relations even formed by long-lived pines (Pinus aristata, P. albicaulis, P. when exogenous geomorphic patterns might be expected to alter it flexilis), subalpine fir (Abies lasiocarpa), and Engelmann spruce (at least within realistic ranges). Bader et al. (2008) found that (Picea engelmannii), which became established at higher elevations positive feedbacks could lead to abrupt transitions that decouple under a warmer-than-present climate many centuries or even rates of advance from climate change (Fig. 2). These modeling millennia ago (e.g., Ives, 1973; Ives and Hansen-Bristow, 1983; efforts are best taken as hypothesis generators, rather than tests, Holtmeier, 1985, 1999, 2009). During the subsequent less and indicate the likely important feedbacks. favorable climatic conditions, subalpine fir and Engelmann spruce were able to reproduce by layering (i.e., formation of adventitious Historical Legacy roots) in krummholz form. Some of these trees now produce viable While self-organization may be maintained in principle, the seeds and facilitate seedling establishment by providing shelter evolution of pattern is also contingent on exogenous forces. The from strong winds. position and structures of present treelines often are the result of Apart from natural landscape processes and feedbacks, historical legacy rather than of the present climate (e.g., human land use has in many regions exerted a strong influence Holtmeier, 1974, 2009; Holtmeier and Broll, 2007). Extreme on treeline patterns. In the European Alps and many other natural events such as severe storms, drought, extremely snow-rich Eurasian high mountains, which were already settled in prehistoric or poor winters, natural and human-induced forest fires, mass time, treeline has been lowered through pastoral use, mining, and outbreaks of leaf-eating insects, debris flows, snow avalanches, burning the high-elevation forest. The present upper limit of the rock avalanches, and volcanic eruptions have long-lasting effects forest has become an ecological boundary that is as distinct as was on current treeline ecotones (e.g. Butler and Walsh, 1994; Daniels the original climatic forest limit, at least in the Alps, Pyrenees, and G. P. MALANSON ET AL. / 171 Andes (Holtmeier, 1965, 1974; Camarero and Gutierrez, 2002; Di change impacts. To pursue this line of research will require analyses that compare geographic areas with different impacts. A Pasquale et al., 2008). A treeline depression by 150 to 300 m below starting point exists in current research, primarily in those the uppermost postglacial level of the climatic treeline can be locations with long records of human occupation, such as the taken for an average value (Holtmeier, 1974, 1986; Burga, 1988; European Alps (e.g., Didier, 2001; Heiri et al., 2006; Dullinger et Tinner et al., 1996; Carcaillet et al., 1998; Burga and Perret, 2001; al., 2003) and in the Andes (e.g., Young, 1993; Sarmiento, 2000; Kaltenrieder et al., 2005). In tropical mountains the history of Sarmiento and Frolich, 2002; Young and Leo ´ n, 2007, and human settlement and its impact on treeline habitats is less clear. references therein), but treelines in Asia and Africa also are part Humans are thought to have spread through South America of unique cultural landscapes. Because treeline forms are before the beginning of the Holocene (e.g., Jackson et al., 2007), historically contingent, extensive sampling will be needed to gain and the earliest evidence of fires at current treeline altitudes stem enough statistical power to make sense of these interactions. While from this time, although clear signals of regional agriculture only some such expansions could be within continents (e.g., Bader et appear in the second half of the Holocene (Di Pasquale et al., al., 2007b [plus an island]; Weiss, 2009), expansion across 2008). Paleoecological records of past treeline altitudes are continents to understudied areas, such as the southern hemisphere heterogeneous, but it is tempting to assume that humans have and remote tropical areas, especially in Africa and Asia, is used tropical alpine habitats from very early times and at least potentially most fruitful (cf. Ohsawa, 1990; Schmidt-Vogt, 1990; locally have slowed or prevented a rise in treeline altitude from late Miehe and Miehe, 1994; Rundel et al., 1994; Schickhoff, 1995, Pleistocene levels (Horn, 1993; Di Pasquale et al., 2008). In some 2005; Winkler, 1997; Wardle et al., 2001; Diaz et al., 2003; regions, however, recent destruction of tropical mountain forests Hofstede et al., 2003; Baker and Moseley, 2007). Moreover, by human land use is evident and has depressed treeline altitude investigation of the treelines on oceanic islands, where the considerably or in some cases has combined with deforestation altitudinal treeline position is usually several hundred meters from below to remove the forest belt altogether (e.g., Miehe and lower than the continental high-elevation treeline at the same Miehe, 2000). latitude, needs to be intensified (e.g., Azores, Canary Islands, Hawaii, etc.; cf. Henning, 1974; Leuschner and Schulte, 1991; Leuschner, 1996; Bader et al., 2007b). Not least, the upper treeline Underexplored Areas and Directions for Future Research in New Guinea would be a valuable site for field research because As noted, a motivation for much ongoing treeline research is it is the largest tropical island with a treeline located above 3000 m anticipated global climate change. The key question resulting from and human-induced fires play an important role for treeline this motivation is, ‘‘Will a warmer world result in globally physiognomy and dynamics (Paijmans and Lo ¨ ffler, 1972). comparable responses of treelines?’’ Given the hypothesized Given the three major conceptual domains that help define general causal control, we would expect a comparable upward and inform the landscape-scale approach to treeline research movement of alpine treeline ecotones worldwide—comparable in (landscape ecology, hierarchy theory, and complexity science), the sense that the rise in elevation of controlling isotherms, though addressing this geographical and historical variation will require a variable, would produce rises in treelines. However, given the research program that reaches across scales. Though a multitude landscape-scale controls, an increase in temperature will change of studies exist that address either, there is a scarcity of research the broad constraint, but the response in any one area will vary aimed at bridging the gap between general and local patterns and through the interaction of the fine- and broad-scale controls. Such causes of treeline (but see Harsch et al., 2009; Harsch and Bader, a varied response is indeed observed when comparing treelines 2011). To build such a bridge, methodologies can be shared worldwide (Harsch et al., 2009). A good example is the observed between the two approaches and specifically multiscale analyses dieback of treeline stands due to drought conditions accompany- can be adopted. ing temperature increases in the Sierra Nevada (Lloyd and In more general terms, we propose that more formal Graumlich, 1997; cf. Johnson et al., 2004; Brodersen et al., 2006; hierarchical statistical methods (e.g., using Bayesian and/or Johnson and Smith, 2007; Millar et al., 2007). Differential multilevel statistics) be applied to link the two approaches that responses, such as the limited advance of treeline in hedges on we have discerned (e.g., Beever et al., 2006; Clark and Gelfand, some areas with only densification but no advance in others in 2006; Qian and Shen, 2007). Multilevel regression could link Glacier National Park, U.S.A., indicates that local-scale controls approaches because of their nested, geographically hierarchical are more important than global temperature control here, at least relationship. Multilevel analyses will be most informative where in the short term (Butler et al., 1994; Klasner and Fagre, 2002; the levels of the hierarchy cross functionally important differences Alftine et al., 2003; Bekker, 2005). So although topography will in underlying and constraining variables. Some work on treeline change the spatial expression of the rise in elevation of any illustrates this approach but more variables are needed (e.g., isotherm, the nonlinear relations created by positive feedbacks, in Harsch et al., 2009; but contrast Gellrich et al., 2007). the context of existing spatial patterns and their legacies, will Research likely to emerge includes reformulations of existing further complicate the dynamics (e.g., Bader and Ruijten, 2008; treeline models that incorporate higher spatial and temporal and Bader et al., 2008; Kharuk et al., 2010). resolution data sets. Increasing the level of detail and geographic One productive area for new research would be in differen- specificity within the present understanding of alpine treeline tiating the responses to current climate change from those of past ecology is warranted because, generally speaking, global-scale human impact; both climate and land use are major aspects of controls on treelines are well understood, (that is, temperature is global change (Vitousek, 1994). Where treelines have been lowered an important control on treelines worldwide, e.g., Ko ¨ rner and by human activities such as grazing and burning, their response to Paulsen, 2004). In contrast, landscape-scale treeline analyses vary release from these impacts may have similarities with responses to greatly in their foci and level of detail, which often makes them climatic warming. Differentiating the two responses could be challenging to compare directly and to synthesize across large informative in terms of understanding the relative importance of geographic areas. Particularly desirable treeline analyses include processes and how they relate to ecological theory as well as those that (1) use theoretically and methodologically consistent providing a sound basis for monitoring and mitigating climate analytical approaches to better define geographic variability in 172 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH Bansal, S., and Germino, M. J., 2008: Carbon balance of conifer treeline pattern-process relationships; (2) explicitly assess the role seedlings at timberline: relative changes in uptake, storage, and that local climatic conditions (e.g., the timing of events like first utilization. Oecologia, 158: 217–227. snowfalls, spring thaw dates, and late and early frosts) play in tree Batllori, E., Camarero, J. J., Ninot, J. 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Taylor & Francis
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Arctic, Antarctic, and Alpine Research
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1938-4246
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1523-0430
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10.1657/1938-4246-43.2.167
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Abstract

Lynn M. Resler{" For over 100 years, mountain treelines have been the subject of varied research endeavors and remain a strong area of investigation. The purpose of this paper is to Maaike Y. Bader{ examine aspects of the epistemology of mountain treeline research—that is, to Friedrich-Karl Holtmeier1 investigate how knowledge on treelines has been acquired and the changes in David R. Butler# knowledge acquisition over time, through a review of fundamental questions and approaches. The questions treeline researchers have raised and continue to raise have Daniel J. Weiss@ undoubtedly directed the current state of knowledge. A continuing, fundamental Lori D. Daniels$ and emphasis has centered on seeking the general cause of mountain treelines, thus Daniel B. Fagre& seeking an answer to the question, ‘‘What causes treeline?’’ with a primary emphasis on searching for ecophysiological mechanisms of low-temperature limitation for tree *Department of Geography, University growth and regeneration. However, treeline research today also includes a rich of Iowa, Iowa City, Iowa 52242, U.S.A. {Department of Geography, Virginia literature that seeks local, landscape-scale causes of treelines and reasons why Tech, Blacksburg, Virginia 24061, treelines vary so widely in three-dimensional patterns from one location to the next, U.S.A. and this approach and some of its consequences are elaborated here. In recent years, {Functional Ecology of Plants, both lines of research have been motivated greatly by global climate change. Given Department of Biology and the current state of knowledge, we propose that future research directions focused on Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, a spatial approach should specifically address cross-scale hypotheses using statistics Germany and simulations designed for nested hierarchies; these analyses will benefit from 1Institute for Landscape Ecology, geographic extension of treeline research. Westfa ¨ lische Wilhelms-Universita ¨t, D-48149 Mu ¨ nster, Germany #Department of Geography, Texas State University–San Marcos, San Marcos, Texas 78666-4616, U.S.A. @Yellowstone Ecological Research Center, 2048 Analysis Drive, Bozeman, Montana 59718, U.S.A. $Department of Geography, University of British Columbia, Vancouver, British Columbia V6T 1Z2, Canada &U.S. Geological Survey, Northern Rocky Mountain Science Center, Glacier National Park, West Glacier, Montana 59936, U.S.A. "Corresponding author: resler@vt.edu DOI: 10.1657/1938-4246-43.2.167 ecological processes and relations) will depend on how they are Introduction approached and thus what we believe we know about this topic. Mountain treeline ecotones represent the spatial transition Our purpose here is not to review what is known, or thought from forested to treeless mountain landscapes and are, therefore, to be known, about the geography, ecology, and plant physiology considered among the most prominent features of mountain of mountain treeline ecotones; Holtmeier (2009) has already environments. Mountain treeline ecotones also represent the upper covered this ground from a worldwide perspective. Instead, our physiological limits of tree species and a lower boundary (though aim is to examine the nature of knowledge, through a review of the not a physiological limit) for alpine herbaceous species. The paths treeline researchers have followed to reach the current state importance of this boundary for nutrient fluxes and biodiversity, of knowledge about alpine treeline ecotones, and also how the the variety and complexity of spatial patterns found at treeline, goals for knowledge acquisition differ. For simplicity, we refer to and the intriguing and still enigmatic life-form limit it represents, these as studies of mountain ‘‘treeline,’’ and distinguish a has given rise to numerous and diverse research endeavors from a conceptual line versus ecotone only where necessary. broad disciplinary base for over 100 years. An important motivation for many recent studies of treeline is ongoing global climate change due to their hypothesized use as an indicator of APPROACHES climate change and to potential loss of biodiversity and ecosystem function as tundra is replaced by woody species. The usefulness of From the variety of research conducted at mountain treelines treeline dynamics as indicators of climate change (or of general worldwide, two major research approaches emerge: searches for 2011 Regents of the University of Colorado G. P. MALANSON ET AL. / 167 1523-0430/11 $7.00 global-scale and landscape-scale causes, wherein the latter are ‘‘descriptio montis fracti’’ (1555) that the altitudinal position of within and modify the limits of the former (Troll, 1973; Wardle, the vegetation belts is related to the decrease of temperature and 1974, 1993; Holtmeier, 2009). Ko ¨ rner (2003) identified these as length of growing season with altitude (Holtmeier, 1965). It is fundamental/global and modulative/regional, and we also see likely that Alexander von Humboldt (von Humboldt and Bonp- these approaches as inquiry at different spatial scales within a land, 1805) was among the first researchers to mention the geographic hierarchy but with the modulation being more local existence of alpine treelines in a manner that connected than regional. Studies exploring the broad causes of treeline observation with causality. His foundational work in physical attempt to answer the fundamental question, ‘‘What causes geography and meteorology was the first to document isotherms treelines?’’ Such studies attempt to understand the factors that and their relationship with elevation and plant geography of cause treelines to exist on a global level, and we refer to it here as mountain slopes (Marek, 1910; Troll, 1962). Additional work the global approach. Furthermore, these studies focus primarily included the early identification of heat (Da ¨ niker, 1923) and tree on limitations to tree growth from an ecophysiological perspective physiology (e.g., Michaelis, 1934; Steiner, 1935; Schmidt, 1936; Pisek because ecophysiology is replicated globally, but ecophysiological and Cartellieri, 1939), and more recent summaries combine the two questions are also examined locally. Studies examining the fine- (Tranquillini, 1979; Wieser and Tausz, 2007). Holtmeier (2009) scale (or local) causes of treelines attempt to answer the question, provides a more complete treatment of more recent research. ‘‘How and why do treelines differ across locations?’’ This Here, we devote most of our analysis to the fundamental approach focuses on landscape patterns, especially the effects of questions and themes in the landscape approach. We conclude by topography and treeline history (such as climatic changes, human outlining needs for future research on mountain treelines impact, and wildfire), because these factors differ within regions, conducted with a spatial perspective. In this paper, our emphasis and we refer to it as the landscape approach. It is not a coincidence on the landscape perspective in no way discredits the excellent that these two approaches have different foci in terms of treeline contributions to understanding treelines offered by the global studies, with the former most concerned with the upper elevational approach, and reflects, rather, the collective training and expertise limit of ‘trees,’ i.e., full upright stems (or at least 2 m; e.g., Wieser of the authors. and Tausz, 2007), and less with tree seedlings, while the latter is most concerned with the three-dimensional gradient or pattern across the ecotone with seedlings at the fore (e.g., Johnson et al., LANDSCAPE FOUNDATIONS 2004). The landscape approach, our main focus, is first and foremost Global treeline research is currently dominated by the search a part of landscape ecology. Much of the early impetus for work in for mechanisms of low-temperature limitations on tree growth the Alps was from the perspective of landscape level management (e.g., Ko ¨ rner, 1998a). Here the discussion centers on the role of problems (Holtmeier, 2009), the root of European landscape carbon acquisition versus use (i.e. source versus sink limitation) at ecology. Much of the pattern-process paradigm in landscape low temperatures (particularly root zone temperature) and/or ecology derived from island biogeography, and so it blossomed in limitations related to the utilization of carbon within trees. In spite the 1970s and boomed in the 1980s (Turner, 1989). This latter of abundant sampling in recent years (Li et al., 2002; Hoch and history is mirrored in the increasing interest in three-dimensional Ko ¨ rner, 2003; Piper et al., 2006; Bansal and Germino, 2008; Shi et pattern in treeline studies (e.g., Humphries et al., 2008; see Fig. 1). al., 2008), there is generally no evidence for a poor carbon status in However, landscape ecology has been affected by ideas formed in treeline trees in the long term, although it may influence local mountain environments, perhaps beginning in the mid-20th pattern (Cairns and Malanson, 1997; Cairns, 2005). Therefore, the century with Troll’s (1971) emphasis on ‘geo-ecology.’ attention is now turned to mechanisms of direct growth The landscape approach, second, includes hierarchy theory in limitations, with an emphasis on understanding how a mean ecology, which developed in the 1980s (e.g., Allen and Starr, 1982) growing season temperature can best correlate with treeline and has been applied to treeline research since the early 1990s. The positions worldwide (Ko ¨ rner and Paulsen, 2004), in spite of this core idea here is that processes and patterns develop at multiple mean being composed of widely differing temperature regimes spatial and temporal scales. Together they are comprised of (Hoch and Ko ¨ rner, 2009). interactions of processes at finer scales, while constrained by Investigations following the landscape approach have typi- patterns at coarser scales (O’Neill et al., 1989). For example, the cally studied treelines from the perspective of spatial pattern and establishment of a forest patch in the treeline ecotone would process (Walsh et al., 1997). Landscape change, linkages among depend on fine-scale processes such as seedling establishment but biotic and physical elements of the environment, and human/ is also constrained by coarse-scale patterns such as mountain environment interactions are often studied (Holtmeier, 1974; Broll topography (slope aspect, exposure to wind). Also, the general et al., 2007; Holtmeier, 2009). Landscape studies may also look at temperature control on treelines can be seen as ‘merely’ a coarser specific ecophysiological causes of treeline patterns, e.g., studies scale constraint on the focal three-dimensional pattern dynamics that address establishment limitations rather than growth in adult driven by population level processes (e.g., Brown et al., 1994; trees (e.g., Germino et al., 2002; Piper et al., 2006; Johnson and Walsh et al., 1997). Smith, 2007; Bader et al., 2007a; Bansal and Germino, 2008; Smith Third, complexity science has, explicitly or implicitly, become et al., 2009). Such studies often find that landscape position and part of many landscape-scale treeline studies. Complexity science facilitation by neighbors are very important. is a body of concepts that examines how higher order pattern or Though a divergence in knowledge ‘camps’ currently exists, structure in systems is produced by few, simple, but nonlinear present-day research has evolved from a similar line of inquiry— interactions at a lower level, and thus includes hierarchy (e.g., Sole one that originated and continues in the consideration of global- and Bascompte, 2006). The higher order patterns could not be scale causes of treelines. A search for broad-scale causes of treelines evolved from the work of great observationists. After predicted from the properties of the lower order units, but instead Leonardo da Vinci (1452–1519) had noticed the altitudinal belts the ‘emergence’ of structure must be determined by observing the and their specific organisms on Monte Rosa (Italian Alps), evolution of the system. Thus, a standard reductionist approach is Conrad Gesner (1516–1565) from Zurich mentioned in his overturned. Such systems can be said to be self-organized in that 168 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH FIGURE 1. Hedges, illustrat- ing positive spatial association, and individual seedlings, illustrat- ing association with microrelief, extend into tundra above the contiguous forest in the treeline ecotone at Lee Ridge, Glacier National Park, Montana, U.S.A. Photo: G. P. Malanson. the higher order structure can be reduced to fewer, but still coarser levels constrain the finer scale dynamics that create and endogenous, nonlinear dimensions or drivers. Complexity science reproduce them (e.g., Malanson et al., 2007). We further see the was developed and distinctly promoted by physicists (Gell-Mann, importance of dynamics as a conceptual framework, with both 1994; Bak, 1996), but quickly spread to other disciplines. straightforward and higher order patterns of change as objects of Malanson (1999) discussed its applicability to treeline studies, study and both endogenous feedbacks and exogenous drivers as and the work by Rietkerk et al. (2002) in spatial ecology hypothetical processes. influenced later advances by Zeng and Malanson (2006) and These epistemological foundations were enabled, if not Bader et al. (2008). Advances in all three domains developed driven, by methodological developments. Common to landscape simultaneously and were clearly being used together by landscape ecology in general, the landscape approach in treeline studies was ecologists. The work of O’Neill (O’Neill et al., 1982, 1986, 1989, advanced by remote sensing—aerial photography at first (e.g., 1992, inter alia) alone illustrates this synergy, and work in Troll, 1939), followed by satellite-based imagery (e.g., Baker and landscape ecology (e.g., Li, 2002) led to the initiation of a new Weisberg, 1995; Allen and Walsh, 1996; Walsh et al., 2003) and journal, Ecological Complexity, in 2004. geographic information systems (GIS) analysis (e.g., Brown, 1994; One can see why linkages among these three areas (landscape Ho ¨ rsch, 2003). Quantification of pattern was an essential ecology, hierarchy theory, and complexity) have been identified contributor to the concepts, and these technologies greatly and are relevant to treeline research. They link spatial heteroge- increased the available data on spatial pattern and simplified its neity and feedbacks (Troll, 1971), scale dependence in a analysis. Computer power itself enabled the development of geographic hierarchy (Pattee, ed., 1973), and self-organization hierarchy theory and complexity science in general (Pagels, 1988). (Haken, 1975). The relations between processes and patterns, in For example, cellular automata, or cellular models with stochas- this case between advances or retreats of treelines on mountain ticity added to neighborhood effects, were core tools in the slopes and the spatial structure of patches and edges (Fig. 1), can development of the complexity approach at ecotones in general be seen to have characteristics in common with other spatial (e.g., Loehle et al., 1996; Li, 2002) and treelines in particular phenomena at higher orders (e.g., fractal patterns or power-law (Alftine and Malanson, 2004; Malanson and Zeng, 2004; Zeng and distributions). Ecologists have found such higher order phenom- Malanson, 2006; Wiegand et al., 2006; Zeng et al., 2007; Bader et ena may explain important characteristics such as metabolic al., 2008). capacity (West et al., 1999) for which evolutionary processes may be revealed. For biogeography (e.g., Deng et al., 2006), including ecotones (Milne et al., 1996), the existence of these higher order Understanding Landscape-Scale Relations patterns may indicate constraints on the variability and explana- In landscape-scale treeline studies, an understanding of the tions possible (i.e., self-organization; Zeng and Malanson, 2006). treeline phenomenon and its local causes is obtained by focusing We might hope that these linkages would provide predictive power on variations of treeline spatial and temporal patterns (e.g., for the response of treelines to climate change, but prediction may Blu ¨ thgen, 1942; Griggs, 1946; Holtmeier, 1965; Smith et al., 2003; be limited. While self-organization, and thus fractal patterns, hold Broll et al., 2007; Malanson et al., 2009; Butler et al., 2009a; see under strong exogenous forcing from either climate change or Holtmeier, 2009, for further references). Like broad-scale ap- underlying geomorphic patterns (Zeng, 2010), the constraint that proaches to treeline, treeline dynamics under global climate patterns will remain fractal does not provide the kind of prediction change scenarios have been a major theme of current research that ecologists or landscape managers desire. from a landscape perspective. However, for landscape-scale Nevertheless, from these three perspectives we have learned to treeline scientists (who acknowledge that heat deficiency is the look at treelines at multiple scales, seeing organisms within ultimate limit on tree growth at high elevation), the problem is patches within landscapes in three dimensions, and how the G. P. MALANSON ET AL. / 169 alpine treeline. Cronartium ribicola, the introduced pathogen that that treeline ecotones present complex patterns in three dimen- causes blister rust in five-needled white pines, has caused sions that are beyond the explanatory power of temperature alone. widespread mortality in whitebark pine (Pinus albicaulis), a The ribbons, hedges, and other patchy patterns (see Holtmeier, foundation and keystone species of northern Rocky Mountain 1982, and Fig. 1), while constrained by temperature limits, require subalpine and treeline ecosystems (Resler and Tomback, 2008), explanation. The landscape approach to treeline is motivated by which has potentially serious and cascading consequences (Tom- interest in these three-dimensional patterns, and processes are seen back and Resler, 2007). Mass outbreaks of the autumnal moth as nested at multiple scales even within the landscape-scale domain (Epirrita autumnata) have repeatedly destroyed vast areas of (e.g., Elliott and Kipfmueller, 2010). mountain birch (Betula pubescens ssp. czerepanovii) forests up to the treeline in Finland, which has been followed by severe soil LOCAL DRIVERS erosion on sandy substrates that now impedes the re-establishment of this species (Holtmeier, 2002; Holtmeier et al., 2003; Broll et al., The primary fine-scale modulators of treeline patterns may be 2007). Mycorrhizae, which increase nutrient availability, may be those that affect heat deficiency, but fine-scale modulators have important to successful seedling establishment, tree growth, and effects on water, nutrients, and disturbance. Temperature defi- afforestation at and above the treeline (Hasselquist et al., 2005; ciency is not only a broad-scale cause of treelines, but can also be a Germino et al., 2006). Birds affect trees in the treeline ecotone by landscape-scale modulator of treeline pattern. At a landscape scale consuming seeds and buds, and dispersing and caching seeds (e.g., it is affected by local modulators such as topography, and it can the nutcrackers [Nucifraga caryocatatces and subspecies in also affect other landscape-scale modulators, such as distribution, Eurasia, Nucifraga columbiana in North America]; Holtmeier, depth, and duration of the winter snowpack. Factors that directly 1966, 2002; Tomback, 1977; Mattes, 1978, 1982, 1985). The reduce temperatures at local scale are wind, cold air drainage, impacts of burrowing animals can be positive because they expose snow cover in summer, and shade. Temperature affects snow cover the mineral soil and thus create open patches that may facilitate directly, in terms of determining the mix of rain vs. snow and the the establishment of seedlings (e.g., Butler et al., 2009b; Butler and rate of melting and, indirectly, by determining snow density and Butler, 2009); conversely they can destroy seedlings and saplings thus snow removal and redeposition by wind. by girdling and by pushing seedlings out of the ground (Holtmeier, Landscape-scale modulators not directly related to heat are 1987, 2002). At the other end of the size scale, grizzly bears tear quite diverse, but the one most important in ecophysiology is swaths of tundra with similar mixed effects (Butler 1992, 1995). water (e.g., Brodersen et al., 2006). Treeline areas vary in the soil Also, the activities of wild-living ungulates are detrimental to moisture available for plant growth at landscape scales, and much treelines (Holtmeier, 2002, 2009; Cairns and Moen, 2004; Cairns et depends on the removal and redeposition of snow by wind. Some al., 2007). These phenomena are spatially variable and their areas are scoured of snow and so experience drought conditions relations with temperature are not well known. much beyond what the regional precipitation pattern would Additionally, competition with alpine herbaceous species can indicate, whereas other areas have multiples of the regional limit or facilitate seedling establishment. Seedlings are facilitated precipitation due to snow collection and eventual melt (e.g., Walsh by a moderate amount of herb cover and limited by too little or et al., 1994; Hiemstra et al., 2002, 2006; Geddes et al., 2005; too much (Germino et al., 2002). These relations can affect pattern Holtmeier, 2005). Other effects of excessive snow cover, which development (Malanson and Butler, 1994). Moreover, allelopathic may impede or even prevent successful seedling establishment, effects of the associated vegetation (e.g., some lichen species or include shortened growing season, physical damage through dwarf shrubs), may impair germination, mycorrhiza development creeping and settling snow, and snow fungi, which may affect and seedling development (Holtmeier, 2009, further references seedlings and saplings of evergreen conifers (e.g., Cunningham et therein). al., 2006). Geomorphology is also a landscape-scale modulator of treeline patterns and dynamics, exerting its influence through SPATIAL DYNAMICS more direct causes like snow, soil, and disturbance (e.g., Kullman, Change through time is integral to explaining three-dimen- 1997; Walsh et al., 2003; Holtmeier and Broll, 2005; Butler et al., sional treeline patterns in a spatial hierarchy (Armand, 1992; 2007; Zeng et al., 2007; Humphries et al., 2008; Butler et al., 2009a, Wiegand et al.. 2006). The feedbacks of growing tree populations 2009b; Bekker and Malanson, 2009; Munier et al., 2010). Local on their neighborhood become increasingly important as the size topography may provide shelter from the wind, as on the lee sides of individuals and patches increases (e.g., Holtmeier, 1999, 2005, of small ridges, solifluction terraces, rocky outcrops and other 2009), but fade as these patches merge into contiguous forest convex sites (e.g., Resler et al., 2005, in the Rocky Mountains; because of reduced edge (Zeng and Malanson, 2006). These Holtmeier et al., 2003; Anschlag et al., 2008, in Finland; see feedbacks imply a strong effect of existing patterns on dynamics Fig. 1). Depressions may be covered too long with snow, however, and indicate self-organization, and more generally, an important and waterlogging can be an additional adverse factor in such role for landscape history. poorly drained places. On wind-exposed convex topography with little or no winter snowpack, wind actions (physiologically, Patterns and Feedbacks mechanically) may cause serious damage to seedlings and saplings (Cairns and Malanson, 1998). Temperatures—their pattern as well Treelines frequently exhibit very discrete patterns, consisting, as limits—also affect local geomorphic processes such as for instance, of abrupt forest edges or distinct dwarf tree or solifluction and frost heaving that may affect seedling establish- krummholz patches (e.g., Walsh et al., 1992; Humphries et al., ment (Butler et al., 2004, 2009b). 2008). The variety of three-dimensional patterns observed are Further local causes of treeline not directly related to heat are unlikely to emerge on environmental gradients of temperature due due to biotic factors such as pathogens, insects, mycorrhizae, to lapse rates unless positive feedback processes amplify initial birds, and mammals. Pathogen outbreaks can cause regional environmental differences (i.e., a positive feedback switch sensu mortality of trees that could ultimately influence spatial pattern at Wilson and Agnew, 1992) or growth responses are nonlinear 170 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH (Batllori et al., 2009). The most notable driver in the pattern- process feedback is wind (e.g., Marr, 1977; Akhalkatsi et al., 2006; Holtmeier and Broll, 2010); shade and protection from sky exposure is probably next (e.g., Orlander, 1993; Germino and Smith, 2000; Germino et al., 2002; Smith et al., 2003; Slot et al., 2005); followed by nutrient enrichment (Holtmeier and Broll, 1992; Seastedt and Adams, 2001; Shiels and Sanford, 2001; Liptzin and Seastedt, 2009) and albedo. The negative feedback (shading and cooler soils) identified by Ko ¨ rner (1998a, 1998b) acts in the opposite direction. However, tree seedlings at treeline are often found preferentially beneath tree canopies (e.g., Griggs, 1946; Ball et al., 1991; Germino and Smith, 1999; Resler and Fonstad, 2009), indicating that the positive feedback through shelter and protection from radiative stress is more important for treeline spatial dynamics than the negative feedback of lower soil temperatures. Several treeline studies have investigated the role of landscape position, including effects of neighboring plants, on demographic processes (e.g., Daly and Shankman, 1985; Bekker, 2005; Resler et al., 2005; Maher and Germino, 2006; Batllori et al., 2009, 2010; Hughes et al., 2009). However, few of these studies have included indices or direct measurements of ecophysiological parameters like FIGURE 2. Emergence of abrupt treeline transitions in time and photosynthetic capacity (Germino and Smith, 1999; Maher et al., space. (a) Tree growth is determined by the microclimate, which is determined by the external macroclimate [e] and modifications 2005) or pigmentation (Akhalkatsi et al., 2006), thus indicating the caused by the vegetation itself [i]. Depending on the strength of i mechanisms by which spatial associations have probably arisen relative to e, the vegetation will be more (large e) or less (large i) (e.g., protection from cold-induced photoinhibition). Combina- coupled to the external climate and hence more or less sensitive to tions of such detailed mechanistic knowledge could strongly climatic changes. (b) Hysteresis fold demonstrating the alternative improve predictions of response to climate change. Another stable states of forest and alpine vegetation existing under the same interesting issue is the effect of landscape position on the growth external environmental conditions (gray area; at treelines the stress and general performance of adult trees, bringing local detail to the factor can be e.g., freezing temperatures, wind, or high solar general ecophysiological principles that may (or may not) be radiation). Abrupt (‘catastrophic’) transitions from forest to alpine applicable at treelines globally (Li and Yang, 2004; Wilmking et vegetation or vice versa can occur at threshold conditions or due to al., 2004). disturbances. (c) Changes in microclimatic conditions as a result of vegetation cover. Some stress factors can be aggravated just outside Given the difficulty of capturing feedbacks in space and time tree stands (gray graph) due to redirection of wind and snow. with field data, several authors have used simulations to try to Adapted from Bader (2007). understand the effects of feedbacks between spatial patterns and dynamics (e.g., Malanson, 1997; Malanson et al., 2001; Alftine and Veblen, 2003; Stueve et al., 2009; Colombaroli et al., 2010). and Malanson, 2004; Malanson and Zeng, 2004; Wiegand et al., For example, the destruction of the soil organic layer by severe 2006; Bader et al., 2008; Elliott, 2009; Diaz-Varela et al., 2010). fires can result in an almost complete loss of nutrient supply, These computer simulations predict that alpine treelines exhibit reduced water-holding capacity of the soils, and consequent unusual dynamics. Zeng and Malanson (2006) found that a model increased surface runoff and soil erosion (e.g., Holtmeier, 2009; that included both positive and negative feedback could generate Holtmeier and Broll, 2005; Holtmeier et al., 2003; Broll et al., many observed patterns (notably fractals, cf. Allen and Walsh, 2007). Thus, the legacy of specific patterns in specific situations 1996) in a single long-term realization driven only by the becomes a dominant local control. At a broader temporal scale, endogenous feedback. Zeng et al. (2007) further found that such many regions of the Rocky Mountains possess ‘relict treelines’ self-organization maintained higher order pattern relations even formed by long-lived pines (Pinus aristata, P. albicaulis, P. when exogenous geomorphic patterns might be expected to alter it flexilis), subalpine fir (Abies lasiocarpa), and Engelmann spruce (at least within realistic ranges). Bader et al. (2008) found that (Picea engelmannii), which became established at higher elevations positive feedbacks could lead to abrupt transitions that decouple under a warmer-than-present climate many centuries or even rates of advance from climate change (Fig. 2). These modeling millennia ago (e.g., Ives, 1973; Ives and Hansen-Bristow, 1983; efforts are best taken as hypothesis generators, rather than tests, Holtmeier, 1985, 1999, 2009). During the subsequent less and indicate the likely important feedbacks. favorable climatic conditions, subalpine fir and Engelmann spruce were able to reproduce by layering (i.e., formation of adventitious Historical Legacy roots) in krummholz form. Some of these trees now produce viable While self-organization may be maintained in principle, the seeds and facilitate seedling establishment by providing shelter evolution of pattern is also contingent on exogenous forces. The from strong winds. position and structures of present treelines often are the result of Apart from natural landscape processes and feedbacks, historical legacy rather than of the present climate (e.g., human land use has in many regions exerted a strong influence Holtmeier, 1974, 2009; Holtmeier and Broll, 2007). Extreme on treeline patterns. In the European Alps and many other natural events such as severe storms, drought, extremely snow-rich Eurasian high mountains, which were already settled in prehistoric or poor winters, natural and human-induced forest fires, mass time, treeline has been lowered through pastoral use, mining, and outbreaks of leaf-eating insects, debris flows, snow avalanches, burning the high-elevation forest. The present upper limit of the rock avalanches, and volcanic eruptions have long-lasting effects forest has become an ecological boundary that is as distinct as was on current treeline ecotones (e.g. Butler and Walsh, 1994; Daniels the original climatic forest limit, at least in the Alps, Pyrenees, and G. P. MALANSON ET AL. / 171 Andes (Holtmeier, 1965, 1974; Camarero and Gutierrez, 2002; Di change impacts. To pursue this line of research will require analyses that compare geographic areas with different impacts. A Pasquale et al., 2008). A treeline depression by 150 to 300 m below starting point exists in current research, primarily in those the uppermost postglacial level of the climatic treeline can be locations with long records of human occupation, such as the taken for an average value (Holtmeier, 1974, 1986; Burga, 1988; European Alps (e.g., Didier, 2001; Heiri et al., 2006; Dullinger et Tinner et al., 1996; Carcaillet et al., 1998; Burga and Perret, 2001; al., 2003) and in the Andes (e.g., Young, 1993; Sarmiento, 2000; Kaltenrieder et al., 2005). In tropical mountains the history of Sarmiento and Frolich, 2002; Young and Leo ´ n, 2007, and human settlement and its impact on treeline habitats is less clear. references therein), but treelines in Asia and Africa also are part Humans are thought to have spread through South America of unique cultural landscapes. Because treeline forms are before the beginning of the Holocene (e.g., Jackson et al., 2007), historically contingent, extensive sampling will be needed to gain and the earliest evidence of fires at current treeline altitudes stem enough statistical power to make sense of these interactions. While from this time, although clear signals of regional agriculture only some such expansions could be within continents (e.g., Bader et appear in the second half of the Holocene (Di Pasquale et al., al., 2007b [plus an island]; Weiss, 2009), expansion across 2008). Paleoecological records of past treeline altitudes are continents to understudied areas, such as the southern hemisphere heterogeneous, but it is tempting to assume that humans have and remote tropical areas, especially in Africa and Asia, is used tropical alpine habitats from very early times and at least potentially most fruitful (cf. Ohsawa, 1990; Schmidt-Vogt, 1990; locally have slowed or prevented a rise in treeline altitude from late Miehe and Miehe, 1994; Rundel et al., 1994; Schickhoff, 1995, Pleistocene levels (Horn, 1993; Di Pasquale et al., 2008). In some 2005; Winkler, 1997; Wardle et al., 2001; Diaz et al., 2003; regions, however, recent destruction of tropical mountain forests Hofstede et al., 2003; Baker and Moseley, 2007). Moreover, by human land use is evident and has depressed treeline altitude investigation of the treelines on oceanic islands, where the considerably or in some cases has combined with deforestation altitudinal treeline position is usually several hundred meters from below to remove the forest belt altogether (e.g., Miehe and lower than the continental high-elevation treeline at the same Miehe, 2000). latitude, needs to be intensified (e.g., Azores, Canary Islands, Hawaii, etc.; cf. Henning, 1974; Leuschner and Schulte, 1991; Leuschner, 1996; Bader et al., 2007b). Not least, the upper treeline Underexplored Areas and Directions for Future Research in New Guinea would be a valuable site for field research because As noted, a motivation for much ongoing treeline research is it is the largest tropical island with a treeline located above 3000 m anticipated global climate change. The key question resulting from and human-induced fires play an important role for treeline this motivation is, ‘‘Will a warmer world result in globally physiognomy and dynamics (Paijmans and Lo ¨ ffler, 1972). comparable responses of treelines?’’ Given the hypothesized Given the three major conceptual domains that help define general causal control, we would expect a comparable upward and inform the landscape-scale approach to treeline research movement of alpine treeline ecotones worldwide—comparable in (landscape ecology, hierarchy theory, and complexity science), the sense that the rise in elevation of controlling isotherms, though addressing this geographical and historical variation will require a variable, would produce rises in treelines. However, given the research program that reaches across scales. Though a multitude landscape-scale controls, an increase in temperature will change of studies exist that address either, there is a scarcity of research the broad constraint, but the response in any one area will vary aimed at bridging the gap between general and local patterns and through the interaction of the fine- and broad-scale controls. Such causes of treeline (but see Harsch et al., 2009; Harsch and Bader, a varied response is indeed observed when comparing treelines 2011). To build such a bridge, methodologies can be shared worldwide (Harsch et al., 2009). A good example is the observed between the two approaches and specifically multiscale analyses dieback of treeline stands due to drought conditions accompany- can be adopted. ing temperature increases in the Sierra Nevada (Lloyd and In more general terms, we propose that more formal Graumlich, 1997; cf. Johnson et al., 2004; Brodersen et al., 2006; hierarchical statistical methods (e.g., using Bayesian and/or Johnson and Smith, 2007; Millar et al., 2007). Differential multilevel statistics) be applied to link the two approaches that responses, such as the limited advance of treeline in hedges on we have discerned (e.g., Beever et al., 2006; Clark and Gelfand, some areas with only densification but no advance in others in 2006; Qian and Shen, 2007). Multilevel regression could link Glacier National Park, U.S.A., indicates that local-scale controls approaches because of their nested, geographically hierarchical are more important than global temperature control here, at least relationship. Multilevel analyses will be most informative where in the short term (Butler et al., 1994; Klasner and Fagre, 2002; the levels of the hierarchy cross functionally important differences Alftine et al., 2003; Bekker, 2005). So although topography will in underlying and constraining variables. Some work on treeline change the spatial expression of the rise in elevation of any illustrates this approach but more variables are needed (e.g., isotherm, the nonlinear relations created by positive feedbacks, in Harsch et al., 2009; but contrast Gellrich et al., 2007). the context of existing spatial patterns and their legacies, will Research likely to emerge includes reformulations of existing further complicate the dynamics (e.g., Bader and Ruijten, 2008; treeline models that incorporate higher spatial and temporal and Bader et al., 2008; Kharuk et al., 2010). resolution data sets. Increasing the level of detail and geographic One productive area for new research would be in differen- specificity within the present understanding of alpine treeline tiating the responses to current climate change from those of past ecology is warranted because, generally speaking, global-scale human impact; both climate and land use are major aspects of controls on treelines are well understood, (that is, temperature is global change (Vitousek, 1994). Where treelines have been lowered an important control on treelines worldwide, e.g., Ko ¨ rner and by human activities such as grazing and burning, their response to Paulsen, 2004). In contrast, landscape-scale treeline analyses vary release from these impacts may have similarities with responses to greatly in their foci and level of detail, which often makes them climatic warming. Differentiating the two responses could be challenging to compare directly and to synthesize across large informative in terms of understanding the relative importance of geographic areas. Particularly desirable treeline analyses include processes and how they relate to ecological theory as well as those that (1) use theoretically and methodologically consistent providing a sound basis for monitoring and mitigating climate analytical approaches to better define geographic variability in 172 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH Bansal, S., and Germino, M. J., 2008: Carbon balance of conifer treeline pattern-process relationships; (2) explicitly assess the role seedlings at timberline: relative changes in uptake, storage, and that local climatic conditions (e.g., the timing of events like first utilization. Oecologia, 158: 217–227. snowfalls, spring thaw dates, and late and early frosts) play in tree Batllori, E., Camarero, J. J., Ninot, J. 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Journal

"Arctic, Antartic and Alpine Research"Taylor & Francis

Published: May 1, 2011

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