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Spatial and Temporal Heterogeneity of Vegetation Properties among Four Tundra Plant Communities at Ivotuk, Alaska, U.S.A.

Spatial and Temporal Heterogeneity of Vegetation Properties among Four Tundra Plant Communities... Howard E. Epstein* Intraseasonal patterns of normalized difference vegetation index (NDVI), leaf area index (LAI), and phytomass were compared for four tundra vegetation types at Ivotuk, Alaska, Donald A. Walker during summer 1999. The vegetation types included moist acidic tundra (MAT), moist David L. Richardson* nonacidic tundra (MNT), mossy tussock tundra, and shrub tundra. The seasonal curves of Monika P. Calef * NDVI were similar among the vegetation types but with varying magnitudes of the peak values. Peak NDVI in the shrub tundra (0.83) was significantly greater than in MAT Erika Edwards and (0.76), which was significantly greater than in MNT (0.71) and mossy tussock tundra Amber Moody (0.70). LAI and phytomass exhibited high temporal variability with distinct seasonality *Department of Environmental Sciences, only in shrub tundra. Seasonal LAI and NDVI patterns were therefore correlated only in University of Virginia, Charlottesville, shrub tundra, which was attributed to the high quantity of deciduous shrub foliage present Virginia 22904-4123, U.S.A. in this community and absent in the other vegetation types. Shrub tundra peak live above- Institute of Arctic Biology, University of ground phytomass (1256 6 123 g m ) was significantly greater than peak live above- Alaska, Fairbanks, Alaska 99775-7000, U.S.A. ground phytomass for MAT, MNT, and mossy tussock tundra (722 6 71, 773 6 53, Corresponding author. 703 6 39 g m respectively, P , 0.05). Relative abundances of deciduous shrubs, hee2b@virginia.edu mosses, and graminoids were revealed as key components controlling differences in NDVI, LAI, and phytomass among tundra vegetation types. fully understood (Bockheim et al., 1998), differences in peat formation Introduction are considered a major control of the acidity of arctic soils (M. D. Recognition of the complexity and importance of arctic tundra Walker et al., 1994). Low-pH soils are hypothesized to occur when peat ecosystems with regard to their sensitivity to climate change (Oechel formation results in restricted drainage and accumulation of acidophilic et al., 1993, 1994; Chapin et al., 1995; Myneni et al., 1997; Arft et al., mosses, which in turn alters the soil chemistry (M. D. Walker et al., 1999; Epstein et al., 2000) has established the need for more detailed 1994). The existence of nonacidic soils can be attributed to several analyses into tundra vegetation. Some recent studies have focused on natural disturbances, including loess deposition, glacial till (Walker understanding landscape heterogeneity of tundra vegetation caused by and Everett, 1991), solifluction, alluvial process (M. D. Walker et al., topography (Giblin et al., 1991; Shaver and Chapin, 1991; Walker and 1994), cryoturbation (Bockheim et al., 1998), and differences in land- Everett, 1991; Shaver et al., 1996) and local variations of substrate (M. scape age (Walker et al., 1998). The differences in soil properties have D. Walker et al., 1994; Gough et al., 2000; D. A. Walker et al., 2001, a substantial effect on ecosystem processes (Walker et al., 1998; Gough 2003a). Total live plant biomass and net primary production are highly et al., 2000; Hobbie and Gough, 2002). Compared to acidic tundra, variable among communities along toposequences (Shaver and Chapin, nonacidic tundra has been shown to have thinner organic horizons, 1991; Walker and Everett, 1991; Shaver et al., 1996); in general, these greater soil heat flux, deeper summer thaw, and greater concentrations system properties are greatest in well-drained riparian areas followed by of calcium, and serves as less of a carbon sink and a smaller methane moist mid-slope tussock tundra communities. Phytomass levels in source (Bockheim et al., 1998; Walker et al., 1998). lowland wet sedge communities are typically considered limited by poor Differing nutrient regimes can have important implications for nutrient supply resulting from anaerobic conditions. Upland dry heath vegetation properties in tundra ecosystems, many of which are communities can exhibit lower levels of phytomass and production, considered nutrient limited (Chapin et al., 1986; Shaver et al., 2001). compared to mid-slope tussock tundra, due to winter desiccation on Compared to acidic tundra communities, nonacidic communities have exposed ridges and its effect on plant community composition (Shaver greater biodiversity (M. D. Walker et al., 1994; Gough et al., 2000). and Chapin, 1991). One study conducted on a toposequence in northern Species composition is also quite different; moist acidic tundra (MAT) Alaska, found that the highest total live plant biomass and second communities are dominated by the sedge, Eriophorum vaginatum, highest productivity occurred in the hilltop birch-heath section of the Sphagnum mosses and dwarf shrubs such as Betula nana, while moist toposequence (Shaver et al., 1996). Local differences in biomass, nonacidic tundra (MNT) communities are generally dominated by the productivity, and species composition may result from variations in sedge, Carex bigelowii, and Dryas integrifolia, a prostrate shrub (M. D. nutrient availability, which are consequences of the interaction of Walker et al., 1994). topography and other environmental variables. Recent findings indicate that the presence of MNT is not an Differences in soil pH can play a key role in affecting the local isolated or small-scale occurrence (Walker et al., 1998, 2001), thus variation of tundra vegetation properties. Distribution patterns of acidic providing further evidence that the natural patchiness of vegetation and nonacidic soils have been observed at the regional scale (Walker types is common across the tundra even on subregional scales. Some et al., 1998), and soil pH in arctic tundra can also vary considerably at previous studies have focused on quantifying differences in vegetation the landscape scale (100s of meters) (Bockheim et al., 1998). While attributes between MNT and MAT (Walker et al., 1998; 2001, 2003a; the ultimate control on the distribution of nonacidic communities is not Gough et al., 2000; Hobbie and Gough, 2002; Jia et al., 2002). The 2005 Regents of the University of Colorado S. M. RIEDEL ET AL. / 25 1523-0430/05 $7.00 TABLE I with nonacidic soils (Table 1). The shrub tundra site is located in a low slope position and drainage channel, compared to mid and upper slope Soil and topography data for four distinct tundra vegetation types at positions for the two other vegetation types found on acidic soils (Table Ivotuk, Alaska. 1). The vegetation types examined in this study are common throughout Soil water the North Slope and have been classified as distinct types in numerous a b Vegetation type Soil pH content (%) Slope studies (Shaver et al., 1996; M. D. Walker et al., 1994; D. A. Walker Moist Acidic Tundra 4.8 461.1 2.58 et al., 1998, 2001, 2003a; Gough et al., 2000, Jia et al., 2004). Moist Nonacidic Tundra 6.3 200.4 3.48 The vegetation at the MAT site is composed of Eriophorum Mossy Tussock Tundra 4.2 443.2 1.28 vaginatum, Sphagnum mosses, and dwarf shrubs, including Betula Shrub Tundra 4.5 295.9 4.58 nana. The MNT site, found on neutral pH soils, has a vegetative community dominated by Carex bigelowii and Dryas integrifolia,as Mean soil pH to a depth of 100 cm (Ping et al., 1998). well as non-Sphagnum mosses. The mossy tussock tundra site is Soil water content of the O horizon (Ping et al., 1998). composed mostly of Sphagnum mosses, Eriophorum vaginatum, Betula nana, and lichens. The shrub tundra site is dominated by Salix pulchra and Betula nana (Fig. 1). majority of the studies have typically focused on comparing vegetation NDVI, LAI, and above-ground phytomass samples were collected properties at the peak of the growing season. Several studies have biweekly from four 100 m 3 100 m grids, each representative of captured the intraseasonal variation of tundra vegetation, but have been a different vegetation type, during the 1999 growing season. Field limited to the use of satellite data (Stow et al., 1993; Jia et al. 2004), and measurements were divided into seven sampling periods beginning 5 therefore the spatial scale at which vegetation properties can be June and ending 27 August. LAI and NDVI measurements were taken examined is bound by resolution of the satellite data. at 20 random points within each grid; the same 20 points were used The objective of this study was to investigate the spatial and throughout the growing season. From prior sampling, we determined temporal patterns of tundra vegetation that result from differences in that 20 random points in a 100 m 3 100 m grid were sufficient for soil properties and topographic location. Seasonal patterns of field- capturing the variability at this spatial scale. NDVI was measured using derived normalized difference vegetation index (NDVI), leaf area index an Analytical Spectral Devices FieldSpec spectroradiometer (Boulder, (LAI), and phytomass have not been compared across an entire growing Colorado). The reflectance data for the pertinent wavelength intervals season for a set of distinct tundra vegetation types that occur within the were used to calculate NDVI from the formula same landscape. Seasonal patterns of NDVI, LAI, and phytomass were analyzed to determine how vegetation properties vary among distinct ðNIR  redÞ NDVI ¼ ð1Þ types that exist as a result of landscape scale heterogeneity. Addi- ðNIR þ redÞ tionally, seasonal patterns of vegetation properties were compared to where NIR is the near infrared reflectance (average reflectance of wave- examine how the level of spatial heterogeneity varied throughout the lengths between 725 and 1060 nm), and ‘‘red’’ is the red reflectance growing season. Finally, this study provides a critical baseline for (average reflectance of wavelengths between 580 and 680 nm) of the ground measurements of tundra vegetation properties against which vegetation. Four replicate measurements were taken 1 m north, east, future measurements from the same site can be compared. south, and west of each grid point and averaged to give a mean NDVI value. The fiber optic sensor of the FieldSpec spectroradiometer was held approximately 1.5 m above the surface of the vegetation, pro- Methods viding a 0.35-m footprint using a 258 field of view. The spectro- Our study site was Ivotuk, Alaska (68.498N, 155.748W), which is radiometer was calibrated with dark and pure white readings before located on the North Slope of the Brooks Mountain Range and is measurements at each grid point. characterized by a growing season of approximately 110 d and a mean LAI which is simply the total leaf area per unit ground surface area 2 2 (m m ) is commonly measured optically, as it was in this study, using July maximum daily temperature of ;128C. Mean annual temperature a LI-COR LAI-2000 Plant Canopy Analyzer (Lincoln, Nebraska). is 10.98C, and mean annual precipitation is 202 mm based on a 3-yr Above- and below-canopy radiation measurements are made with dataset (1991–2001; Hinzman, unpublished data). Ivotuk was a key a ‘‘fish eye’’ optical sensor with a 5-ring 1488 angle of view. Below- sampling location for the NSF Arctic Transitions in the Land- canopy measurements were made from the top of the moss layer. LAI is Atmosphere System (ATLAS) study as part of a western North Slope transect from Point Barrow to the Seward Peninsula. Ivotuk was chosen calculated using a model of radiative transfer in vegetation canopies as a research site because it has one of the few airstrips in all of and then stored in the unit data logger. At each of the 20 grid points, northwestern Alaska, and it is comparable to Toolik Lake, a National one above canopy reading was taken, and four below canopy readings Science Foundation Long-Term Ecological Research site found were taken (1 m north, east, south, and west of the grid point) to yield approximately 200 km to the east. Additionally, the study site was a single LAI value. chosen because four tundra vegetation types, moist acidic tundra Above-ground phytomass was collected from 10 randomly (MAT), moist nonacidic tundra (MNT), mossy tussock tundra, and selected 20 cm 3 50 cm plots within each grid, for a total of 1 m per shrub tundra exist within a 2 km area. Examining the heterogeneity of vegetation type for each of the first six sample periods. Phytomass tundra vegetation was accomplished by comparing ecosystem proper- sampling occurred near, but not within, the footprints of LAI and NDVI ties across distinct vegetation types that occur largely as a result of measurements. On the fourth sample period (15–26 July), the number of differences in hydrologic regime and soil pH. In this study the MNT site phytomass plots harvested was doubled for MAT and mossy tussock differed from the other three vegetation types in that it is the only site tundra in order to more accurately capture peak biomass. On the fifth FIGURE 1. Vegetation maps of the shrub tundra (ST), moist nonacidic tundra (MNT) and mossy tussock tundra (MT) 100 3 100 m grids. A vegetation map of the moist acidic tundra (MAT) grid was not developed because the corners of the grid could not be adequately georeferenced from the aerial photographs of the site. 26 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH S. M. RIEDEL ET AL. / 27 FIGURE 3. (a) Peak quantities of total live above-ground phytomass for the 1999 growing season for four vegetation types found at Ivotuk, Alaska. Distinct hatching represents specific components of above- ground phytomass. (b) The percent of peak live above-ground phytomass comprised by each component plant type for four vegetation types at Ivotuk, Alaska. Distinct hatching represents specific components of above-ground phytomass. munity in the nonacidic tundra. The graminoid and shrub samples were sorted further into subcategories. Graminoid phytomass was divided into live and dead material. Shrub phytomass was divided into evergreen and deciduous, which were then separated into woody, foliar live, and foliar dead components. Dead material was combined into a single pool, referred to as ‘‘standing dead.’’ Therefore when we refer FIGURE 2. (a) Seasonal patterns of NDVI across the 1999 growing to specific components of phytomass or total phytomass, we are season at Ivotuk, Alaska. Curves represent four distinct vegetation referring only to the above-ground live fraction. types. Error bars represent the standard error of the mean (n ¼ 20); Comparison of seasonal patterns of LAI, NDVI, and phytomass (b) Seasonal patterns of LAI across the 1999 growing season at Ivotuk, among the four vegetation types was accomplished using a repeated Alaska. Curves represent four distinct vegetation types. Error bars represent the standard error of the mean (n¼ 20); (c) Seasonal patterns measures general linear model (SPSS 8.0). Peak LAI, NDVI, and of total live above-ground phytomass across the 1999 growing season phytomass among the four vegetation types were compared using at Ivotuk, Alaska. Curves represent four distinct vegetation types. Error ANOVA. Above-ground net primary productivity (ANPP) was bars represent the standard error of the mean (n ¼ 10 or n ¼ 20*). estimated by subtracting initial phytomass from peak phytomass. Coefficients of variation were calculated for mid-growing season sample period (27 July–7 August), the number of phytomass plots phytomass values to investigate the level of heterogeneity within harvested was doubled for MNT and shrub tundra, for the same reason. vegetation types. Results were used to determine the controls that local Vascular plants were clipped at the top of the moss surface; mosses substrate and topography have on vegetation patterns. Finally, we were clipped at the base of the green layer. Phytomass samples were compared biomass and ANPP data from Ivotuk to other similar datasets sorted into six main categories (horsetails, other forbs, graminoids, collected near the Toolik Lake Long-Term Ecological Research site lichens, mosses, and shrubs). Horsetails were separated from other forbs within the Kuparuk River Basin, located approximately 200 km to the because they can represent a substantial component of the plant com- east of Ivotuk (Shaver and Chapin, 1991; Shippert et al., 1995; Walker 28 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH 2 1 et al., 1995; Shaver et al., 1996, 2001; Williams and Rastetter, 1999, was considerably less productivity observed in MNT (357 g m yr ) Hobbie et al., 2002; Walker et al. 2003a, 2003b; Epstein et al., 2004). than in MAT and shrub tundra, and the mossy tussock tundra had the 2 1 lowest rate of ANPP of all vegetation types (140 g m yr ). Spatial coefficients of variation were relatively similar across vegetation types Results (Table 4). All four tundra types had high coefficients of variation for The seasonal patterns of NDVI were significantly different among lichen biomass (78–222%) and low coefficients of variation for shrub the four vegetation types (repeated measures GLM, P , 0.05) (Fig. 2a). biomass (36–44%). Spatial variation in forb biomass was high for Early season (7–10 June) values of NDVI were generally similar for all MAT (182%) and MNT (149%). vegetation types, with no statistical difference between MAT, mossy Total above-ground live biomass values for MAT at Ivotuk (using tundra, and shrub tundra. Peak season NDVI values occurred at the same data from this study and Walker et al. [2003b]) averaged 780 g m and time for three vegetation types (16 July), the exception being the mossy fell within the range of values (although on the high side) estimated for tussock tundra (27 July). The peak value of NDVI in shrub tundra areas near Toolik Lake (444–789 g m ). In general, greater moss, (0.83 6 0.01; mean 6 one standard error) was significantly greater than deciduous shrub and graminoid biomass at Ivotuk accounted for the in the MAT (0.76 6 0.01), which was significantly greater than in the relatively greater total biomass at Ivotuk compared to Toolik Lake. MNT (0.71 6 0.01) and the mossy tundra (0.70 6 0.01) (P , 0.05). ANPP estimates of MAT from Ivotuk were substantially greater than 2 1 Late season (22–26 August) NDVI values were again not statistically those estimated for the Toolik Lake area, 479 g m yr at Ivotuk 2 1 different for MAT, mossy tundra, and shrub tundra, while MNT values compared to a range of 144–266 g m yr for Toolik. Total above- were significantly lower. In all vegetation types the shapes of the ground live biomass values for MNT was approximately 60% greater at seasonal patterns were generally similar; however, the magnitude of the Ivotuk compared to the Toolik Lake area, with average values of 644 g 2 2 seasonal increase was dependent on vegetation type. For example, m at Ivotuk compared to 401 g m at Toolik. Toolik MNT had NDVI increased by 44% from early to mid-growing season in the shrub somewhat greater shrub and graminoid biomass compared to Ivotuk, tundra, while increasing by only 20% in the mossy tussock tundra. however, substantially greater moss biomass at Ivotuk (465 g m The seasonal patterns of LAI were significantly different among compared to 115 g m at Toolik) accounted for the difference. the four vegetation types (repeated measures GLM, P , 0.05) (Fig. 2b). Vascular plant ANPP for MNT at Ivotuk was estimated to be 95 g m 1 2 1 Peak values of LAI were observed at different times throughout the yr compared to an estimate of 127 g m yr from Toolik Lake growing season, depending on vegetation type. Shrub tundra was the (Hobbie et al., 2002). Total above-ground biomass in shrub tundra at only vegetation type in which the seasonal pattern of LAI exhibited Ivotuk was calculated to be 1037 g m , which was relatively similar to a notable peak at mid-growing season (24 July). LAI values in the MNT the mean of 1077 g m found near Toolik Lake (range of 750–1394 g 2 2 1 showed little variation over the course of the growing season with m ). ANPP estimates for shrub tundra at Ivotuk were 459 g m yr 2 1 a slight peak relatively early (5 July). LAI values in the MAT and mossy compared to 310 g m yr at Toolik Lake; greater shrub and tundra were variable throughout the course of the growing season. In nonvascular plant productivity at Ivotuk compared to Toolik Lake both MAT and mossy tundra, peak LAI values were observed at the end accounted for the difference. of our sampling season (27 August). The peak value of LAI in the shrub tundra (2.93 6 0.30) was significantly greater than peak values of LAI Discussion in MAT (2.26 6 0.23) and in mossy tussock tundra (1.75 6 0.23), which were significantly greater than those in the MNT (0.71 6 0.10) NDVI was relatively similar among the vegetation types both early (P , 0.05). and late in the growing season, but with varying magnitudes of the peak The seasonal patterns of total phytomass were also significantly values. Our results are consistent with two studies that used satellite different among the four vegetation types (repeated measures GLM, P data to compare seasonal patterns of NDVI among arctic tundra , 0.05) (Fig. 2c). In the shrub tundra and MNT, quantities of total vegetation types (Hope et al., 2003; Jia et al., 2004). Jia et al. (2004) phytomass peaked during mid-season (8 July and 10 July, respectively), also found the highest NDVI in the shrub tundra, and both studies found while total phytomass quantities in MAT and mossy tundra peaked NDVI in MAT to be greater than in MNT. The larger increase of NDVI later in the growing season (29 July and July 30, respectively). The peak from early to mid-season observed in the shrub tundra versus MAT, total phytomass in shrub tundra (1256 6 123 g m ) was significantly could have been due in part to the larger increase in the total quantity of greater than peak total live phytomass for MAT, MNT, and mossy phytomass, but was most likely due to increases in specific components tundra (722 6 71, 773 6 53, 703 6 39 g m respectively, P , 0.05) of phytomass. Several studies have shown that the presence of shrub (Fig. 3a). phytomass has a significant influence on variations of NDVI in tundra The two dominant life forms found across tundra vegetation types vegetation (Hope et al., 1993; Walker et al., 1995; 2003a, Riedel et al., were shrubs and mosses, which comprised over 80% of the total peak 2005). The seasonal quantities of shrub phytomass found in each phytomass for all four of the tundra types (Table 2). However, the vegetation type coincided with the seasonal levels of NDVI (the relative abundance of the specific phytomass components and the quantity of shrub phytomass and seasonal magnitude of NDVI were seasonal patterns of these components showed significant variation significantly greater in the shrub tundra than in the MAT, which were among the vegetation types (Fig. 3b). Moss phytomass made up a large significantly greater than in both the MNT and mossy tussock tundra) proportion of total peak phytomass in the MNT (80%) and mossy (Table 2, Fig. 2). These results suggest that vegetation types with tussock tundra (64%), unlike in the MAT and shrub tundra, where only different total phytomass quantities and plant community structure can approximately 30% of the total phytomass was moss. In the MAT and exhibit similar NDVI values during the early and late portions of the shrub tundra, the shrubs were the dominant life form comprising over growing season. However, over the course of the growing season 50% of the total phytomass. Shrub phytomass comprised less than one differences in shrub phytomass yield differences in foliar production fifth of the total phytomass in MNT (8%) and mossy tundra (18%). and therefore a separation of the patterns of NDVI, or an increased Graminoid phytomass made up 11 and 15% of total phytomass in the ‘‘greening up,’’ in areas with high abundance of shrubs, particularly MAT and mossy tundra respectively, but in the MNT and shrub tundra deciduous shrubs. less than 4% of total phytomass was composed of graminoids. Observing similar NDVI values among the vegetation types early 2 1 MAT exhibited the highest ANPP (479 g m yr ), only slightly and late in the growing season indicated that if the spatial pattern of 2 1 greater than the shrub tundra (459 g m yr ) (Table 3). There NDVI were examined at Ivotuk at these time periods, the landscape S. M. RIEDEL ET AL. / 29 TABLE 2 Summary of live aboveground phytomass and standing dead quantities for the 1999 growing season for four vegetation types at Ivotuk, Alaska. Phytomass 6 S.E. (g m ) 6 June–11 June 18 June–25 June 2 July–10 July 15 July–26 July 29 July–6 Aug. 13 Aug.–20 Aug. Forb* MAT 0.0 6 0.0 1.3 6 0.8 1.1 6 0.4 2.9 6 1.2 3.8 6 2.8 3.2 6 1.3 MNT 3.6 6 1.2 4.5 6 1.1 14.1 6 3.9 14.7 6 7.3 8.7 6 1.7 6.0 6 1.9 MT 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 ST 2.9 6 1.3 4.8 6 0.9 10.3 6 2.7 21.7 6 4.3 7.5 6 1.3 5.9 6 1.0 Graminoid MAT 10.2 6 3.1 88.5 6 27.8 71.7 6 12.7 90.6 6 16.0 78.3 6 17.3 61.2 6 13.2 MNT 7.1 6 1.1 24.3 6 4.0 24.0 6 2.6 39.1 6 9.0 25.9 6 2.5 16.8 6 3.3 MT 59.7 6 12.6 75.6 6 9.2 87.2 6 12.2 114.3 6 10.2 104.0 6 14.8 81.4 6 11.8 ST 27.9 6 17.3 21.4 6 10.4 17.5 6 5.9 34.1 6 10.4 51.2 6 13.1 32.2 6 12.8 Horsetail MAT 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 MNT 9.4 6 4.0 16.8 6 4.1 27.2 6 8.3 17.5 6 4.4 12.1 6 2.3 17.2 6 3.0 MT 0.0 6 0.0 2.2 6 2.2 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 ST 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 Lichen MAT 19.0 6 8.0 28.4 6 9.8 46.4 6 14.4 30.6 6 8.4 43.5 6 30.7 56.5 6 29.8 ab MNT 19.3 6 6.1 14.6 6 5.7 28.5 6 8.9 4.7 6 2.7 31.1 6 9.1 90.4 6 23.1 ab MT 34.3 6 11.6 18.6 6 4.1 15.1 6 3.8 24.4 6 4.3 26.7 6 6.7 15.8 6 6.6 ST 8.4 6 4.5 13.9 6 9.7 8.9 6 4.2 29.6 6 20.8 21.4 6 7.7 16.4 6 12.3 Moss MAT 111.5 6 32.9 214.3 6 50.4 111.2 6 43.1 145.1 6 29.1 229.1 6 53.2 226.0 6 64.0 MNT 345.8 6 55.8 440.5 6 45.4 618.7 6 47.2 414.8 6 78.0 513.9 6 44.8 436.4 6 47.9 MT 368.3 6 42.1 338.9 6 74.7 155.5 6 23.0 272.1 6 32.4 448.0 6 48.3 426.9 6 59.9 ST 269.3 6 35.5 252.2 6 58.7 371.1 6 30.4 265.7 6 28.3 350.1 6 48.0 400.6 6 60.2 Deciduous Woody Material MAT 50.8 6 12.1 134.3 6 33.3 58.5 6 16.0 97.9 6 16.8 141.9 6 17.8 143.8 6 30.1 MNT 2.9 6 1.0 5.7 6 1.3 6.3 6 3.3 7.2 6 2.3 11.4 6 2.7 23.5 6 3.8 MT 6.7 6 2.4 9.1 6 2.8 6.4 6 1.8 5.0 6 1.4 8.9 6 2.2 9.7 6 3.7 ST 483.8 6 142.1 512.8 6 77.7 711.3 6 136.1 537.2 6 77.2 527.8 6 79.6 565.8 6 90.5 Deciduous Foliar Material MAT 0.1 6 0.1 19.3 6 8.3 19.1 6 3.8 29.5 6 4.0 29.1 6 5.7 23.4 6 4.8 MNT 2.7 6 0.7 14.8 6 2.3 10.3 6 4.3 13.1 6 2.5 13.6 6 2.5 11.9 6 1.9 MT 0.2 6 0.1 8.7 6 2.0 11.8 6 1.4 11.8 6 1.0 13.8 6 1.9 5.9 6 2.0 ST 0.5 6 0.3 87.1 6 10.9 125.1 6 17.7 109.6 6 16.6 71.1 6 12.9 16.8 6 5.1 Evergreen Woody Material MAT 42.9 6 7.8 61.5 6 8.7 47.4 6 9.2 79.3 6 9.7 72.2 6 16.0 68.9 6 11.8 MNT 0.0 6 0.0 0.0 6 0.0 10.4 6 3.7 3.1 6 1.4 19.2 6 3.6 36.7 6 5.8 MT 52.4 6 7.9 37.1 6 4.2 38.2 6 3.9 55.3 6 7.0 50.3 6 9.0 42.7 6 6.1 ST 2.3 6 1.6 1.3 6 0.9 7.0 6 5.3 8.8 6 8.6 4.4 6 2.7 3.4 6 2.3 Evergreen Foliar Material MAT 22.4 6 5.4 116.6 6 16.3 86.5 6 10.7 98.9 6 10.1 123.7 6 37.0 114.5 6 15.9 MNT 25.5 6 8.6 31.9 6 5.9 34.0 6 12.5 59.7 6 9.5 22.1 6 4.7 32.9 6 7.7 MT 40.5 6 5.0 31.7 6 5.8 45.3 6 5.9 52.7 6 5.3 51.4 6 6.9 37.6 6 5.0 ST 1.9 6 1.9 0.4 6 0.3 4.6 6 4.3 8.3 6 8.0 3.9 6 2.8 7.1 6 4.1 Shrub Total MAT 116.2 6 17.8 331.8 6 48.2 211.5 6 27.0 305.6 6 24.8 367.0 6 50.4 350.5 6 47.0 MNT 31.1 6 9.5 52.4 6 6.9 60.9 6 15.6 83.1 6 12.1 66.3 6 5.9 105.0 6 12.8 MT 99.9 6 8.7 86.6 6 4.5 101.8 6 7.8 124.8 6 11.5 124.3 6 16.0 95.9 6 10.2 ST 488.4 6 141.0 601.6 6 86.0 848.1 6 146.0 663.8 6 80.4 607.2 6 89.3 593.0 6 89.2 Total MAT 256.9 6 34.1 664.2 6 72.3 441.8 6 54.3 574.7 6 42.5 721.6 6 71.3 697.2 6 83.3 MNT 416.2 6 54.8 553.0 6 46.1 773.4 6 52.7 573.9 6 88.8 658.0 6 44.5 671.8 6 43.5 MT 562.2 6 35.8 522.0 6 68.2 359.6 6 19.2 535.6 6 38.7 703.0 6 39.2 619.9 6 52.5 ST 796.9 6 142.0 893.9 6 72.2 1255.9 6 122.8 1014.9 6 89.5 1037.4 6 95.3 1048.1 6 106.8 30 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH TABLE 2 (Cont.) Phytomass 6 S.E. (g m ) 6 June–11 June 18 June–25 June 2 July–10 July 15 July–26 July 29 July–6 Aug. 13 Aug.–20 Aug. Standing Dead MAT 161.5 6 49.4 291.3 6 75.4 207.0 6 68.2 264.4 6 47.6 217.0 6 58.8 170.2 6 36.6 MNT 91.7 6 10.1 96.1 6 10.9 75.7 6 11.6 73.3 6 18.1 153.4 6 29.5 127.0 6 12.8 MT 246.7 6 40.3 170.0 6 29.0 154.3 6 16.8 219.2 6 29.5 177.7 6 25.5 237.9 6 57.5 ST 119.4 6 14.0 108.9 6 22.7 79.8 6 13.0 75.7 6 9.7 158.2 6 31.8 166.0 6 20.2 * Seasonal trends of specific phytomass components with different lower case letters (a,b,c) among tundra vegetation types are significantly different. would look homogenous. However, at peak season there was a dif- used. As the growing season progresses the effect of the high abundance ference in NDVI between shrub tundra and MNT of 0.12, which was of graminoids on LAI will increase due to continued plant growth similar to that found in a study comparing vegetation types at a coarser combined with the accumulation of graminoid standing dead, and scale (1 km ) using NOAA’s Advanced Very High Resolution consequently the peak in LAI occurred at the end of the growing season Radiometer (AVHRR) (Jia et al., 2004). The level of spatial variability in the MAT and mossy tundra. in peak NDVI from our study was approximately equal to the temporal Across the growing season, LAI values in the MAT and mossy change in NDVI from early June to peak season found in the MNT and tussock tundra were highly variable, and the peak values of LAI did not almost half the temporal change in NDVI found in the shrub tundra coincide with the peak greenness (NDVI). Some of this is likely due to during that period. The distinct seasonal patterns observed in this study LAI sampling variability associated with time of day, cloudiness, and illustrate the variability of NDVI patterns among the vegetation types sampling inconsistencies, even though we tried to minimize these. and demonstrate that there is a significant amount of spatial variation However, part of the discrepancy between the LAI and NDVI curves is even at the landscape scale. due to the specific phytomass components that are included in each of We assumed that quantities of live foliar deciduous shrub these measurements. LAI measurements were taken above the moss phytomass would also have the strongest influence on seasonal patterns layer, therefore mosses are not part of LAI, but they will contribute to of LAI, resulting in a seasonal trend that increases to a mid-season peak NDVI values. Standing dead tissue will be included in LAI, but will not (coinciding with mid season foliar growth) and declines late in the contribute in large part to NDVI, which is essentially an index of green growing season due to leaf senescence. The results from this study vegetation. In the mossy tussock tundra, high quantities of mosses and support the fact that shrub foliar material has an important control on graminoids outweigh any minor peak in green shrub material that LAI, but also suggest that the presence of graminoids and standing dead occurred at mid-season, suggesting that we should not expect a temporal may substantially influence LAI, when estimated using these optical, correlation between LAI and NDVI. It was more surprising that peak nondestructive methods. The only notable mid-season peak in LAI LAI did not coincide with peak NDVI in the MAT, however again this occurred in the shrub tundra. Not surprisingly the shrub tundra was the was due to the plant structure present at that site. Even though MAT had only vegetation type with a relatively high productivity of deciduous high levels of shrub phytomass, the majority was either in woody or shrubs (Table 3). A smaller mid season increase of LAI was observed in evergreen material, neither of which have a substantial peak at mid- the MAT, coinciding with the productivity of evergreen shrubs (Table season. As a result there was no definitive mid-season peak in LAI, and 3) and a peak in graminoid phytomass. consequently the peak greenness from NDVI did not coincide with the In both the MAT and mossy tussock tundra the highest LAI values peak of LAI. occurred at the end of our sampling season (end of August). This is most In MNT, the seasonal LAI had low values and showed little likely attributed ultimately to the absence of substantial deciduous foliar variation. This resulted from the very low quantity of shrub phytomass, shrub phytomass and the relatively high quantities of graminoid resulting in low shrub foliar production (Table 2) coupled with the phytomass found in these two vegetation types (Table 2). The standing relatively high quantity of moss phytomass. In the MNT 76% of total dead pool in these communities consists mostly of dead graminoids, phytomass was composed of mosses which, again due to the sampling and consequently where there were large amounts of graminoid method, are not included in the LAI measurements. phytomass, we found large amounts of standing dead. MAT and mossy Seasonal and peak quantities of total phytomass were similar in all tundra had considerably higher ratios of standing dead to total vegetation types except for shrub tundra, which had significantly more phytomass than found in MNT and shrub tundra. Standing dead total phytomass. The total phytomass was between 60 and 80% greater material blocks incoming solar radiation and therefore contributes to in the shrub tundra versus the other three vegetation types. The LAI when these particular methods (attenuation of solar radiation) are similarity in quantities of total phytomass observed in the other three TABLE 3 Estimates of above-ground net primary productivity for four vegetation types at Ivotuk, Alaska. 2 1 Aboveground net primary productivity (g m yr ) Deciduous Evergreen Shrub total Forb Graminoid Horsetail Lichen Moss foliar foliar (includes woody) Total MAT 4 80 0 37 133 29 102 252 479 MNT 11 32 18 24 273 11 34 52 357 MT 0 55 0 0 80 14 12 25 141 ST 19 23 0 21 131 125 6 360 459 S. M. RIEDEL ET AL. / 31 TABLE 4 permanent grids established at Ivotuk allow for temporal replication of this study. Therefore, these data provide a critical baseline against Spatial coefficients of variation (CV) of mean above-ground live which future studies may be compared. phytomass and standing dead. CVs calculated for mid growing season (15 July–26 July) measurements for four vegetation types at Ivotuk, Alaska. Acknowledgments Aboveground live phytomass components This project was funded by the NSF Arctic System Sciences, Standing Forb Graminoid Horsetail Lichen Moss Shrub Total Land-Atmosphere-Ice Interactions Program in a grant to Donald A. dead Walker (OPP-9908829) as part of the Arctic Transitions in the Land- MAT 182 79 0 123 90 36 33 80 Atmosphere System (ATLAS) effort. We thank Meg Miller and Elaine MNT 149 69 76 173 56 44 46 74 Smid for lab assistance at the University of Virginia. MT 0 40 0 78 53 41 32 60 ST 63 96 0 222 34 38 28 41 References Cited Arft, A. M., Walker, M. D., Gurevitch, J., Alatalo, J. M., Bret-Harte, M. S., Dale, M., Diemer, M., Guferli, F., Henery, G. H. R., Jones, tundra types was unexpected. In some research comparing plant M. H., Hollister, R. D., Jonsdottir, I. S., Laine, K., Levesque, E., biomass quantities in MAT and MNT conducted in the Arctic, MAT Marion, G. M., Molau, U., Molgaard, P., Nordenhall, U., Raszhivin, was found to have greater above-ground plant biomass than MNT V., Robinson, C. H., Starr, G., Stenstrom, A., Stenstrom, M., (Shippert et al., 1995; Walker et al., 2001, Hobbie et al., 2002). Our Totland, O., Tuner, P. L., Walker, L. J., Webber, P. J., Welker, J. M., total live phytomass quantities and those from the 1998 growing season and Wookey, P. A., 1999: Responses of tundra plants to at Ivotuk (Walker et al., 2003a) were more consistent with a study experimental warming: a meta analysis of the International Tundra comparing MAT and MNT at their transition zone in the Kuparuk River Experiment. Ecological Monographs, 69: 491–511. basin, where no significant difference in above-ground plant biomass Bockheim, J. G., Walker, D. A., Everett, L. R., Nelson, F. E., and was found (Walker et al., 1998). Shiklomanov, N. I., 1998: Soils and cryoturbation in moist nonacidic and acidic tundra in the Kurparuk River Basin, Arctic Alaska, USA. The high temporal variability in the phytomass patterns probably Arctic and Alpine Research, 30: 166–174. resulted from a combination of the sampling method and the high level Chapin, F. S., III, McKendrick, J. D., and Johnson, D. A., 1986: of spatial heterogeneity in tundra vegetation found even within Seasonal changes in carbon fractions in Alaskan tundra plants of vegetation types. The high spatial variability found at Ivotuk is differing growth form: implications for herbivores. Journal of demonstrated by the relatively high coefficients of variation observed Ecology, 74: 707–731. for mean mid season phytomass values within each vegetation type Chapin, F. S., III, Shaver, G. R., Giblin, A. E., Nadelhoffer, K. J., and (Table 4). Using destructive sampling to estimate phytomass obviously Laundre, J. A., 1995: Responses of arctic tundra to experimental and does not allow for repetition of measurements at the exact same point observed changes in climate. Ecology, 76: 694–711. throughout the growing season; rather samples are collected nearby Epstein, H. E., Walker, M. D., Chapin, F. S., III, and Starfield, A. M., previous sample locations. Therefore in landscapes with high spatial 2000: A transient nutrient-based model of Arctic plant community response to climatic warming. Ecological Applications, 10: 824–841. variability at scales of a few meters, such as those at Ivotuk, when Epstein, H. E., Calef, M. P., Walker, M. D., Chapin, F. S., III, and temporal patterns are examined at slightly different locations Starfield, A. M., 2004: Detecting changes in arctic plant commu- throughout the course of the sampling, the spatial variability is nities in response to warming over decadal time scales. Global incorporated into the temporal patterns. By increasing the replicates of Change Biology, 10: 1325–1334. phytomass harvests we could have reduced some of the effects of Giblin, A. E., Nadelhoffer, K. J., Shaver, G. R., Laundre, J. A., and spatial variability that were included in the seasonal patterns, and McKerrow, A. J., 1991: Biogeochemical diversity along a riverside consequently we might have observed a more predictable pattern toposequence in arctic Alaska. Ecological Monographs, 61: 415–435. similar to that of NDVI. Gough, L., Shaver, G. R., Carroll, J., Royer, D. L., and Laundre, J. A., Above-ground net primary production (ANPP) was also different 2000: Vascular plant species richness in Alaskan arctic tundra: the among vegetation types and significantly influenced by plant importance of pH. Journal of Ecology, 88: 54–66. Hobbie, S. E. and Gough, L., 2002: Foliar and soil nutrients in tundra community structure. The higher abundance of shrubs in the MAT on glacial landscapes of contrasting ages in northern Alaska. and shrub tundra (ST), compared to MNT and mossy tussock tundra, 2 1 2 Oecologia, 131: 453–462. led to higher levels of ANPP (MAT¼ 479 g m yr ,ST¼ 459 g m 1 Hobbie, S. E., Miley, T. A., and Weiss, M. S., 2002. Carbon and yr ). In the MNT and mossy tussock tundra, shrub phytomass made up nitrogen cycling in soils from acidic and nonacidic tundra with less than 20% of the total live phytomass, consequently lower estimates different glacial histories in northern Alaska. Ecosystems, 5: 2 1 of ANPP were observed in the MNT (357 g m yr ) and mossy tundra 761–774. 2 1 (140 g m yr ). However, the absence of substantial shrub Hope, A. S., Kimball, J. S., and Stow, D. A., 1993: The relationship productivity found in the MNT and mossy tussock tundra was partially between tussock tundra spectral reflectance properties and biomass offset in the MNT by high productivity of mosses. and vegetation composition. International Journal of Remote The results from this study show that seasonal patterns of tundra Sensing, 14: 1861–1874. Hope, A. S., Boynton, W. L., Stow, D. A., and Douglas, D. C., 2003. vegetation properties can vary substantially among vegetation types Interannual growth dynamics of vegetation in the Kuparuk River that occur in close proximity. Investigation of how these vegeta- watershed, Alaska based on the Normalized Difference Vegetation tion properties vary temporally identified key components of tundra Index. International Journal of Remote Sensing, 24: 3413–3425. ecosystems, in this case deciduous shrubs, mosses, and graminoids, Jia, G. J., Epstein, H. E., and Walker, D. 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Journal of Ecology, 87: 885–898. characteristics of arctic tundra vegetation from airborne radiometry. Revised ms submitted July 2004 International Journal of Remote Sensing, 14: 1239–1244. S. M. RIEDEL ET AL. / 33 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Arctic, Antartic and Alpine Research" Taylor & Francis

Spatial and Temporal Heterogeneity of Vegetation Properties among Four Tundra Plant Communities at Ivotuk, Alaska, U.S.A.

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Taylor & Francis
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© 2005 Institute of Arctic and Alpine Research (INSTAAR), University of Colorado
ISSN
1938-4246
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1523-0430
DOI
10.1657/1523-0430(2005)037[0025:SATHOV]2.0.CO;2
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Abstract

Howard E. Epstein* Intraseasonal patterns of normalized difference vegetation index (NDVI), leaf area index (LAI), and phytomass were compared for four tundra vegetation types at Ivotuk, Alaska, Donald A. Walker during summer 1999. The vegetation types included moist acidic tundra (MAT), moist David L. Richardson* nonacidic tundra (MNT), mossy tussock tundra, and shrub tundra. The seasonal curves of Monika P. Calef * NDVI were similar among the vegetation types but with varying magnitudes of the peak values. Peak NDVI in the shrub tundra (0.83) was significantly greater than in MAT Erika Edwards and (0.76), which was significantly greater than in MNT (0.71) and mossy tussock tundra Amber Moody (0.70). LAI and phytomass exhibited high temporal variability with distinct seasonality *Department of Environmental Sciences, only in shrub tundra. Seasonal LAI and NDVI patterns were therefore correlated only in University of Virginia, Charlottesville, shrub tundra, which was attributed to the high quantity of deciduous shrub foliage present Virginia 22904-4123, U.S.A. in this community and absent in the other vegetation types. Shrub tundra peak live above- Institute of Arctic Biology, University of ground phytomass (1256 6 123 g m ) was significantly greater than peak live above- Alaska, Fairbanks, Alaska 99775-7000, U.S.A. ground phytomass for MAT, MNT, and mossy tussock tundra (722 6 71, 773 6 53, Corresponding author. 703 6 39 g m respectively, P , 0.05). Relative abundances of deciduous shrubs, hee2b@virginia.edu mosses, and graminoids were revealed as key components controlling differences in NDVI, LAI, and phytomass among tundra vegetation types. fully understood (Bockheim et al., 1998), differences in peat formation Introduction are considered a major control of the acidity of arctic soils (M. D. Recognition of the complexity and importance of arctic tundra Walker et al., 1994). Low-pH soils are hypothesized to occur when peat ecosystems with regard to their sensitivity to climate change (Oechel formation results in restricted drainage and accumulation of acidophilic et al., 1993, 1994; Chapin et al., 1995; Myneni et al., 1997; Arft et al., mosses, which in turn alters the soil chemistry (M. D. Walker et al., 1999; Epstein et al., 2000) has established the need for more detailed 1994). The existence of nonacidic soils can be attributed to several analyses into tundra vegetation. Some recent studies have focused on natural disturbances, including loess deposition, glacial till (Walker understanding landscape heterogeneity of tundra vegetation caused by and Everett, 1991), solifluction, alluvial process (M. D. Walker et al., topography (Giblin et al., 1991; Shaver and Chapin, 1991; Walker and 1994), cryoturbation (Bockheim et al., 1998), and differences in land- Everett, 1991; Shaver et al., 1996) and local variations of substrate (M. scape age (Walker et al., 1998). The differences in soil properties have D. Walker et al., 1994; Gough et al., 2000; D. A. Walker et al., 2001, a substantial effect on ecosystem processes (Walker et al., 1998; Gough 2003a). Total live plant biomass and net primary production are highly et al., 2000; Hobbie and Gough, 2002). Compared to acidic tundra, variable among communities along toposequences (Shaver and Chapin, nonacidic tundra has been shown to have thinner organic horizons, 1991; Walker and Everett, 1991; Shaver et al., 1996); in general, these greater soil heat flux, deeper summer thaw, and greater concentrations system properties are greatest in well-drained riparian areas followed by of calcium, and serves as less of a carbon sink and a smaller methane moist mid-slope tussock tundra communities. Phytomass levels in source (Bockheim et al., 1998; Walker et al., 1998). lowland wet sedge communities are typically considered limited by poor Differing nutrient regimes can have important implications for nutrient supply resulting from anaerobic conditions. Upland dry heath vegetation properties in tundra ecosystems, many of which are communities can exhibit lower levels of phytomass and production, considered nutrient limited (Chapin et al., 1986; Shaver et al., 2001). compared to mid-slope tussock tundra, due to winter desiccation on Compared to acidic tundra communities, nonacidic communities have exposed ridges and its effect on plant community composition (Shaver greater biodiversity (M. D. Walker et al., 1994; Gough et al., 2000). and Chapin, 1991). One study conducted on a toposequence in northern Species composition is also quite different; moist acidic tundra (MAT) Alaska, found that the highest total live plant biomass and second communities are dominated by the sedge, Eriophorum vaginatum, highest productivity occurred in the hilltop birch-heath section of the Sphagnum mosses and dwarf shrubs such as Betula nana, while moist toposequence (Shaver et al., 1996). Local differences in biomass, nonacidic tundra (MNT) communities are generally dominated by the productivity, and species composition may result from variations in sedge, Carex bigelowii, and Dryas integrifolia, a prostrate shrub (M. D. nutrient availability, which are consequences of the interaction of Walker et al., 1994). topography and other environmental variables. Recent findings indicate that the presence of MNT is not an Differences in soil pH can play a key role in affecting the local isolated or small-scale occurrence (Walker et al., 1998, 2001), thus variation of tundra vegetation properties. Distribution patterns of acidic providing further evidence that the natural patchiness of vegetation and nonacidic soils have been observed at the regional scale (Walker types is common across the tundra even on subregional scales. Some et al., 1998), and soil pH in arctic tundra can also vary considerably at previous studies have focused on quantifying differences in vegetation the landscape scale (100s of meters) (Bockheim et al., 1998). While attributes between MNT and MAT (Walker et al., 1998; 2001, 2003a; the ultimate control on the distribution of nonacidic communities is not Gough et al., 2000; Hobbie and Gough, 2002; Jia et al., 2002). The 2005 Regents of the University of Colorado S. M. RIEDEL ET AL. / 25 1523-0430/05 $7.00 TABLE I with nonacidic soils (Table 1). The shrub tundra site is located in a low slope position and drainage channel, compared to mid and upper slope Soil and topography data for four distinct tundra vegetation types at positions for the two other vegetation types found on acidic soils (Table Ivotuk, Alaska. 1). The vegetation types examined in this study are common throughout Soil water the North Slope and have been classified as distinct types in numerous a b Vegetation type Soil pH content (%) Slope studies (Shaver et al., 1996; M. D. Walker et al., 1994; D. A. Walker Moist Acidic Tundra 4.8 461.1 2.58 et al., 1998, 2001, 2003a; Gough et al., 2000, Jia et al., 2004). Moist Nonacidic Tundra 6.3 200.4 3.48 The vegetation at the MAT site is composed of Eriophorum Mossy Tussock Tundra 4.2 443.2 1.28 vaginatum, Sphagnum mosses, and dwarf shrubs, including Betula Shrub Tundra 4.5 295.9 4.58 nana. The MNT site, found on neutral pH soils, has a vegetative community dominated by Carex bigelowii and Dryas integrifolia,as Mean soil pH to a depth of 100 cm (Ping et al., 1998). well as non-Sphagnum mosses. The mossy tussock tundra site is Soil water content of the O horizon (Ping et al., 1998). composed mostly of Sphagnum mosses, Eriophorum vaginatum, Betula nana, and lichens. The shrub tundra site is dominated by Salix pulchra and Betula nana (Fig. 1). majority of the studies have typically focused on comparing vegetation NDVI, LAI, and above-ground phytomass samples were collected properties at the peak of the growing season. Several studies have biweekly from four 100 m 3 100 m grids, each representative of captured the intraseasonal variation of tundra vegetation, but have been a different vegetation type, during the 1999 growing season. Field limited to the use of satellite data (Stow et al., 1993; Jia et al. 2004), and measurements were divided into seven sampling periods beginning 5 therefore the spatial scale at which vegetation properties can be June and ending 27 August. LAI and NDVI measurements were taken examined is bound by resolution of the satellite data. at 20 random points within each grid; the same 20 points were used The objective of this study was to investigate the spatial and throughout the growing season. From prior sampling, we determined temporal patterns of tundra vegetation that result from differences in that 20 random points in a 100 m 3 100 m grid were sufficient for soil properties and topographic location. Seasonal patterns of field- capturing the variability at this spatial scale. NDVI was measured using derived normalized difference vegetation index (NDVI), leaf area index an Analytical Spectral Devices FieldSpec spectroradiometer (Boulder, (LAI), and phytomass have not been compared across an entire growing Colorado). The reflectance data for the pertinent wavelength intervals season for a set of distinct tundra vegetation types that occur within the were used to calculate NDVI from the formula same landscape. Seasonal patterns of NDVI, LAI, and phytomass were analyzed to determine how vegetation properties vary among distinct ðNIR  redÞ NDVI ¼ ð1Þ types that exist as a result of landscape scale heterogeneity. Addi- ðNIR þ redÞ tionally, seasonal patterns of vegetation properties were compared to where NIR is the near infrared reflectance (average reflectance of wave- examine how the level of spatial heterogeneity varied throughout the lengths between 725 and 1060 nm), and ‘‘red’’ is the red reflectance growing season. Finally, this study provides a critical baseline for (average reflectance of wavelengths between 580 and 680 nm) of the ground measurements of tundra vegetation properties against which vegetation. Four replicate measurements were taken 1 m north, east, future measurements from the same site can be compared. south, and west of each grid point and averaged to give a mean NDVI value. The fiber optic sensor of the FieldSpec spectroradiometer was held approximately 1.5 m above the surface of the vegetation, pro- Methods viding a 0.35-m footprint using a 258 field of view. The spectro- Our study site was Ivotuk, Alaska (68.498N, 155.748W), which is radiometer was calibrated with dark and pure white readings before located on the North Slope of the Brooks Mountain Range and is measurements at each grid point. characterized by a growing season of approximately 110 d and a mean LAI which is simply the total leaf area per unit ground surface area 2 2 (m m ) is commonly measured optically, as it was in this study, using July maximum daily temperature of ;128C. Mean annual temperature a LI-COR LAI-2000 Plant Canopy Analyzer (Lincoln, Nebraska). is 10.98C, and mean annual precipitation is 202 mm based on a 3-yr Above- and below-canopy radiation measurements are made with dataset (1991–2001; Hinzman, unpublished data). Ivotuk was a key a ‘‘fish eye’’ optical sensor with a 5-ring 1488 angle of view. Below- sampling location for the NSF Arctic Transitions in the Land- canopy measurements were made from the top of the moss layer. LAI is Atmosphere System (ATLAS) study as part of a western North Slope transect from Point Barrow to the Seward Peninsula. Ivotuk was chosen calculated using a model of radiative transfer in vegetation canopies as a research site because it has one of the few airstrips in all of and then stored in the unit data logger. At each of the 20 grid points, northwestern Alaska, and it is comparable to Toolik Lake, a National one above canopy reading was taken, and four below canopy readings Science Foundation Long-Term Ecological Research site found were taken (1 m north, east, south, and west of the grid point) to yield approximately 200 km to the east. Additionally, the study site was a single LAI value. chosen because four tundra vegetation types, moist acidic tundra Above-ground phytomass was collected from 10 randomly (MAT), moist nonacidic tundra (MNT), mossy tussock tundra, and selected 20 cm 3 50 cm plots within each grid, for a total of 1 m per shrub tundra exist within a 2 km area. Examining the heterogeneity of vegetation type for each of the first six sample periods. Phytomass tundra vegetation was accomplished by comparing ecosystem proper- sampling occurred near, but not within, the footprints of LAI and NDVI ties across distinct vegetation types that occur largely as a result of measurements. On the fourth sample period (15–26 July), the number of differences in hydrologic regime and soil pH. In this study the MNT site phytomass plots harvested was doubled for MAT and mossy tussock differed from the other three vegetation types in that it is the only site tundra in order to more accurately capture peak biomass. On the fifth FIGURE 1. Vegetation maps of the shrub tundra (ST), moist nonacidic tundra (MNT) and mossy tussock tundra (MT) 100 3 100 m grids. A vegetation map of the moist acidic tundra (MAT) grid was not developed because the corners of the grid could not be adequately georeferenced from the aerial photographs of the site. 26 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH S. M. RIEDEL ET AL. / 27 FIGURE 3. (a) Peak quantities of total live above-ground phytomass for the 1999 growing season for four vegetation types found at Ivotuk, Alaska. Distinct hatching represents specific components of above- ground phytomass. (b) The percent of peak live above-ground phytomass comprised by each component plant type for four vegetation types at Ivotuk, Alaska. Distinct hatching represents specific components of above-ground phytomass. munity in the nonacidic tundra. The graminoid and shrub samples were sorted further into subcategories. Graminoid phytomass was divided into live and dead material. Shrub phytomass was divided into evergreen and deciduous, which were then separated into woody, foliar live, and foliar dead components. Dead material was combined into a single pool, referred to as ‘‘standing dead.’’ Therefore when we refer FIGURE 2. (a) Seasonal patterns of NDVI across the 1999 growing to specific components of phytomass or total phytomass, we are season at Ivotuk, Alaska. Curves represent four distinct vegetation referring only to the above-ground live fraction. types. Error bars represent the standard error of the mean (n ¼ 20); Comparison of seasonal patterns of LAI, NDVI, and phytomass (b) Seasonal patterns of LAI across the 1999 growing season at Ivotuk, among the four vegetation types was accomplished using a repeated Alaska. Curves represent four distinct vegetation types. Error bars represent the standard error of the mean (n¼ 20); (c) Seasonal patterns measures general linear model (SPSS 8.0). Peak LAI, NDVI, and of total live above-ground phytomass across the 1999 growing season phytomass among the four vegetation types were compared using at Ivotuk, Alaska. Curves represent four distinct vegetation types. Error ANOVA. Above-ground net primary productivity (ANPP) was bars represent the standard error of the mean (n ¼ 10 or n ¼ 20*). estimated by subtracting initial phytomass from peak phytomass. Coefficients of variation were calculated for mid-growing season sample period (27 July–7 August), the number of phytomass plots phytomass values to investigate the level of heterogeneity within harvested was doubled for MNT and shrub tundra, for the same reason. vegetation types. Results were used to determine the controls that local Vascular plants were clipped at the top of the moss surface; mosses substrate and topography have on vegetation patterns. Finally, we were clipped at the base of the green layer. Phytomass samples were compared biomass and ANPP data from Ivotuk to other similar datasets sorted into six main categories (horsetails, other forbs, graminoids, collected near the Toolik Lake Long-Term Ecological Research site lichens, mosses, and shrubs). Horsetails were separated from other forbs within the Kuparuk River Basin, located approximately 200 km to the because they can represent a substantial component of the plant com- east of Ivotuk (Shaver and Chapin, 1991; Shippert et al., 1995; Walker 28 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH 2 1 et al., 1995; Shaver et al., 1996, 2001; Williams and Rastetter, 1999, was considerably less productivity observed in MNT (357 g m yr ) Hobbie et al., 2002; Walker et al. 2003a, 2003b; Epstein et al., 2004). than in MAT and shrub tundra, and the mossy tussock tundra had the 2 1 lowest rate of ANPP of all vegetation types (140 g m yr ). Spatial coefficients of variation were relatively similar across vegetation types Results (Table 4). All four tundra types had high coefficients of variation for The seasonal patterns of NDVI were significantly different among lichen biomass (78–222%) and low coefficients of variation for shrub the four vegetation types (repeated measures GLM, P , 0.05) (Fig. 2a). biomass (36–44%). Spatial variation in forb biomass was high for Early season (7–10 June) values of NDVI were generally similar for all MAT (182%) and MNT (149%). vegetation types, with no statistical difference between MAT, mossy Total above-ground live biomass values for MAT at Ivotuk (using tundra, and shrub tundra. Peak season NDVI values occurred at the same data from this study and Walker et al. [2003b]) averaged 780 g m and time for three vegetation types (16 July), the exception being the mossy fell within the range of values (although on the high side) estimated for tussock tundra (27 July). The peak value of NDVI in shrub tundra areas near Toolik Lake (444–789 g m ). In general, greater moss, (0.83 6 0.01; mean 6 one standard error) was significantly greater than deciduous shrub and graminoid biomass at Ivotuk accounted for the in the MAT (0.76 6 0.01), which was significantly greater than in the relatively greater total biomass at Ivotuk compared to Toolik Lake. MNT (0.71 6 0.01) and the mossy tundra (0.70 6 0.01) (P , 0.05). ANPP estimates of MAT from Ivotuk were substantially greater than 2 1 Late season (22–26 August) NDVI values were again not statistically those estimated for the Toolik Lake area, 479 g m yr at Ivotuk 2 1 different for MAT, mossy tundra, and shrub tundra, while MNT values compared to a range of 144–266 g m yr for Toolik. Total above- were significantly lower. In all vegetation types the shapes of the ground live biomass values for MNT was approximately 60% greater at seasonal patterns were generally similar; however, the magnitude of the Ivotuk compared to the Toolik Lake area, with average values of 644 g 2 2 seasonal increase was dependent on vegetation type. For example, m at Ivotuk compared to 401 g m at Toolik. Toolik MNT had NDVI increased by 44% from early to mid-growing season in the shrub somewhat greater shrub and graminoid biomass compared to Ivotuk, tundra, while increasing by only 20% in the mossy tussock tundra. however, substantially greater moss biomass at Ivotuk (465 g m The seasonal patterns of LAI were significantly different among compared to 115 g m at Toolik) accounted for the difference. the four vegetation types (repeated measures GLM, P , 0.05) (Fig. 2b). Vascular plant ANPP for MNT at Ivotuk was estimated to be 95 g m 1 2 1 Peak values of LAI were observed at different times throughout the yr compared to an estimate of 127 g m yr from Toolik Lake growing season, depending on vegetation type. Shrub tundra was the (Hobbie et al., 2002). Total above-ground biomass in shrub tundra at only vegetation type in which the seasonal pattern of LAI exhibited Ivotuk was calculated to be 1037 g m , which was relatively similar to a notable peak at mid-growing season (24 July). LAI values in the MNT the mean of 1077 g m found near Toolik Lake (range of 750–1394 g 2 2 1 showed little variation over the course of the growing season with m ). ANPP estimates for shrub tundra at Ivotuk were 459 g m yr 2 1 a slight peak relatively early (5 July). LAI values in the MAT and mossy compared to 310 g m yr at Toolik Lake; greater shrub and tundra were variable throughout the course of the growing season. In nonvascular plant productivity at Ivotuk compared to Toolik Lake both MAT and mossy tundra, peak LAI values were observed at the end accounted for the difference. of our sampling season (27 August). The peak value of LAI in the shrub tundra (2.93 6 0.30) was significantly greater than peak values of LAI Discussion in MAT (2.26 6 0.23) and in mossy tussock tundra (1.75 6 0.23), which were significantly greater than those in the MNT (0.71 6 0.10) NDVI was relatively similar among the vegetation types both early (P , 0.05). and late in the growing season, but with varying magnitudes of the peak The seasonal patterns of total phytomass were also significantly values. Our results are consistent with two studies that used satellite different among the four vegetation types (repeated measures GLM, P data to compare seasonal patterns of NDVI among arctic tundra , 0.05) (Fig. 2c). In the shrub tundra and MNT, quantities of total vegetation types (Hope et al., 2003; Jia et al., 2004). Jia et al. (2004) phytomass peaked during mid-season (8 July and 10 July, respectively), also found the highest NDVI in the shrub tundra, and both studies found while total phytomass quantities in MAT and mossy tundra peaked NDVI in MAT to be greater than in MNT. The larger increase of NDVI later in the growing season (29 July and July 30, respectively). The peak from early to mid-season observed in the shrub tundra versus MAT, total phytomass in shrub tundra (1256 6 123 g m ) was significantly could have been due in part to the larger increase in the total quantity of greater than peak total live phytomass for MAT, MNT, and mossy phytomass, but was most likely due to increases in specific components tundra (722 6 71, 773 6 53, 703 6 39 g m respectively, P , 0.05) of phytomass. Several studies have shown that the presence of shrub (Fig. 3a). phytomass has a significant influence on variations of NDVI in tundra The two dominant life forms found across tundra vegetation types vegetation (Hope et al., 1993; Walker et al., 1995; 2003a, Riedel et al., were shrubs and mosses, which comprised over 80% of the total peak 2005). The seasonal quantities of shrub phytomass found in each phytomass for all four of the tundra types (Table 2). However, the vegetation type coincided with the seasonal levels of NDVI (the relative abundance of the specific phytomass components and the quantity of shrub phytomass and seasonal magnitude of NDVI were seasonal patterns of these components showed significant variation significantly greater in the shrub tundra than in the MAT, which were among the vegetation types (Fig. 3b). Moss phytomass made up a large significantly greater than in both the MNT and mossy tussock tundra) proportion of total peak phytomass in the MNT (80%) and mossy (Table 2, Fig. 2). These results suggest that vegetation types with tussock tundra (64%), unlike in the MAT and shrub tundra, where only different total phytomass quantities and plant community structure can approximately 30% of the total phytomass was moss. In the MAT and exhibit similar NDVI values during the early and late portions of the shrub tundra, the shrubs were the dominant life form comprising over growing season. However, over the course of the growing season 50% of the total phytomass. Shrub phytomass comprised less than one differences in shrub phytomass yield differences in foliar production fifth of the total phytomass in MNT (8%) and mossy tundra (18%). and therefore a separation of the patterns of NDVI, or an increased Graminoid phytomass made up 11 and 15% of total phytomass in the ‘‘greening up,’’ in areas with high abundance of shrubs, particularly MAT and mossy tundra respectively, but in the MNT and shrub tundra deciduous shrubs. less than 4% of total phytomass was composed of graminoids. Observing similar NDVI values among the vegetation types early 2 1 MAT exhibited the highest ANPP (479 g m yr ), only slightly and late in the growing season indicated that if the spatial pattern of 2 1 greater than the shrub tundra (459 g m yr ) (Table 3). There NDVI were examined at Ivotuk at these time periods, the landscape S. M. RIEDEL ET AL. / 29 TABLE 2 Summary of live aboveground phytomass and standing dead quantities for the 1999 growing season for four vegetation types at Ivotuk, Alaska. Phytomass 6 S.E. (g m ) 6 June–11 June 18 June–25 June 2 July–10 July 15 July–26 July 29 July–6 Aug. 13 Aug.–20 Aug. Forb* MAT 0.0 6 0.0 1.3 6 0.8 1.1 6 0.4 2.9 6 1.2 3.8 6 2.8 3.2 6 1.3 MNT 3.6 6 1.2 4.5 6 1.1 14.1 6 3.9 14.7 6 7.3 8.7 6 1.7 6.0 6 1.9 MT 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 ST 2.9 6 1.3 4.8 6 0.9 10.3 6 2.7 21.7 6 4.3 7.5 6 1.3 5.9 6 1.0 Graminoid MAT 10.2 6 3.1 88.5 6 27.8 71.7 6 12.7 90.6 6 16.0 78.3 6 17.3 61.2 6 13.2 MNT 7.1 6 1.1 24.3 6 4.0 24.0 6 2.6 39.1 6 9.0 25.9 6 2.5 16.8 6 3.3 MT 59.7 6 12.6 75.6 6 9.2 87.2 6 12.2 114.3 6 10.2 104.0 6 14.8 81.4 6 11.8 ST 27.9 6 17.3 21.4 6 10.4 17.5 6 5.9 34.1 6 10.4 51.2 6 13.1 32.2 6 12.8 Horsetail MAT 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 MNT 9.4 6 4.0 16.8 6 4.1 27.2 6 8.3 17.5 6 4.4 12.1 6 2.3 17.2 6 3.0 MT 0.0 6 0.0 2.2 6 2.2 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 ST 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 0.0 6 0.0 Lichen MAT 19.0 6 8.0 28.4 6 9.8 46.4 6 14.4 30.6 6 8.4 43.5 6 30.7 56.5 6 29.8 ab MNT 19.3 6 6.1 14.6 6 5.7 28.5 6 8.9 4.7 6 2.7 31.1 6 9.1 90.4 6 23.1 ab MT 34.3 6 11.6 18.6 6 4.1 15.1 6 3.8 24.4 6 4.3 26.7 6 6.7 15.8 6 6.6 ST 8.4 6 4.5 13.9 6 9.7 8.9 6 4.2 29.6 6 20.8 21.4 6 7.7 16.4 6 12.3 Moss MAT 111.5 6 32.9 214.3 6 50.4 111.2 6 43.1 145.1 6 29.1 229.1 6 53.2 226.0 6 64.0 MNT 345.8 6 55.8 440.5 6 45.4 618.7 6 47.2 414.8 6 78.0 513.9 6 44.8 436.4 6 47.9 MT 368.3 6 42.1 338.9 6 74.7 155.5 6 23.0 272.1 6 32.4 448.0 6 48.3 426.9 6 59.9 ST 269.3 6 35.5 252.2 6 58.7 371.1 6 30.4 265.7 6 28.3 350.1 6 48.0 400.6 6 60.2 Deciduous Woody Material MAT 50.8 6 12.1 134.3 6 33.3 58.5 6 16.0 97.9 6 16.8 141.9 6 17.8 143.8 6 30.1 MNT 2.9 6 1.0 5.7 6 1.3 6.3 6 3.3 7.2 6 2.3 11.4 6 2.7 23.5 6 3.8 MT 6.7 6 2.4 9.1 6 2.8 6.4 6 1.8 5.0 6 1.4 8.9 6 2.2 9.7 6 3.7 ST 483.8 6 142.1 512.8 6 77.7 711.3 6 136.1 537.2 6 77.2 527.8 6 79.6 565.8 6 90.5 Deciduous Foliar Material MAT 0.1 6 0.1 19.3 6 8.3 19.1 6 3.8 29.5 6 4.0 29.1 6 5.7 23.4 6 4.8 MNT 2.7 6 0.7 14.8 6 2.3 10.3 6 4.3 13.1 6 2.5 13.6 6 2.5 11.9 6 1.9 MT 0.2 6 0.1 8.7 6 2.0 11.8 6 1.4 11.8 6 1.0 13.8 6 1.9 5.9 6 2.0 ST 0.5 6 0.3 87.1 6 10.9 125.1 6 17.7 109.6 6 16.6 71.1 6 12.9 16.8 6 5.1 Evergreen Woody Material MAT 42.9 6 7.8 61.5 6 8.7 47.4 6 9.2 79.3 6 9.7 72.2 6 16.0 68.9 6 11.8 MNT 0.0 6 0.0 0.0 6 0.0 10.4 6 3.7 3.1 6 1.4 19.2 6 3.6 36.7 6 5.8 MT 52.4 6 7.9 37.1 6 4.2 38.2 6 3.9 55.3 6 7.0 50.3 6 9.0 42.7 6 6.1 ST 2.3 6 1.6 1.3 6 0.9 7.0 6 5.3 8.8 6 8.6 4.4 6 2.7 3.4 6 2.3 Evergreen Foliar Material MAT 22.4 6 5.4 116.6 6 16.3 86.5 6 10.7 98.9 6 10.1 123.7 6 37.0 114.5 6 15.9 MNT 25.5 6 8.6 31.9 6 5.9 34.0 6 12.5 59.7 6 9.5 22.1 6 4.7 32.9 6 7.7 MT 40.5 6 5.0 31.7 6 5.8 45.3 6 5.9 52.7 6 5.3 51.4 6 6.9 37.6 6 5.0 ST 1.9 6 1.9 0.4 6 0.3 4.6 6 4.3 8.3 6 8.0 3.9 6 2.8 7.1 6 4.1 Shrub Total MAT 116.2 6 17.8 331.8 6 48.2 211.5 6 27.0 305.6 6 24.8 367.0 6 50.4 350.5 6 47.0 MNT 31.1 6 9.5 52.4 6 6.9 60.9 6 15.6 83.1 6 12.1 66.3 6 5.9 105.0 6 12.8 MT 99.9 6 8.7 86.6 6 4.5 101.8 6 7.8 124.8 6 11.5 124.3 6 16.0 95.9 6 10.2 ST 488.4 6 141.0 601.6 6 86.0 848.1 6 146.0 663.8 6 80.4 607.2 6 89.3 593.0 6 89.2 Total MAT 256.9 6 34.1 664.2 6 72.3 441.8 6 54.3 574.7 6 42.5 721.6 6 71.3 697.2 6 83.3 MNT 416.2 6 54.8 553.0 6 46.1 773.4 6 52.7 573.9 6 88.8 658.0 6 44.5 671.8 6 43.5 MT 562.2 6 35.8 522.0 6 68.2 359.6 6 19.2 535.6 6 38.7 703.0 6 39.2 619.9 6 52.5 ST 796.9 6 142.0 893.9 6 72.2 1255.9 6 122.8 1014.9 6 89.5 1037.4 6 95.3 1048.1 6 106.8 30 / ARCTIC,ANTARCTIC, AND ALPINE RESEARCH TABLE 2 (Cont.) Phytomass 6 S.E. (g m ) 6 June–11 June 18 June–25 June 2 July–10 July 15 July–26 July 29 July–6 Aug. 13 Aug.–20 Aug. Standing Dead MAT 161.5 6 49.4 291.3 6 75.4 207.0 6 68.2 264.4 6 47.6 217.0 6 58.8 170.2 6 36.6 MNT 91.7 6 10.1 96.1 6 10.9 75.7 6 11.6 73.3 6 18.1 153.4 6 29.5 127.0 6 12.8 MT 246.7 6 40.3 170.0 6 29.0 154.3 6 16.8 219.2 6 29.5 177.7 6 25.5 237.9 6 57.5 ST 119.4 6 14.0 108.9 6 22.7 79.8 6 13.0 75.7 6 9.7 158.2 6 31.8 166.0 6 20.2 * Seasonal trends of specific phytomass components with different lower case letters (a,b,c) among tundra vegetation types are significantly different. would look homogenous. However, at peak season there was a dif- used. As the growing season progresses the effect of the high abundance ference in NDVI between shrub tundra and MNT of 0.12, which was of graminoids on LAI will increase due to continued plant growth similar to that found in a study comparing vegetation types at a coarser combined with the accumulation of graminoid standing dead, and scale (1 km ) using NOAA’s Advanced Very High Resolution consequently the peak in LAI occurred at the end of the growing season Radiometer (AVHRR) (Jia et al., 2004). The level of spatial variability in the MAT and mossy tundra. in peak NDVI from our study was approximately equal to the temporal Across the growing season, LAI values in the MAT and mossy change in NDVI from early June to peak season found in the MNT and tussock tundra were highly variable, and the peak values of LAI did not almost half the temporal change in NDVI found in the shrub tundra coincide with the peak greenness (NDVI). Some of this is likely due to during that period. The distinct seasonal patterns observed in this study LAI sampling variability associated with time of day, cloudiness, and illustrate the variability of NDVI patterns among the vegetation types sampling inconsistencies, even though we tried to minimize these. and demonstrate that there is a significant amount of spatial variation However, part of the discrepancy between the LAI and NDVI curves is even at the landscape scale. due to the specific phytomass components that are included in each of We assumed that quantities of live foliar deciduous shrub these measurements. LAI measurements were taken above the moss phytomass would also have the strongest influence on seasonal patterns layer, therefore mosses are not part of LAI, but they will contribute to of LAI, resulting in a seasonal trend that increases to a mid-season peak NDVI values. Standing dead tissue will be included in LAI, but will not (coinciding with mid season foliar growth) and declines late in the contribute in large part to NDVI, which is essentially an index of green growing season due to leaf senescence. The results from this study vegetation. In the mossy tussock tundra, high quantities of mosses and support the fact that shrub foliar material has an important control on graminoids outweigh any minor peak in green shrub material that LAI, but also suggest that the presence of graminoids and standing dead occurred at mid-season, suggesting that we should not expect a temporal may substantially influence LAI, when estimated using these optical, correlation between LAI and NDVI. It was more surprising that peak nondestructive methods. The only notable mid-season peak in LAI LAI did not coincide with peak NDVI in the MAT, however again this occurred in the shrub tundra. Not surprisingly the shrub tundra was the was due to the plant structure present at that site. Even though MAT had only vegetation type with a relatively high productivity of deciduous high levels of shrub phytomass, the majority was either in woody or shrubs (Table 3). A smaller mid season increase of LAI was observed in evergreen material, neither of which have a substantial peak at mid- the MAT, coinciding with the productivity of evergreen shrubs (Table season. As a result there was no definitive mid-season peak in LAI, and 3) and a peak in graminoid phytomass. consequently the peak greenness from NDVI did not coincide with the In both the MAT and mossy tussock tundra the highest LAI values peak of LAI. occurred at the end of our sampling season (end of August). This is most In MNT, the seasonal LAI had low values and showed little likely attributed ultimately to the absence of substantial deciduous foliar variation. This resulted from the very low quantity of shrub phytomass, shrub phytomass and the relatively high quantities of graminoid resulting in low shrub foliar production (Table 2) coupled with the phytomass found in these two vegetation types (Table 2). The standing relatively high quantity of moss phytomass. In the MNT 76% of total dead pool in these communities consists mostly of dead graminoids, phytomass was composed of mosses which, again due to the sampling and consequently where there were large amounts of graminoid method, are not included in the LAI measurements. phytomass, we found large amounts of standing dead. MAT and mossy Seasonal and peak quantities of total phytomass were similar in all tundra had considerably higher ratios of standing dead to total vegetation types except for shrub tundra, which had significantly more phytomass than found in MNT and shrub tundra. Standing dead total phytomass. The total phytomass was between 60 and 80% greater material blocks incoming solar radiation and therefore contributes to in the shrub tundra versus the other three vegetation types. The LAI when these particular methods (attenuation of solar radiation) are similarity in quantities of total phytomass observed in the other three TABLE 3 Estimates of above-ground net primary productivity for four vegetation types at Ivotuk, Alaska. 2 1 Aboveground net primary productivity (g m yr ) Deciduous Evergreen Shrub total Forb Graminoid Horsetail Lichen Moss foliar foliar (includes woody) Total MAT 4 80 0 37 133 29 102 252 479 MNT 11 32 18 24 273 11 34 52 357 MT 0 55 0 0 80 14 12 25 141 ST 19 23 0 21 131 125 6 360 459 S. M. RIEDEL ET AL. / 31 TABLE 4 permanent grids established at Ivotuk allow for temporal replication of this study. Therefore, these data provide a critical baseline against Spatial coefficients of variation (CV) of mean above-ground live which future studies may be compared. phytomass and standing dead. CVs calculated for mid growing season (15 July–26 July) measurements for four vegetation types at Ivotuk, Alaska. Acknowledgments Aboveground live phytomass components This project was funded by the NSF Arctic System Sciences, Standing Forb Graminoid Horsetail Lichen Moss Shrub Total Land-Atmosphere-Ice Interactions Program in a grant to Donald A. dead Walker (OPP-9908829) as part of the Arctic Transitions in the Land- MAT 182 79 0 123 90 36 33 80 Atmosphere System (ATLAS) effort. We thank Meg Miller and Elaine MNT 149 69 76 173 56 44 46 74 Smid for lab assistance at the University of Virginia. MT 0 40 0 78 53 41 32 60 ST 63 96 0 222 34 38 28 41 References Cited Arft, A. M., Walker, M. D., Gurevitch, J., Alatalo, J. M., Bret-Harte, M. S., Dale, M., Diemer, M., Guferli, F., Henery, G. H. 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Journal

"Arctic, Antartic and Alpine Research"Taylor & Francis

Published: Feb 1, 2005

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