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INTERNATIONAL JOURNAL OF BIODIVERSITY SCIENCE, ECOSYSTEM SERVICES & MANAGEMENT, 2016 VOL. 12, NOS. 1–2, 83–95 http://dx.doi.org/10.1080/21513732.2016.1168868 Special Issue: Synergies between biodiversity and timber management Enrichment of Central African logged forests with high-value tree species: testing a new approach to regenerating degraded forests a b a a a,e Jean-Louis Doucet , Kasso Daïnou , Gauthier Ligot , Dakis-Yaoba Ouédraogo , Nils Bourland , c d d a Sheila E. Ward , Patrice Tekam , Paul Lagoute and Adeline Fayolle a b Gestion des Ressources forestières, Département BIOSE, Gembloux Agro-Bio Tech, Université de Liège, Gembloux, Belgium; Nature + asbl, Foresterie tropicale, Département BIOSE, Gembloux Agro-Bio Tech, Université de Liège, Gembloux, Belgium; Mahogany for the d e Future, Inc., San Juan, PR, USA; Pallisco SARL, Douala, Cameroon; Service of Wood Biology - Royal Museum for Central Africa, Tervuren, Belgium ABSTRACT ARTICLE HISTORY Received 17 April 2015 In natural forests of Central Africa, several studies indicate a dramatic decrease in commercial Accepted 17 March 2016 trees, including species of concern for conservation. Enrichment planting with these species will favor both the long-term recovery of their populations and biodiversity conservation in EDITED BY logged forests. In this study, we analyzed the survival and growth of 23 species in plantations. Nicholas Brokaw Fourteen 0.2–1.1 ha mixed species plantations consisting of single-species 15 × 15 m blocks KEYWORDS were studied for 5 years in a logging concession of southeastern Cameroon. The plantation Silviculture; mixture; design considered both species light requirements and sensitivity to damage by pests. To plantation; rehabilitation; identify the best species for enrichment planting, we assessed both species performance and high-value species; plantation costs. We also tested for relationships between species traits and species perfor- functional trait; cost; mance. Mean annual diameter growth increments ranged from 1.67 to 42.9 mm. No sig- regeneration nificant relationship was found between growth and survival. Herbivory by wild Bovidae was the main cause of mortality and should be carefully considered in rehabilitation efforts. We found a significant negative relationship between wood density and maximum growth rate. The other traits tested were not good predictors of species performance in plantations. The two best-performing species, Triplochiton scleroxylon and Terminalia superba, could reach the minimum cutting diameter during a 30-year cutting cycle. Costs were high and mechanized site preparation is suggested to reduce them. Widespread adoption of such plantations will only occur if financial incentives or national regulations for assuring regeneration are implemented. 2012). In most countries, logging companies must com- 1. Introduction ply with an updated legal framework, including a More than 400 million hectares of tropical forest are detailed management plan based on tree inventories, designated for timber production. Most of them have estimates of recovery rates, planned felling cycles, and been or will be selectively logged. In their meta-analysis, site-specific minimum cutting diameters (MCDs) (Bayol Putz et al. (2012) emphasized that although 85–100% of &Borie 2004;Fargeot et al. 2004). In the Congo basin, species of mammals, birds, invertebrates, and plants logging is very selective, with only six logged species remain after selective logging, timber volumes decline accounting for 75% of total timber volume (Ruiz Pérez by about 65% after the first harvest if the same species are et al. 2005). Timber extraction rates per unit area are low again harvested. This depletion of logged species is of (on average 6 m and less than two trees per hectare), growing concern and many commercial species are now associated damages are about 10%, and vegetation recov- listed in the International Union for Conservation of ery is rather fast (Ruiz Pérez et al. 2005). Nature (IUCN) red list (e.g., Bourland et al. 2012). In spite of the logging regulatory framework and Post-logging silvicultural treatments, including planting limited extraction, the continuity of supply of key tim- of high-value species, would need to be applied in order ber species in Central Africa is in question. Although to reestablish or conserve the timber biodiversity of biomass can recover quickly after logging (i.e., 20 years), forest stands. This paper analyzes the survival and regaining exploitable wood volume is much slower growth of such high-value species in simple mixture in (Chazdon 2003). In a long-term study at the M’Baïki plantations in Central Africa. silvicultural experiment in the Central African Central African moist forests cover around 180 mil- Republic, trees available for exploitation decreased dra- lion hectares, and 26% of this area is licensed for com- matically after a first cutting cycle of 24 years (Gourlet- mercial use by logging companies (De Wasseige et al. Fleury et al. 2013). For two important timber species, CONTACT Jean-Louis Doucet email@example.com Gestion des Ressources forestières, Département BIOSE, Gembloux Agro-Bio Tech, Université de Liège, Passage des Déportés, 2, 5030 Gembloux, Belgium © 2016 Informa UK Limited, trading as Taylor & Francis Group 84 J.-L. DOUCET ET AL. (Parrotta et al. 1997). Approaches to restoring tropical modeling the populations dynamics under a repeated forest ecosystems vary depending on the amount of felling regime showed a decrease in exploitable stock linked to decline of the target populations, indicating forest and soil degradation, residual vegetation, and desired restoration outcomes (Chazdon 2008). that the traditional logging regime is far from sustain- Stanturf et al. (2014) proposed the term ‘rehabili- able (Karsenty & Gourlet-Fleury 2006). In Central Africa, forest managers face two major tation’ for restoring desired species composition, structure, or processes to an existing but degraded issues: (i) the difficulty of recovering the timber stock ecosystem. There are several methods of rehabilita- 20–30 years after logging (i.e., after a complete cutting cycle) and (ii) the lack of regeneration in the pioneer tion including enrichment planting or planting after partial overstory removal. In such planting, species species that make up most of the harvest because of mixtures are desirable for both improving biodiver- natural decline of their populations with increasing stand age. According to Gourlet-Fleury et al. (2013), sity and the range of goods and services as well as reducing the felling intensity or even doubling the limiting pest-induced damages (Piotto et al. 2004; Potvin & Gotelli 2008). Different kinds of mixtures length of the cutting cycle will not ensure recovery of these pioneer species, but only hasten their decline due exist. Simple mixtures consist of two or more species to further natural reductions in regeneration. planted in single-species blocks or rows (Stanturf Silviculture may help recover timber stock. In a 24- et al. 2014). Such mixtures are useful on sites with year experiment, Gourlet-Fleury et al. (2013) assessed distinct gradients in environmental factors such as −1 the effect of thinning (mean of 21 trees thinned ha drainage or light. Species must thus be selected 2 −1 according to the site conditions (Stanturf et al. 2014). and 6.6 m ha of basal area removed) on forest However, there is little data on site tolerance and structure and dynamics. They concluded that thinning growth in Africa as emphasized in the meta-analysis of combined with logging significantly increased gain in Piotto (2008). The use of functional traits could help to biomass, but it had no effect on gain in timber stock at predict species performance in plantations. Plant func- the end of the first felling cycle. However, these treat- ments should promote gains in timber stock in sub- tional traits, in particular specific leaf area (SLA), seed sequent cycles. In the same experiment, Ouédraogo size, wood density, and tree height at maturity (H ), max et al. (2011) explained that thinning enhanced the are often good predictors of tree growth rates within growth and survival of nonpioneer light demanders communities (Poorter et al. 2008). For example, func- or shade-bearers, to the detriment of pioneer species. tional traits could predict tree growth and survival in If thinning fosters the growth of nonpioneer sap- plantings in Mexico (Martínez-Garza et al. 2013). While lings, other techniques are necessary to regenerate the relationships may not hold over a broad area, among pioneer timber species, which probably establish tree species in a local area, those with high SLA, small after larger perturbations such as cultivation and seeds, low wood density, and high H tend to have fast max subsequent abandonment (Doucet 2003; Van growth rates (Poorter et al. 2008; Paine et al. 2015). Gemerden et al. 2003; Biwolé et al. 2015). In some In this study, we test a forest rehabilitation method areas, late secondary growth rather than pristine for- using simple mixtures of high-value timber species ests are indicated by abundant large pioneer tree planted after manual removal of the understory in species such as Pericopsis elata and Triplochiton scler- degraded forests in southeastern Cameroon. In 2014, oxylon, which were included in our study (Gillet & we analyzed the survival and growth of 23 species Doucet 2013; Biwolé et al. 2015; Bourland et al. 2015). planted between 2009 and 2012 in 14 mixed-species Management techniques such as enrichment planting plots. We addressed the following questions: (i) How have been recommended to promote these species do the selected species perform in simple mixtures? (Fayolle et al. 2014; Ouédraogo et al. 2014). (ii) Is it possible to use plant functional traits to select Planting success was a major preoccupation for for- the best candidates for mixed plantation? (iii) What is esters in the mid-twentieth century in Africa. Many trials the cost of these plantations? were basedona feworevenasinglespecies (e.g., Khaya spp., Tarrietia utilis, Dupuy & Koua 1993; Dupuy & Chézeaux 1994) sometimes planted on huge areas (e.g., 2. Methods Aucoumea klaineana in Gabon, Brunck et al. 1990). In 2.1. Study area spite of good results, these plantations were abandoned because of labor costs and because management plans The study area is located in the province of Eastern were considered sufficient to ensure sustainable forest Cameroon (between 3°01′ N and 3°44′ N; 13°20′ E management. Today, increasing pressure on tropical and 14°31′ E). The study plantations were established forests and global concerns about the maintenance of in Forest Management Units 10–041, 10–042, ecosystem services (including timber production, biodi- 10–044, 10–039, 10–030, and 10–031, managed by versity, and carbon), again focus attention on yield, and the FSC-certified Pallisco Company. According to also on restoring biodiversity in production forests Worldclim (2015), annual rainfall is ca. 1640 mm INTERNATIONAL JOURNAL OF BIODIVERSITY SCIENCE, ECOSYSTEM SERVICES & MANAGEMENT 85 2.2. Study species with two distinct rainy seasons (August–November and March–June), and the mean annual temperature We selected 23 high-value species for planting is 23.1°C (Hijmans et al. 2005). The topography is (Table 1), based on their use as high-value timber or undulating with elevation varying between 500 and nontimber forest products (fruits, edible caterpillars, 650 m. The geological substrate consists of volcanic medicine, etc.). Many of them are considered as threa- intrusions and metamorphic rocks, and soils are clas- tened by the IUCN (http://:www.iucnredlist.org/), and sified as Ferralsols (Jones et al. 2013). The forest is one is included in CITES Appendix II (P. elata, https:// mostly semi-deciduous and has been classified as www.cites.org/eng/app/appendices.php). We gathered Celtis forest by Fayolle et al. (2014). It is spatially information on qualitative traits (deciduousness, regen- heterogeneous in species composition and degraded eration guild, dispersal) from Hawthorne (1995)and by recent (<50 years) or long-term (>200 years) Meunier et al. (2015). We extracted information on human disturbances (Morin-Rivat et al. 2014). Due quantitative traits from several databases. Seed mass to the lack of regeneration of commercial light- was obtained from Seed Information Database demanding species, enrichment plantings have been Version 7.1 (available from: http://data.kew.org/sid/). recommended for this forest by Fayolle et al. (2014). Wood density was extracted from the Dryad global The host forest company has included enrichment wood density database (Chave et al. 2009; Zanne et al. plantings in its environmental policy since 2009 in 2009). Maximum height and diameter were computed order to promote the regeneration of the logged from Meunier et al. (2015). If data were not available at species in the framework for certification. the species level, we used the most frequent or mean Table 1. Characteristics and functional traits of the species used in this study. Seed IUCN Leaf Dispersal Wood density D H mass max max 1 2 3 –3 4 5 Species Family status Main uses phénology Guild mechanism (g·cm ) (m) (m) (mg) Afzelia bipindensis Harms Fabaceae VU Timber De NPLD Animal 0.73 1.5 40 4977 Amphimas pterocarpoides Fabaceae NA Timber De NPLD Wind 0.66 1 45 Harms Autranella congolensis (De Sapotaceae CR Timber De NPLD Animal 0.78 2 50 NA Wild.) A. Chev. Baillonella toxisperma Pierre Sapotaceae VU Timber/fruits/ De NPLD Animal 0.72 3 60 NA oil Bobgunnia fistuloides (Harms) Fabaceae EN Timber Ev P Animal 0.87 0.8 25 96 J.H. Kirkbr. & Wiersema Cylicodiscus gabunensis Harms Fabaceae NA Timber De P Wind 0.78 2 60 NA Detarium macrocarpum Harms Fabaceae NA Timber De P Animal 0.57 1.5 50 9613 Distemonanthus benthamianus Fabaceae NA Timber De P Wind 0.61 1.2 40 NA Baill. Entandrophragma angolense Meliaceae VU Timber De NPLD Wind 0.46 1.5 50 376.6 (Welw.) C. DC. Entandrophragma cylindricum Meliaceae VU Timber/edible De NPLD Wind 0.57 2 60 311.5 (Sprague) Sprague caterpillars Entandrophragma utile (Dawe & Meliaceae VU Timber De NPLD Wind 0.53 2 60 425 Sprague) Sprague Erythrophleum suaveolens (Guill. Fabaceae NA Timber/edible De P Unassisted 0.84 1.5 40 702.2 & Perr.) Brenan caterpillars Khaya anthotheca (Welw.) C. Meliaceae VU Timber De NPLD Wind 0.50 1.5 60 258 DC. Lophira alata Banks ex C. F. Ochnaceae VU Timber/edible De P Wind 0.88 1.5 50 1300 Gaertn. caterpillars Mansonia altissima (A. Chev.) A. Sterculiaceae NA Timber De NPLD Wind 0.56 0.9 30 322 Chev. Milicia excelsa (Welw.) C.C. Berg Moraceae NT Timber De P Animal 0.56 1.5 55 3 Nauclea diderrichii (De Wild. & Rubiaceae VU Timber Ev P Animal 0.65 1.2 35 1.7 T. Durand) Merr. Pericopsis elata (Harms) Fabaceae EN Timber De P Wind 0.64 1.3 40 333 Meeuwen Piptadeniastrum africanum Fabaceae NA Timber De NPLD Wind 0.61 2 60 NA (Hook. f.) Brenan Pseudospondias microcarpa (A. Anacardiaceae NA Fruits De NPLD Animal NA 0.6 20 NA Rich.) Engl. Pterocarpus soyauxii Taub. Fabaceae NA Timber De NPLD Wind 0.66 1.5 50 531 Terminalia superba Engl. & Diels Combretaceae NA Timber De P Wind 0.46 1.5 50 95 Triplochiton scleroxylon K. Malvaceae LC Timber/edible De P Wind 0.33 2 50 301.2 Schum. caterpillars Values in bold were calculated at the genus level. IUCN status: CR, critically endangered; LC, least concern; NT, near threatened; VU, vulnerable; EN, endangered. Deciduousness: De, deciduous; Ev, evergreen. Guild: P, pioneer; NPLD, nonpioneer light demanding. Maximum diameter. Maximum height. 86 J.-L. DOUCET ET AL. each plot, species were established in blocks according value at the genus level (Slik et al. 2008). Most of the to the visually evaluated canopy cover by the team focal species (20 out of 23) were deciduous, pioneer or nonpioneer light demanders, and dispersed by wind or supervisor following the recommendations of Hawthorne (1995) and Meunier et al. (2015). Planting animals. Among the study species, seed mass varied was done in 15 × 15 m single-species blocks using 25 between 2 (Nauclea diderrichii) and 9613 mg (Detarium macrocarpum). Wood density ranged from seedlings per species planted 3 m apart in a block −3 (Figure 2). Planted seedlings were around 50 cm high. 0.33 (T. scleroxylon)to0.88 g·cm (Lophira alata), with −3 Within each plot, species blocks were alternated to an average of 0.63 g·cm . Maximum diameter at breast minimize the effects of parasites and predation. Milicia height (dbh) ranged from 0.6 (Pseudospondias micro- excelsa, Entandrophragma spp., and K. anthotheca are carpa)to 3.0m(Baillonella toxisperma) and maximum extremely sensitive to parasites (gall-forming insects height ranged from 20 (P. microcarpa) to 60 m (B. and/or shootborers) (Bosu et al. 2006; Opuni- toxisperma, Cylicodiscus gabunensis, Entandrophragma cylindricum, Entandrophragma utile, Khaya anthotheca, Frimpong et al. 2008). In the first year, maintenance cleaning was done every four months. In the second Piptadeniastrum africanum). year, two cleanings were performed every six months, and one cleaning was done at the end of the third year. Future thinning will be necessary to promote the best 2.3. Experimental design tree in each species block, the width of a block (15 m) From 1 January 2009 to 31 December 2014, during being roughly the diameter of one mature crown. the wet season, the forest company planted 53 plots We analyzed the performance of the 23 species in (totaling 44.4 ha) with seedlings grown in nurseries. 14 plots planted between 2009 and 2012, out of the 53 These plots were established in the most degraded original plots (Table 2). The size of the selected plots areas along principal or secondary roads (Figure 1). ranged from 0.2 to 1.1 ha, and the number of species These locations facilitated plot maintenance and pre- varied from 2 to 10 per plot. At plantation establish- served forest cover in less disturbed areas. ment, canopy cover was estimated above each planted The planting design was a ‘simple mixture, multiple seedling with a clinometer. The presence (1) or species, single cohort planting’ as defined by Stanturf absence (0) of vegetation in the following height et al. (2014). Planting followed these four steps. First, the intervals – 0–10 m, 10–20 m, and >20 m – was high-value species and all other species with a dbh visually assessed. The sum was calculated to obtain >50 cm were identified and protected. Second, the a canopy cover index in a range from 0 (full open) to understory was clear cut by a team of 10 workers 3 (full cover). After plantation establishment, the using machetes or a chainsaw. Third, the species planted proportion of seedlings planted in fully open areas were selected based on their shade tolerance (P, NPLD (total cover index = 0) was 75% (SD = 18%), includ- sensu Hawthorne 1995, Table 1), their availability in the ing 81% for pioneer and 62% for nonpioneer species. nurseries, and canopy openness at the sites. Fourth, for Figure 1. Location of the enrichment plantings between 2009 and 2014 in the Pallisco management units, southeastern Cameroon. INTERNATIONAL JOURNAL OF BIODIVERSITY SCIENCE, ECOSYSTEM SERVICES & MANAGEMENT 87 Bt Bt Bt Bt Bt Ea Ea Ea Ea Ea Ma Ma Ma Ma Ma Ka Ka Ka Ka Ka Me Me Me Me Me Ma Ma Ma Ma Ma Bt Bt Bt Bt Bt 42 Bt Bt Bt Bt Bt Ea Ea Ea Ea Ea Ma Ma Ma Ma Ma Ka Ka Ka Ka Ka Me Me Me Me Me Ma Ma Ma Ma Ma Bt Bt Bt Bt Bt 39 Bt Bt Bt Bt Bt Ea Ea Ea Ea Ea Ma Ma Ma Ma Ma Ka Ka Ka Ka Ka Me Me Me Me Me Ma Ma Ma Ma Ma Bt Bt Bt Bt Bt 36 Bt Bt Bt Bt Bt Ea Ea Ea Ea Ea Ma Ma Ma Ma Ma Ka Ka Ka Ka Ka Me Me Me Me Me Ma Ma Ma Ma Ma Bt Bt Bt Bt Bt 33 Bt Bt Bt Bt Bt Ea Ea Ea Ea Ea Ma Ma Ma Ma Ma Ka Ka Ka Ka Ka Me Me Me Me Me Ma Ma Ma Ma Ma Bt Bt Bt Bt Bt 30 Ec Ec Ec Ec Ec Nd Nd Nd Nd Nd Ts Ts Ts Ts Ts Me Me Me Me Me Ka Ka Ka Ka Ka Ts Ts Ts Ts Ts Ec Ec Ec Ec Ec 27 Ec Ec Ec Ec Ec Nd Nd Nd Nd Nd Ts Ts Ts Ts Ts Me Me Me Me Me Ka Ka Ka Ka Ka Ts Ts Ts Ts Ts Ec Ec Ec Ec Ec 24 Ec Ec Ec Ec Ec Nd Nd Nd Nd Nd Ts Ts Ts Ts Ts Me Me Me Me Me Ka Ka Ka Ka Ka Ts Ts Ts Ts Ts Ec Ec Ec Ec Ec 21 Ec Ec Ec Ec Ec Nd Nd Nd Nd Nd Ts Ts Ts Ts Ts Me Me Me Me Me Ka Ka Ka Ka Ka Ts Ts Ts Ts Ts Ec Ec Ec Ec Ec 18 Ec Ec Ec Ec Ec Nd Nd Nd Nd Nd Ts Ts Ts Ts Ts Me Me Me Me Me Ka Ka Ka Ka Ka Ts Ts Ts Ts Ts Ec Ec Ec Ec Ec 15 Ma Ma Ma Ma Ma Ka Ka Ka Ka Ka Eu Eu Eu Eu Eu Db Db Db Db Db Nd Nd Nd Nd Nd Me Me Me Me Me Ma Ma Ma Ma Ma 12 Ma Ma Ma Ma Ma Ka Ka Ka Ka Ka Eu Eu Eu Eu Eu Db Db Db Db Db Nd Nd Nd Nd Nd Me Me Me Me Me Ma Ma Ma Ma Ma 9 Ma Ma Ma Ma Ma Ka Ka Ka Ka Ka Eu Eu Eu Eu Eu Db Db Db Db Db Nd Nd Nd Nd Nd Me Me Me Me Me Ma Ma Ma Ma Ma 6 Ma Ma Ma Ma Ma Ka Ka Ka Ka Ka Eu Eu Eu Eu Eu Db Db Db Db Db Nd Nd Nd Nd Nd Me Me Me Me Me Ma Ma Ma Ma Ma 3 Ma Ma Ma Ma Ma Ka Ka Ka Ka Ka Eu Eu Eu Eu Eu Db Db Db Db Db Nd Nd Nd Nd Nd Me Me Me Me Me Ma Ma Ma Ma Ma 0 AI AH AG AF AE AD AC AB AA Z Y X W V U T S R Q P O N M L K J I H G F E D C B A Figure 2. Example of a multispecies plot consisting of 21 single-species blocks. Each block includes 25 seedlings planted 3 m apart. Each seedling is identified by a combination of a letter (X axis) and a distance (Y axis). Bt, Baillonella toxisperma; Db, Distemonanthus benthamianus; Ea, Entandrophragma angolense; Ec, Entandrophragma cylindricum; Eu, Entandrophragma utile; Ka, Khaya anthotheca; Ma, Mansonia altissima; Me, Milicia excelsa; Nd, Nauclea diderrichii; Ts, Terminalia superba. The survival and diameter of each seedling was establishment varied among plots and species from recorded each year. For 2–5 years, depending on the 690 to 1740 days, survival at t = 690 days was deter- plot, diameters of 6540 tagged seedlings were mea- mined for all species. sured at 10 cm above the stem base with a caliper. At A total of 17,794 diameter measurements were the end of the experiment, the crown exposure of made on the 4621 trees alive at the end of the mon- each seedling was classified following Dawkins itoring period. The diameter growth was modeled (1958). Code 1 was assigned to fully shaded unders- with a linear mixed model. The time period (up to tory trees, 2 to upper understory trees partly exposed ca. 5 years) was not long enough to justify the use of a to direct light, 3 to lower canopy trees partly exposed nonlinear model (Paine et al. 2012). Because mea- to direct light, 4 to canopy trees fully exposed to light sures of tree diameter were repeated through time, from above, and 5 to fully emergent with no other these observations are dependent and correlated with vegetation in an inverted vertical cone of 45°. All field each other. Consequently, including random effects measurements were performed by the same team. to account for individual tree variability was required. Plantations yields and costs were estimated for the It was achieved using a mixed modeling procedure. 53 plots using data from the logging company Models with fixed and random effects, including a between 2009 and 2014. The costs for planted and random intercept (α) and a random slope (β), were maintained areas were calculated each year. Costs tested. The best model was selected based on Akaike were classified into wages, transport (fuel, mainte- information criterion comparison of models, and the nance, and depreciation), and materials (machetes, significance of the difference between pairs of models chainsaw, etc.). Costs were also divided into the was tested with the likelihood ratio test. We first main operation stages (seedling production, site pre- identified the best random structure (random slope) paration, planting, and maintenance). and then the best fixed effects (species and species– time interaction) in accordance with Zuur et al. (2009). The equation that was found to give the best 2.4. Data analysis mixed linear model, based on the diameter (D) of the seedling (i) belonging to species (s) at the time (t) was To quantify tree survival, we estimated the survival function S(t) over time using the nonparametric ¼ a þ c Time þ β Time þ ε ist s s ist is Kaplan–Meier estimator (Harrell 2001), which gives with a and c as fixed parameters, β as a random the probability of an individual seedling surviving to s s is parameter, and ε as the error. time t, the time since the beginning of the ist We then extracted the fixed model parameters for experiment: each species corresponding to the mean growth value SðtÞ¼ t tð1 d =n Þ i i i and the best linear unbiased estimate of the random where t is the time interval, d is the number of effects (BLUPS) for each individual in order to com- i i deaths that occur in the interval t , n is the number pute the mean of the growth rate of the best-perform- i i of seedlings that are alive at the end of the interval t , ing seedlings (i.e., best 10% of stems). and Π is the product operator across all cases less Species performance was represented by mean than or equal to t. Since the time from plantation growth rates (all stems), maximum growth rates 88 J.-L. DOUCET ET AL. Table 2. Size, openness, planting date, and species combinations (number of planted seedlings) for experimental plots. Plot number: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Total Area (ha) 0.4 0.8 1.1 0.3 0.3 0.2 0.5 0.3 0.3 0.5 0.3 0.2 0.3 0.3 5.9 % remaining canopy cover 48.0 42.3 40.0 70.5 13.3 8.2 11.3 15.6 10.0 19.5 17.7 11.9 26.7 21.7 25.5 Planting date 2009 2009 2010 2010 2011 2012 2012 2012 2012 2012 2012 2012 2012 2012 July October March August November February February February February March March February June June Amphimas pterocarpoides 50 25 75 Autranella congolensis 25 25 Baillonella toxisperma 75 50 50 50 50 99 50 50 75 549 Bobgunnia fistuloides 50 25 25 100 Cylicodiscus gabunensis 100 2 102 Detarium macrocarpum 50 50 Distemonanthus 25 50 25 50 150 benthamianus Entandrophragma angolense 25 25 50 Entandrophragma 75 75 25 50 50 50 75 25 25 50 500 cylindricum Entandrophragma utile 40 75 25 50 25 50 50 23 25 25 388 Erythrophleum suaveolens 25 25 Khaya anthotheca 75 75 50 200 Lophira alata 25 25 50 Mansonia altissima 300 650 396 75 75 25 100 50 75 151 50 50 75 2072 Milicia excelsa 75 75 Nauclea diderrichii 50 75 125 Pericopsis elata 60 50 50 160 Piptadeniastrum africanum 50 50 Pseudospondias microcarpa 20 20 Pterocarpus soyauxii 25 25 25 75 Terminalia superba 275 454 50 50 25 50 25 50 75 270 25 50 100 1499 Triplochiton scleroxylon 25 50 25 25 125 INTERNATIONAL JOURNAL OF BIODIVERSITY SCIENCE, ECOSYSTEM SERVICES & MANAGEMENT 89 (average of best 10% of stems), and mortality rates 3.2. Relationships between species performance and (all stems). traits We tested for significant relationships between We observed only weak correlations between species species traits and species performance using Pearson performance and most functional traits (Table 4). correlation for quantitative traits and Kruskal–Wallis However, we found a significant relationship between analysis for qualitative traits. All statistical analyses the wood density and the maximum growth rate, with were performed within the open source R environ- fast-growing species tending to have lighter wood. If ment (R version 2.14.1). The ‘lme4’ package was used the fastest growing species (T. scleroxylon) was for fitting mixed models (Bates et al. 2015). removed from the analysis, the relationship was no longer significant. We also identified a weak, non- significant, correlation between leaf phenology and 3. Results survival, with deciduous species tending to have a better survival rate than evergreen species. There 3.1. Species survival and growth were however only two evergreen species among the By the end of the experiment, some species experi- 23 studied species, N. diderrichii and Bobgunnia fis- enced high mortality, while others had high survival tuloides, with probabilities of survival of 31.2% and (Figure 3). The probability of survival at 690 days 66.0%, respectively, at t = 690 days, below the 73.7% (Table 3) ranged from 31.2% for Nauclea diderrichii average of the 23 studied species. (88 dead, 37 living seedlings) to 100% for Autranella congolensis (0 dead, 25 living seedlings). After 690 3.3. Costs days, the survival rate stabilized (e.g., P. elata, at ca. 70%) or decreased further (e.g., E. cylindricum). The forest planting team consisted of 13 people, includ- In the first years, growth trajectories tended to be ing two nurserymen and one supervisor. This team was linear for the 23 study species (Figure 4). The mean only in charge of the silvicultural activities. As a con- annual diameter growth increment (c ) ranged from sequence, the cost per hectare was specific to this team. 1.67 mm for M. excelsa to 42.90 mm for T. sclerox- The team was estimated to be able to plant 10 ha per ylon (Table 3). Although both were planted according year and to maintain ca. 60 ha per year (three cleanings to their shade tolerance guild under low canopy on 10 ha the first year following planting, two cleanings cover, the proportion of fully emergent stems (fifth on 10 ha in the second year, and the last cleaning on category of Dawkins) was 3% for M. excelsa and 10 ha in the third year). The total estimated cost per 61.3% for T. scleroxylon (Table 3). For all species hectare was $7038 (€5585) distributed as follows: wages considered together, the relationship between survival 69%, vehicles 26% (including depreciation, fuel, and and growth was positive but not significant maintenance), and materials 5%. If the total cost was (r = 0.291, p = 0.178). split among the main operation stages, the results were Figure 3. Kaplan–Meier survival estimates over time, with 95% confidence bounds for the 23 studied species. The vertical gray line indicates survival at t = 690 days for comparison among species. 90 J.-L. DOUCET ET AL. Table 3. For each species, number of observations (N), average canopy cover index (from 0, full open, to 3, full cover), average Dawkins index (from 1, lower understory, to 5, emergent), proportion (%) of emergent stems (fifth category of Dawkins), survival −1 at 690 days (% of stems alive), maximum and mean diameter growth rates (in mm year ), minimum cutting diameter (Cameroon national regulation), and time to reach the minimum cutting diameter given the mean diameter growth rate. Average cover Average Dawkins % % Max. Growth MCD Time Species N index index emergent survival growth Growth SD (cm) (years) Afzelia bipindensis 75 0.97 3.4 18.2 54.7 11.21 5.75 1.27 80 139 Amphimas pterocarpoides 75 0.42 3.2 21.1 43.7 5.42 2.42 1.25 50 207 Autranella congolensis 25 0.84 2.4 0.0 100.0 8.76 6.41 1.59 60 94 Baillonella toxisperma 549 0.29 3.5 16.1 58.7 11.65 6.76 0.47 100 148 Bobgunnia fistuloides 100 0.09 3.7 18.5 66.0 16.10 7.77 0.96 50 64 Cylicodiscus gabunensis 102 1.62 2.6 1.2 83.3 9.08 5.10 0.80 60 118 Detarium macrocarpum 50 1.00 2.7 2.2 92.0 6.71 4.53 1.08 50 110 Distemonanthus 150 0.21 3.6 23.4 76.0 21.16 10.46 0.73 60 57 benthamianus Entandrophragma 50 0.33 2.9 0.0 56.0 9.88 3.84 1.48 80 208 angolense Entandrophragma 500 0.72 2.7 1.6 72.6 5.10 1.71 0.48 100 585 cylindricum Entandrophragma utile 387 0.59 2.9 3.3 73.4 12.22 5.00 0.48 80 160 Erythrophleum suaveolens 25 0.27 3.5 22.7 88.0 12.41 7.30 1.64 50 68 Khaya anthotheca 200 0.96 2.8 3.5 74.0 17.20 7.59 0.63 80 105 Lophira alata 50 0.19 3.8 27.1 96.0 18.73 13.64 1.15 60 44 Mansonia altissima 2072 0.93 2.7 6.8 86.4 13.64 7.34 0.18 60 82 Milicia excelsa 75 0.09 3.3 3.0 44.0 2.83 1.67 1.34 100 599 Nauclea diderrichii 125 0.24 3.7 21.6 31.2 17.08 9.54 1.27 80 84 Pericopsis elata 160 0.50 3.4 18.3 73.8 16.12 9.33 0.69 90 96 Piptadeniastrum 50 1.00 2.8 0.0 92.0 19.41 11.42 1.07 60 53 africanum Pseudospondias 20 0.00 4.6 70.6 85.0 21.74 16.77 1.87 50 30 microcarpa Pterocarpus soyauxii 75 0.33 3.2 0.0 81.3 11.80 5.01 1.06 60 120 Terminalia superba 1499 0.45 3.4 20.9 84.0 33.50 19.80 0.22 60 30 Triplochiton scleroxylon 125 0.09 4.4 61.3 83.0 65.96 42.90 0.75 80 19 Figure 4. Growth trajectory of the studied species. Number of monitored seedlings (ind.) and total number of measurements (obs.) are given for each species. Depending on the installation date (between 2009 and 2012), the monitoring period varied between plots and ranged from 690 to 1740 days. as follows: seedling production 11%, site preparation mature tree from seed was approximated at $160 53%, planting 8%, and maintenance 28%. (€127) in 2014. This figure is probably underesti- Because of the plantation design, the number of mated because thinning costs were not included. In expected crop trees was 44 per hectare (44 blocks per Cameroon, the MCD ranges from 50 to 100 cm, hectare with one crop tree per block remaining after depending on the species (Table 3). Any projections thinning). Therefore, the total cost for raising a about the time needed for trees to reach this diameter INTERNATIONAL JOURNAL OF BIODIVERSITY SCIENCE, ECOSYSTEM SERVICES & MANAGEMENT 91 Table 4. Relationship between the performance of 23 species were similar or slightly lower than those reported in (mean and maximum growth, and survival) in plantations other studies (Dupuy & Koua 1993; Koumba Zaou and species functional traits. et al. 1998; Onyekwelu 2007; Addo-Danso 2010). Maximum The positive (but nonsignificant) relationship Growth growth Survival between survival and growth was not always consistent Quantitative traits Wood-specific gravity r = −0.41 r = −0.46 r = 0.01 with results from other studies. For example, Beckage p = 0.058 p = 0.030 p = 0.650 and Clark (2003) found that species with the highest Seed mass r = −0.17 r = −0.22 r = 0.22 p = 0.540 p = 0.415 p = 0.406 mortality rates outperformed the other species. On the Maximum diameter r = 0.03 r = 0.04 r = 0.11 otherhand, in ourstudy,for some species(e.g., M. p = 0.883 p = 0.868 p = 0.609 excelsa and Entandrophragma spp.), low growth incre- Maximum height r = −0.14 r = −0.10 r = 0.08 p = 0.539 p = 0.638 p = 0.710 ments were associated with high mortality rates. Qualitative traits Attempts to grow M. excelsa in plantations have gener- 2 2 2 Regeneration guild (df = 1) χ = 2.37 χ = 1.37 χ = 0.21 ally failed due to attacks by Phytolyma lata, a gall-form- p = 0.124 p = 0.242 p = 0.640 2 2 2 Deciduousness (df = 1) χ = 0.76 χ = 0.43 χ = 3.05 ing insect. Shade during the first 12–18 months p = 0.383 p = 0.513 p = 0.081 2 2 2 minimizes the development of galls and associated die- Dispersal (df = 2) χ = 0.21 χ = 1.38 χ = 1.72 p = 0.901 p = 0.501 p = 0.422 back, but repeated attacks in open areas have frequently Pearson correlation coefficients (r) and associated p-values are given for resulted in high mortality (Nichols et al. 1999;Bosuetal. quantitative traits. Kruskal–Wallis χ and associated p-values are given 2006). Fayolle et al. (2015) have recently recommended for qualitative traits. Significant relationships are shown in bold. planting M. excelsa in logging gaps rather than in open areas. Similarly, Hypsipyla shoot borers often hamper plan tation success for Meliaceae (Khaya spp. and are imprecise. But if the average observed growth Entandrophragma spp.)inopenareas.In our study, rates could be maintained with thinning (see Section planting K. anthotheca underlight shade(Table 3) 4), the time to the MCD would be ca. 19 years for the allowed for good growth and reduced mortality, con- best-performing species (T. scleroxylon) and between firming the results obtained by Opuni-Frimpong et al. 30 and 160 years for most other species. (2008)inGhana. The high mortality rates obtained for the other Meliaceae species (Entandrophragma spp.) were not 4. Discussion due to shoot borer attack but rather the result of 4.1. Identification of the best species for shoots being browsed by wild Bovidae (mainly enrichment planting Tragelaphus spekii and Cephalophus spp.) (personal observation). Such damage in fallow forests with Many plantation methods have been tested since the early twentieth century, but few data are available on canopy structure very similar to young plantations has also been reported by Hall (2008). Due to their species performance in young plantations, even for low re-sprouting capacity, the growth of the injured important timbers like Entandrophragma spp. (Dupuy & Mille 1993) or species of concern for con- trees was probably reduced by competing, overtop- ping vegetation (Table 3). The tradeoffs among sur- servation like P. elata (on CITES Appendix II), A. vival, growth, and herbivory present a serious congolensis, and B. fistuloides (classified CR and EN challenge for these species (Goodale et al. 2014). in the IUCN red list, respectively). Most of the quan- titative information available is from projections of early species growth derived from older trees in plan- 4.2. Relationships between functional traits and tations of a known age, but with little information on species performance plantation maintenance (Ndongo et al. 2009; Ebuy et al. 2011). Moreover, to avoid bias due to environ- The lack of significant correlations between qualita- mental and time variations, multispecies comparison tive functional traits and performance measures in the same site during the same period has been might be due to several factors. Variation in regen- needed, as was carried out in this study. eration guild was limited; most of the species used We found promising growth rates and high survi- were deciduous and light-demanding (either pioneer val rates for some species (e.g., T. scleroxylon, T. or nonpioneer), only two species were evergreen, and superba, and L. alata). In comparison to other plan- none was a shade bearer. Martínez-Garza et al. (2013) tations of the same age (maximum 10 years), our found that regeneration guild could be used to pre- estimates of mean annual diameter increment were dict tree growth, but they also emphasized that some (i) higher for T. scleroxylon (Lapido et al. 1951; nonpioneer species performed nearly as well as pio- Dupuy & Koua 1993), (ii) similar for T. superba neers. Functional guilds are not discrete because there (Tariel & Groulez 1958; Appiah 2012), and (iii) is a continuum in such traits as light requirements slightly higher for L. alata (Biwolé et al. 2012). For (Agyeman et al. 1999). The maximum growth of most of the other species, the values we observed nonpioneer species also occurs in partial to full 92 J.-L. DOUCET ET AL. vertical illumination (Martínez-Garza et al. 2005). height in forest canopies, and the crowns of taller species receive more light than those of shorter spe- Nursery-grown seedlings of nonpioneer species can cope with high light levels and a low water supply, cies (Poorter et al. 2008). Nearly all the studied spe- and they grow faster in the open conditions than in cies were able to reach the canopy and the variance in sizes was probably too small to detect any relation- shade (Fayolle et al. 2015). The type of dispersal (Table 1) was also a poor predictor of species growth, ship between height and growth. Another architec- perhaps because the main dispersal categories can be tural feature, such as crown length, might have been a better predictor. Long crown species have a larger leaf found in both fast- and slow-growing tropical tree species (Agyeman et al. 1999). area and are more self-shading at midday, thus mini- The weak relationship between quantitative func- mizing photoinhibition due to high leaf temperature (Martínez-Garza et al. 2013). tional traits (Table 1) and growth is more surprising because wood density, seed mass, and adult stature are usually considered to be significant predictors of 4.3. Perspectives on supply and costs tree growth and mortality for trees with diameter above 10 cm (Poorter et al. 2008). Wood density Without long-term monitoring, any growth projec- was negatively correlated with maximum growth tion is imprecise, but useful estimates can be made. If rate, but its relationship with mean growth rate was unthinned, growth for any of the studied species may not significant, possibly because too few species were not be linear over a long time period (Paine et al. sampled (Poorter et al. 2008). 2012). Obiang et al. (2014) showed that growth rates Growth and mortality rates are often negatively of two species (A. klaineana and L. alata) decrease correlated with seed mass. In our experiment, the when their stems reach 50–70 cm in diameter. relationship, although negative, was not statistically However, in mixed plantations, Dupuy and Koua significant. Seed size affects survival and indirectly (1993) observed a linear growth rate of Khaya spp. growth since small-seeded species have limited until at least 50 cm in diameter. In 50-year-old reserves and must quickly deploy roots and leaves unthinned plantations, diameter growth rates for the to become autotrophic (Poorter et al. 2008). Many best-performing stems of Distemonanthus bentha- small-seeded species have photosynthetic cotyledons mianus (at 825 surviving stems per hectare (SSH)), and a high SLA, which boosts juvenile growth. But E. cylindricum (800 SSH), L. alata (1375 SSH), correlation between seed size and growth disappears Mansonia altissima (300 SSH), and T. scleroxylon −1 over time as the initial SLA differences are reduced. (800 SSH) were 10, 7, 9, 10, and 14 mm·year , Variation in SLA can be a main driver of inter- respectively (Ndongo 2006). Given such high growth specific variation in seedling growth rate (Poorter & rates after 50 years, we can assume that the average Bongers 2006), although SLA is only a good predictor growth rates we observed can be maintained up to at a low irradiance level and not in open sites the legal cutting diameter (Table 3) if thinning is (Martínez-Garza et al. 2013). Other leaf traits, like performed. The possible time to reach this diameter leaf dry mass content (LDMC), could be better pre- ranged from 19 years (T. scleroxylon) to 599 years (M. dictors for open sites, at least for nonpioneer species, excelsa). This latter value can be explained by the low because a high LDMC is an adaptation to survive in growth rate of M. excelsa under repeated attacks by P. dry open area conditions (Martínez-Garza et al. lata in open areas. 2013). Leaves with higher LDMC have higher moduli In Central Africa, forests remain the property of of elasticity and thicker and more rigid lignified cell the State and concessions are licensed to private log- walls than species with lower LDMC. These charac- ging companies for a cutting cycle. In Cameroon, the teristics allow them to maintain leaf turgor in dry duration of the cutting cycle is 30 years. Over this conditions (Martínez-Garza et al. 2013). For the spe- period, only two of the studied species would prob- cies studied, however, LDMC may not predict growth ably reach the MCDs: T. scleroxylon and T. superba well due to the overlap in LDMC values between the (Table 3). Their average volumes at the MCDs should fast-growing and slow-growing species used in this be 6.3 and 2.8 m , if calculated with the volume tables 2,35 study. Vanhal (2013) reported LDMC values of 0.51, used by the government, V = 0.000209D and −1 2,28 0.31, and 0.36 g·g for fast-growers T. scleroxylon, T. V = 0.000252D , respectively (Fayolle et al. 2013). superba, and L. alata, respectively, versus LDMC The per tree cost we estimated for raising a mature −1 values of 0.30, 0.33, and 0.36 g·g , for slow growers tree (US$160) is roughly equivalent to the market Afzelia bipindensis, E. cylindricum, and Pterocarpus sales price for 1 m of round wood: average log soyauxii, respectively. FOB (Free On Board) prices $163 for T. scleroxylon Maximum size reached by the studied species was (ITTO 2012) and $128 for T. superba (http://data also a poor predictor of performance, probably base.prota.org). The FOB prices for other species because the relationship between height and growth can be much higher (e.g., ca. $350 for P. elata, depends on the ontogenic stage. Light increases with http://database.prota.org) but their growth rates are INTERNATIONAL JOURNAL OF BIODIVERSITY SCIENCE, ECOSYSTEM SERVICES & MANAGEMENT 93 lower (0.8 m for P. elata after 30 years if calculated Funding with the volume table recommended by Fayolle et al. This research was supported by the FRFC project (F.R.S./ 2013). While our estimates of production costs are FNRS, No. 2.4577.10) Dynamics of light-demanding tree rough, they help explain the current financial disin- species and grasses in the humid forests of Central Africa centive for implementing silvicultural operations, as in relationship with past anthropogenic and climate dis- turbances. Dakis Ouédraogo was funded by the research shown in Amazonia by Schulze (2008). The high cost unit for the management of forest resources, BIOSE of planting has always been cited as one of the main Department, Gembloux Agro-Bio Tech, Liège University, reasons for the abandonment of this practice in tro- Belgium. pical regions. Mechanization could reduce the expen- sive preparation of the sites (53% of the total cost) and should be tested. Alternatively, gap enrichment References planting is relatively inexpensive (Doucet et al. 2009; Schulze 2008). Planting in logging gaps should be Addo-Danso SD 2010. Survival and growth in a moist-semi deciduous forest in Ghana: comparison of monoculture favored for some species that have shown good per- and mixed-species plantations [Thesis]. Freiburg, formance in this environment, such as M. excelsa or Germany: Faculty of Forest and Environmental B. toxisperma (Fayolle et al. 2015). Science, Albert-Ludwigs University. Agyeman VK, Swaine MD, Thompson J. 1999. Responses of tropical forest tree seedlings to irradiance and the 5. Conclusions derivation of a light response index. J Ecol. 87:815–827. Appiah M. 2012. Changes in plant species composition Simple mixtures could be an effective way to restore within a planted forest in a deciduous agroecosystem biodiversity of timber species and to maintain a long- in Ghana. Agrofor Syst. 85:57–74. Bates D, Maechler M, Bolker B. 2012. Fitting Linear Mixed- term supply of goods and services from Central Effects Models Using lme4. J Stat Soft.67:1–48 African moist logged forests. 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International Journal of Biodiversity Science, Ecosystem Services & Management – Taylor & Francis
Published: Jan 2, 2016
Keywords: Silviculture; mixture; plantation; rehabilitation; high-value species; functional trait; cost; regeneration
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