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ABSTRACT: Hunting affects a considerably greater area of the tropical forest biome than deforestation and logging combined. Often even large remote protected areas are depleted of a substantial proportion of their vertebrate fauna. However, understanding of the long-term ecological consequences of defaunation in tropical forests remains poor. Using tree census data from a large-scale plot monitored over a 15-year period since the approximate onset of intense hunting, we provide a comprehensive assessment of the immediate consequences of defaunation for a tropical tree community. Our data strongly suggest that over-hunting has engendered pervasive changes in tree population spatial structure and dynamics, leading to a consistent decline in local tree diversity over time. However, we do not find any support for suggestions that over-hunting reduces above-ground biomass or biomass accumulation rate in this forest. To maintain critical ecosystem processes in tropical forests increased efforts are required to protect and restore wildlife populations.
Ecology Letters 03/2013; · 17.56 Impact Factor
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Claire A Baldeck,
Kyle E Harms,
Joseph B Yavitt,
Robert John,
Benjamin L Turner,
Renato Valencia,
Hugo Navarrete, Stuart J Davies,
George B Chuyong,
David Kenfack,
Duncan W Thomas,
Sumedha Madawala,
Nimal Gunatilleke,
Savitri Gunatilleke,
Sarayudh Bunyavejchewin,
Somboon Kiratiprayoon,
Adzmi Yaacob,
Mohd N Nur Supardi,
James W Dalling
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ABSTRACT: Both habitat filtering and dispersal limitation influence the compositional structure of forest communities, but previous studies examining the relative contributions of these processes with variation partitioning have primarily used topography to represent the influence of the environment. Here, we bring together data on both topography and soil resource variation within eight large (24-50 ha) tropical forest plots, and use variation partitioning to decompose community compositional variation into fractions explained by spatial, soil resource and topographic variables. Both soil resources and topography account for significant and approximately equal variation in tree community composition (9-34% and 5-29%, respectively), and all environmental variables together explain 13-39% of compositional variation within a plot. A large fraction of variation (19-37%) was spatially structured, yet unexplained by the environment, suggesting an important role for dispersal processes and unmeasured environmental variables. For the majority of sites, adding soil resource variables to topography nearly doubled the inferred role of habitat filtering, accounting for variation in compositional structure that would previously have been attributable to dispersal. Our results, illustrated using a new graphical depiction of community structure within these plots, demonstrate the importance of small-scale environmental variation in shaping local community structure in diverse tropical forests around the globe.
Proceedings of the Royal Society B: Biological Sciences 01/2013; 280(1753):20122532. · 5.41 Impact Factor
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ABSTRACT: Drought and pests are primary abiotic and biotic factors proposed as selective filters acting on species distributions along rainfall gradients in tropical forests and may contribute importantly to species distributional limits, performance, and diversity gradients. Recent research demonstrates linkages between species distributions along rainfall gradients and physiological drought tolerance; corresponding experimental examinations of the contribution of pest pressure to distributional limits and potential interactions between drought and herbivory are limited. This study aims to quantitate differential performance and herbivory as a function of species range limits across a climatic and floristic transition in Southeast Asia. Khao Chong Botanical Garden, Thailand and Pasoh Forest Reserve, Malaysia straddle the Kangar-Pattani Line. A reciprocal transplantation across a seasonality gradient was established using two groups of species ("widespread" taxa whose distributions include seasonally dry forests and "aseasonal" taxa whose distributions are limited to aseasonal forests). Growth, biomass allocation, survival, and herbivory were monitored for 19 months. Systematic differences in performance were a function of species distribution in relation to rainfall seasonality. In aseasonal Pasoh, aseasonal species had both greater growth and survivorship than widespread species. These differences were not a function of differential herbivory as widespread and aseasonal species experienced similar damage in the aseasonal forest. In seasonally dry Khao Chong, widespread species showed higher survivorship than aseasonal species, but these differences were only apparent during drought. We link this differential performance to physiological mechanisms as well as differential tolerance of biotic pressure during drought stress. Systematic decreases in seedling survival in aseasonal taxa during drought corresponded with previously documented physiological differences and may be exacerbated by herbivore damage. These results have important implications for tropical diversity and community composition in light of predicted increases in the frequency and severity of drought in hyperdiverse tropical forests.
Ecology and evolution. 11/2012; 2(11):2682-94.
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ABSTRACT: The response of tropical forests to global climate variability and change remains poorly understood. Results from long-term studies of permanent forest plots have reported different, and in some cases opposing trends in tropical forest dynamics. In this study, we examined changes in tree growth rates at four long-term permanent tropical forest research plots in relation to variation in solar radiation, temperature and precipitation. Temporal variation in the stand-level growth rates measured at five-year intervals was found to be positively correlated with variation in incoming solar radiation and negatively related to temporal variation in night-time temperatures. Taken alone, neither solar radiation variability nor the effects of night-time temperatures can account for the observed temporal variation in tree growth rates across sites, but when considered together, these two climate variables account for most of the observed temporal variability in tree growth rates. Further analysis indicates that the stand-level response is primarily driven by the responses of smaller-sized trees (less than 20 cm in diameter). The combined temperature and radiation responses identified in this study provide a potential explanation for the conflicting patterns in tree growth rates found in previous studies.
Proceedings of the Royal Society B: Biological Sciences 07/2012; 279(1744):3923-31. · 5.41 Impact Factor
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Nathan G Swenson,
James C Stegen, Stuart J Davies,
David L Erickson,
Jimena Forero-Montaña,
Allen H Hurlbert,
W John Kress,
Jill Thompson,
María Uriarte,
S Joseph Wright,
Jess K Zimmerman
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ABSTRACT: The degree to which turnover in biological communities is structured by deterministic or stochastic factors and the identities of influential deterministic factors are fundamental, yet unresolved, questions in ecology. Answers to these questions are particularly important for projecting the fate of forests with diverse disturbance histories worldwide. To uncover the processes governing turnover we use species-level molecular phylogenies and functional trait data sets for two long-term tropical forest plots with contrasting disturbance histories: one forest is older-growth, and one was recently disturbed. Having both phylogenetic and functional information further allows us to parse out the deterministic influences of different ecological filters. With the use of null models we find that compositional turnover was random with respect to phylogeny on average, but highly nonrandom with respect to measured functional traits. Furthermore, as predicted by a deterministic assembly process, the older-growth and disturbed forests were characterized by less than and greater than expected functional turnover, respectively. These results suggest that the abiotic environment, which changes due to succession in the disturbed forest, strongly governs the temporal dynamics of disturbed and undisturbed tropical forests. Predicting future changes in the composition of disturbed and undisturbed forests may therefore be tractable when using a functional-trait-based approach.
Ecology 03/2012; 93(3):490-9. · 4.85 Impact Factor
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ABSTRACT: Long-term studies have revealed that the structure and dynamics of many tropical forests are changing, but the causes and consequences of these changes remain debated. To learn more about the forces driving changes within tropical forests, we investigated shifts in tree species composition over the past 25 years within the 50-ha Forest Dynamics Plot on Barro Colorado Island (BCI), Panama, and examined how observed patterns relate to predictions of (1) random population fluctuations, (2) carbon fertilization, (3) succession from past disturbance, (4) recovery from an extreme El Niño drought at the start of the study period, and (5) long-term climate change. We found that there have been consistent and directional changes in the tree species composition. These shifts have led to increased relative representations of drought-tolerant species as determined by the species' occurrence both across a gradient of soil moisture within BCI and across a wider precipitation gradient from a dry forest near the Pacific coast of Panama to a wet forest near its Caribbean coast. These nonrandom changes cannot be explained by stochastic fluctuations or carbon fertilization. They may be the legacy of the El Niño drought, or alternatively, potentially reflect increased aridity due to long-term climate change. By investigating compositional changes, we increased not only our understanding of the ecology of tropical forests and their responses to large-scale disturbances, but also our ability to predict how future global change will impact some of the critical services provided by these important ecosystems.
Ecology 04/2011; 92(4):871-82. · 4.85 Impact Factor
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S Joseph Wright,
Kaoru Kitajima,
Nathan J B Kraft,
Peter B Reich,
Ian J Wright,
Daniel E Bunker,
Richard Condit,
James W Dalling, Stuart J Davies,
Sandra Díaz,
Bettina M J Engelbrecht,
Kyle E Harms,
Stephen P Hubbell,
Christian O Marks,
Maria C Ruiz-Jaen,
Cristina M Salvador,
Amy E Zanne
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ABSTRACT: A trade-off between growth and mortality rates characterizes tree species in closed canopy forests. This trade-off is maintained by inherent differences among species and spatial variation in light availability caused by canopy-opening disturbances. We evaluated conditions under which the trade-off is expressed and relationships with four key functional traits for 103 tree species from Barro Colorado Island, Panama. The trade-off is strongest for saplings for growth rates of the fastest growing individuals and mortality rates of the slowest growing individuals (r2 = 0.69), intermediate for saplings for average growth rates and overall mortality rates (r2 = 0.46), and much weaker for large trees (r2 < or = 0.10). This parallels likely levels of spatial variation in light availability, which is greatest for fast- vs. slow-growing saplings and least for large trees with foliage in the forest canopy. Inherent attributes of species contributing to the trade-off include abilities to disperse, acquire resources, grow rapidly, and tolerate shade and other stresses. There is growing interest in the possibility that functional traits might provide insight into such ecological differences and a growing consensus that seed mass (SM), leaf mass per area (LMA), wood density (WD), and maximum height (H(max)) are key traits among forest trees. Seed mass, LMA, WD, and H(max) are predicted to be small for light-demanding species with rapid growth and mortality and large for shade-tolerant species with slow growth and mortality. Six of these trait-demographic rate predictions were realized for saplings; however, with the exception of WD, the relationships were weak (r2 < 0.1 for three and r2 < 0.2 for five of the six remaining relationships). The four traits together explained 43-44% of interspecific variation in species positions on the growth-mortality trade-off; however, WD alone accounted for > 80% of the explained variation and, after WD was included, LMA and H(max) made insignificant contributions. Virtually the full range of values of SM, LMA, and H(max) occurred at all positions on the growth-mortality trade-off. Although WD provides a promising start, a successful trait-based ecology of tropical forest trees will require consideration of additional traits.
Ecology 12/2010; 91(12):3664-74. · 4.85 Impact Factor
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ABSTRACT: • Premise of the study: Quantifying variation in functional traits associated with shifts in the species composition of plant communities along resource gradients helps identify environmental attributes important for community assembly. Stomates regulate the balance between carbon assimilation and water status in plants. If environmental attributes affecting photosynthetic water-use efficiency govern species distribution along an edaphic gradient, then adaptive variation in stomatal traits of plant species specializing on different soils should reflect belowground resource availability. • Methods: We tested this hypothesis by quantifying stomatal trait variation in understory saplings of 28 Bornean tree species in relation to gas exchange and water-use efficiency (WUE). • Key results: Comparisons between congeneric specialists of the more fertile, moister clay and the less fertile, well-drained sandy loam revealed little evidence of similar shifts in stomatal traits across genera, nor was stomatal pore index correlated with g(max), A(max), or WUE (A(max)/g(max) or Δ(13)C), suggesting that stomates may be overbuilt in these shaded juveniles. A(max) was higher on sandy loam, likely due to higher understory irradiance there, but there were no other significant differences in gas exchange or WUE. • Conclusions: Despite substantial diversity in stomatal anatomy, there were few strong relationships between stomatal, photosynthetic, and WUE traits in relation to soil resources. Routine differences in water availability therefore may not exert a dominant control on the distributions of these Bornean tree species. Furthermore, the clades represented by these 12 genera may possess alternative functional designs enabling photosynthetic WUE that is sufficient to these humid, understory environments, due to whole plant-functional integration of stomatal traits with other, unmeasured traits influencing gas exchange.
American Journal of Botany 07/2010; 97(7):1109-20. · 2.66 Impact Factor
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ABSTRACT: Drought is a critical factor in plant species distributions. Much research points to its relevance even in moist tropical regions. Recent studies have begun to elucidate mechanisms underlying the distributions of tropical tree species with respect to drought; however, how such desiccation tolerance mechanisms correspond with the coordination of hydraulic and photosynthetic traits in determining species distributions with respect to rainfall seasonality deserves attention. In the present study, we used a common garden approach to quantify inherent differences in wood anatomical and foliar physiological traits in 21 tropical tree species with either widespread (occupying both seasonal and aseasonal climates) or southern (restricted to aseasonal forests) distributions with respect to rainfall seasonality. Use of congeneric species pairs and phylogenetically independent contrast analyses allowed examination of this question in a phylogenetic framework. Widespread species opted for wood traits that provide biomechanical support and prevent xylem cavitation and showed associated reductions in canopy productivity and consequently growth rates compared with southern species. These data support the hypothesis that species having broader distributions with respect to climatic variability will be characterized by traits conducive to abiotic stress tolerance. This study highlights the importance of the well-established performance vs. stress tolerance trade-off as a contributor to species distributions at larger scales.
American Journal of Botany 12/2009; 96(12):2214-23. · 2.66 Impact Factor
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ABSTRACT: *Relatively little is known about diversity or structure of tropical ectomycorrhizal communities or their roles in tropical ecosystem dynamics. In this study, we present one of the largest molecular studies to date of an ectomycorrhizal community in lowland dipterocarp rainforest. *We sampled roots from two 0.4 ha sites located across an ecotone within a 52 ha forest dynamics plot. Our plots contained > 500 tree species and > 40 species of ectomycorrhizal host plants. Fungi were identified by sequencing ribosomal RNA genes. *The community was dominated by the Russulales (30 species), Boletales (17), Agaricales (18), Thelephorales (13) and Cantharellales (12). Total species richness appeared comparable to molecular studies of temperate forests. Community structure changed across the ecotone, although it was not possible to separate the role of environmental factors vs host plant preferences. Phylogenetic analyses were consistent with a model of community assembly where habitat associations are influenced by evolutionary conservatism of functional traits within ectomycorrhizal lineages. *Because changes in the ectomycorrhizal fungal community parallel those of the tree community at this site, this study demonstrates the potential link between the distribution of tropical tree diversity and the distribution of tropical ectomycorrhizal diversity in relation to local-scale edaphic variation.
New Phytologist 10/2009; 185(2):529-42. · 6.64 Impact Factor
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Functional Ecology 12/2008; 23(2):284 - 291. · 4.57 Impact Factor
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Jérôme Chave,
Richard Condit,
Helene C Muller-Landau,
Sean C Thomas,
Peter S Ashton,
Sarayudh Bunyavejchewin,
Leonardo L Co,
Handanakere S Dattaraja, Stuart J Davies,
Shameema Esufali, [......],
Hebbalalu S Suresh,
Sylvester Tan,
Jill Thompson,
Ma Dolores C Tongco,
Renato Valencia,
Martha Vallejo,
Gorky Villa,
Takuo Yamakura,
Jess K Zimmerman,
Elizabeth C Losos
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ABSTRACT: In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and in primary productivity, as well as shifts in plant species composition favouring fast-growing species over slow-growing ones. This pervasive alteration of mature tropical forests was attributed to global environmental change, such as an increase in atmospheric CO2 concentration, nutrient deposition, temperature, drought frequency, and/or irradiance. We used standardized, repeated measurements of over 2 million trees in ten large (16-52 ha each) forest plots on three continents to evaluate the generality of these findings across tropical forests. Aboveground biomass increased at seven of our ten plots, significantly so at four plots, and showed a large decrease at a single plot. Carbon accumulation pooled across sites was significant (+0.24 MgC ha(-1) y(-1), 95% confidence intervals [0.07, 0.39] MgC ha(-1) y(-1)), but lower than reported previously for Amazonia. At three sites for which we had data for multiple census intervals, we found no concerted increase in biomass gain, in conflict with the increased productivity hypothesis. Over all ten plots, the fastest-growing quartile of species gained biomass (+0.33 [0.09, 0.55] % y(-1)) compared with the tree community as a whole (+0.15 % y(-1)); however, this significant trend was due to a single plot. Biomass of slow-growing species increased significantly when calculated over all plots (+0.21 [0.02, 0.37] % y(-1)), and in half of our plots when calculated individually. Our results do not support the hypothesis that fast-growing species are consistently increasing in dominance in tropical tree communities. Instead, they suggest that our plots may be simultaneously recovering from past disturbances and affected by changes in resource availability. More long-term studies are necessary to clarify the contribution of global change to the functioning of tropical forests.
PLoS Biology 04/2008; 6(3):e45. · 11.45 Impact Factor
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Journal of Ecology 12/2007; 96(1):192 - 203. · 4.69 Impact Factor
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ABSTRACT: The responses of tropical forests to global anthropogenic disturbances remain poorly understood. Above-ground woody biomass in some tropical forest plots has increased over the past several decades, potentially reflecting a widespread response to increased resource availability, for example, due to elevated atmospheric CO2 and/or nutrient deposition. However, previous studies of biomass dynamics have not accounted for natural patterns of disturbance and gap phase regeneration, making it difficult to quantify the importance of environmental changes. Using spatially explicit census data from large (50 ha) inventory plots, we investigated the influence of gap phase processes on the biomass dynamics of four 'old-growth' tropical forests (Barro Colorado Island (BCI), Panama; Pasoh and Lambir, Malaysia; and Huai Kha Khaeng (HKK), Thailand). We show that biomass increases were gradual and concentrated in earlier-phase forest patches, while biomass losses were generally of greater magnitude but concentrated in rarer later-phase patches. We then estimate the rate of biomass change at each site independent of gap phase dynamics using reduced major axis regressions and ANCOVA tests. Above-ground woody biomass increased significantly at Pasoh (+0.72% yr(-1)) and decreased at HKK (-0.56% yr(-1)) independent of changes in gap phase but remained stable at both BCI and Lambir. We conclude that gap phase processes play an important role in the biomass dynamics of tropical forests, and that quantifying the role of gap phase processes will help improve our understanding of the factors driving changes in forest biomass as well as their place in the global carbon budget.
Proceedings of the Royal Society B: Biological Sciences 12/2007; 274(1627):2857-64. · 5.41 Impact Factor
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ABSTRACT: The impacts of global change on tropical forests remain poorly understood. We examined changes in tree growth rates over the past two decades for all species occurring in large (50-ha) forest dynamics plots in Panama and Malaysia. Stem growth rates declined significantly at both forests regardless of initial size or organizational level (species, community or stand). Decreasing growth rates were widespread, occurring in 24-71% of species at Barro Colorado Island, Panama (BCI) and in 58-95% of species at Pasoh, Malaysia (depending on the sizes of stems included). Changes in growth were not consistently associated with initial growth rate, adult stature, or wood density. Changes in growth were significantly associated with regional climate changes: at both sites growth was negatively correlated with annual mean daily minimum temperatures, and at BCI growth was positively correlated with annual precipitation and number of rainfree days (a measure of relative insolation). While the underlying cause(s) of decelerating growth is still unresolved, these patterns strongly contradict the hypothesized pantropical increase in tree growth rates caused by carbon fertilization. Decelerating tree growth will have important economic and environmental implications.
Ecology Letters 07/2007; 10(6):461-9. · 17.56 Impact Factor
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ABSTRACT: It is critical to understand the responses of tropical tree species to ongoing anthropogenic disturbances. Given the longevity of large trees and the scarcity of appropriately long-term demographic data, standing size distributions are a potential tool for predicting species' responses to disturbances and resultant changes in population structure. Here we test the utility of several different measures of size distribution for predicting subsequent population changes at the intraspecific level using demographic records from two subsampled 50-ha tree plots in Malaysia (Pasoh and Lambir). Most measures of size distribution failed to successfully predict population change better than random; however, the ‘coefficient of skewness’ (a measure of the relative proportion of small vs. large stems in a population) was able to correctly predict the direction of population change for approximately three-quarters of species at both sites. At Pasoh, the magnitude of this relationship decreased with adult stature and rate of turnover, but was unrelated to sapling growth rates at either site. Finally, using data for species common at both forests, we found that size distributions were generally uninformative of subsequent differences in population change between sites (only median dbh correctly predicted the direction of change for more species than random). Based on these results we conclude that some measures of intraspecific differences in size distribution are potentially informative of population trends within forests but have limited utility across broader spatial scales.
Journal of Tropical Ecology 02/2007; 23(02):191 - 198. · 1.40 Impact Factor
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James V LaFrankie,
Peter S Ashton,
George B Chuyong,
Leonardo Co,
Richard Condit, Stuart J Davies,
Robin Foster,
Stephen P Hubbell,
David Kenfack,
Daniel Lagunzad,
Elizabeth C Losos,
Noor Supardi Md Nor,
Sylvester Tan,
Duncan W Thomas,
Renato Valencia,
Gorky Villa
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ABSTRACT: In large samples of trees > or = 1 cm dbh (more than 1 million trees and 3000 species), in six lowland tropical forests on three continents, we assigned species with >30 individuals to one of six classes of stature at maturity (SAM). We then compared the proportional representation of understory trees (1-2 cm dbh) among these classes. The understory of the three Asian sites was predominantly composed of the saplings of large-canopy trees whereas the African and American sites were more richly stocked with trees of the smaller SAM classes. Differences in class representation were related to taxonomic families that were present exclusively in one continent or another. Families found in the Asian plots but not in the American plot (e.g., Dipterocarpaceae, Fagaceae) were predominantly species of the largest SAM classes, whereas families exclusive to the American plots (e.g., Melastomataceae sensu stricto, Piperaceae, and Malvaceae [Bombacacoidea]) were predominantly species of small classes. The African plot was similar to Asia in the absence of those American families rich in understory species, while similar to America in lacking the Asian families rich in canopy species. The numerous understory species of Africa were chiefly derived from families shared with Asia and/or America. The ratio of saplings (1-2 cm dbh) to conspecific canopy trees (>40 cm dbh) was lower in American plots than in the Asian plots. Possible explanations for these differences include phenology, moisture and soil fertility regimes, phyletic constraints, and the role of early successional plants in forest development. These results demonstrate that tropical forests that appear similar in tree number, basal area, and the family taxonomy of canopy trees nonetheless differ in ecological structure in ways that may impact the ecology of pollinators, dispersers, and herbivores and might reflect fundamental differences in canopy tree regeneration.
Ecology 10/2006; 87(9):2298-305. · 4.85 Impact Factor
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Journal of Ecology 03/2006; 94(3):670 - 680. · 4.69 Impact Factor
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Journal of Ecology 06/2005; 93(5):879 - 889. · 4.69 Impact Factor
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ABSTRACT: Dispersal-assembly theories of species coexistence posit that environmental factors play no role in explaining community diversity and structure. Dispersal-assembly theories shed light on some aspects of community structure such as species-area and species-abundance relationships. However, species' environmental associations also affect these measures of community structure. Measurements of species' niche breadth and overlap address this influence. Using a new continuous measure of niche and a dispersal-assembly null model that maintains species' niche breadth and aggregation, we tested two hypotheses assessing the effects of habitat heterogeneity on the ability of dispersal-assembly theories to explain community niche structure. We found that in both homogenous and heterogeneous environments dispersal-assembly theories cannot fully explain observed niche structure. The performance of the dispersal-assembly null models was particularly poor in heterogeneous environments. These results indicate that non-dispersal based mechanisms are in part responsible for observed community structure and measures of community structure which include species' environmental associations should be used to test theories of species diversity.
Oecologia 06/2004; 139(3):446-53. · 3.41 Impact Factor
