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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: Summary1. Nutrients are a critical resource for plant growth, but the elements limiting growth in tropical forests have rarely been determined.2. We investigated the influence of nitrogen (N), phosphorus (P), potassium (K) and micronutrients on seedling biomass and nutrient allocation in a factorial nutrient fertilization experiment in lowland tropical forest at the Barro Colorado Nature Monument, Panama. We also measured 8 years of herbivory and growth for 1800 seedlings. We sought to determine the identity of limiting elements and possible nutrient interactions.3. The five study species were Alseis blackiana, Desmopsis panamensis, Heisteria concinna, Sorocea affinis and Tetragastris panamensis. Plants grew in deeply shaded understorey with a mean canopy openness of 4.9% (±0.7%; 1 SE).4. Tissue N concentration increased by 11% with N addition. Tissue P concentration increased by 16% with P addition. Tissue K increased by 4% with K addition. K addition reduced root-to-shoot biomass ratio. There was no significant effect of fertilization on specific leaf area or leaf area ratio.5. The proportion of leaves damaged and the mean level of damage by herbivory increased with P and K addition and showed a significant P × K interaction.6.7ensp;Across all species and years, relative growth rate of height increased with K addition and with N and P in combination. Relative growth rate of leaf count trended 8.5% higher with K addition (P = 0.076).7. We also added micronutrients in a parallel experiment. There was no effect of micronutrient addition on any seedling parameter.8. Synthesis. K addition affected seedlings by enhancing tissue nutrient concentration, increasing herbivory, reducing root-to-shoot biomass ratio and increasing height growth. Additional effects of N or P on tissue chemistry, herbivory and growth offer support for the multiple limiting resources hypothesis. Our results suggest that seedling growth is limited by nutrients, especially K, even under highly shaded conditions in this lowland tropical forest.
Journal of Ecology 02/2012; 100(2):309 - 316. · 4.69 Impact Factor
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Cory C Cleveland,
Alan R Townsend,
Philip Taylor,
Silvia Alvarez-Clare,
Mercedes M C Bustamante,
George Chuyong,
Solomon Z Dobrowski,
Pauline Grierson, Kyle E Harms,
Benjamin Z Houlton,
Alison Marklein,
William Parton,
Stephen Porder,
Sasha C Reed,
Carlos A Sierra,
Whendee L Silver,
Edmund V J Tanner,
William R Wieder
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ABSTRACT: Tropical rain forests play a dominant role in global biosphere-atmosphere CO(2) exchange. Although climate and nutrient availability regulate net primary production (NPP) and decomposition in all terrestrial ecosystems, the nature and extent of such controls in tropical forests remain poorly resolved. We conducted a meta-analysis of carbon-nutrient-climate relationships in 113 sites across the tropical forest biome. Our analyses showed that mean annual temperature was the strongest predictor of aboveground NPP (ANPP) across all tropical forests, but this relationship was driven by distinct temperature differences between upland and lowland forests. Within lowland forests (< 1000 m), a regression tree analysis revealed that foliar and soil-based measurements of phosphorus (P) were the only variables that explained a significant proportion of the variation in ANPP, although the relationships were weak. However, foliar P, foliar nitrogen (N), litter decomposition rate (k), soil N and soil respiration were all directly related with total surface (0-10 cm) soil P concentrations. Our analysis provides some evidence that P availability regulates NPP and other ecosystem processes in lowland tropical forests, but more importantly, underscores the need for a series of large-scale nutrient manipulations - especially in lowland forests - to elucidate the most important nutrient interactions and controls.
Ecology Letters 09/2011; 14(9):939-47. · 17.56 Impact Factor
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S Joseph Wright,
Joseph B Yavitt,
Nina Wurzburger,
Benjamin L Turner,
Edmund V J Tanner,
Emma J Sayer,
Louis S Santiago,
Michael Kaspari,
Lars O Hedin, Kyle E Harms,
Milton N Garcia,
Marife D Corre
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ABSTRACT: We maintained a factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment for 11 years in a humid lowland forest growing on a relatively fertile soil in Panama to evaluate potential nutrient limitation of tree growth rates, fine-litter production, and fine-root biomass. We replicated the eight factorial treatments four times using 32 plots of 40 x 40 m each. The addition of K was associated with significant decreases in stand-level fine-root biomass and, in a companion study of seedlings, decreases in allocation to roots and increases in height growth rates. The addition of K and N together was associated with significant increases in growth rates of saplings and poles (1-10 cm in diameter at breast height) and a further marginally significant decrease in stand-level fine-root biomass. The addition of P was associated with a marginally significant (P = 0.058) increase in fine-litter production that was consistent across all litter fractions. Our experiment provides evidence that N, P, and K all limit forest plants growing on a relatively fertile soil in the lowland tropics, with the strongest evidence for limitation by K among seedlings, saplings, and poles.
Ecology 08/2011; 92(8):1616-25. · 4.85 Impact Factor
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George B Chuyong,
David Kenfack, Kyle E Harms,
Duncan W Thomas,
Richard Condit,
Liza S Comita,
G B Chuyong,
D Kenfack,
K E Harms,
D W Thomas,
L S Comita
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ABSTRACT: Niche differentiation with respect to hab-itat has been hypothesized to shape patterns of diversity and species distributions in plant communi-ties. African forests have been reported to be relatively less diverse compared to highly diversed regions of the Amazonian or Southeast Asian forests, and might be expected to have less niche differentiation. We exam-ined patterns of structural and floristic differences among five topographically defined habitats for 494 species with stems C1 cm dbh in a 50-ha plot in Korup National Park, Cameroon. In addition, we tested for species–habitat associations for 272 species (with more than 50 individuals in the plot) using Torus translation randomization tests. Tree density and basal area were lowest in areas with negative convexity, which contained streams or were inundated during rainy periods and highest in moist well-drained hab-itats. Species composition and diversity varied along the topographical gradient from low flat to ridge top habitats. The low depression and low flat habitats were characterized by high diversity and similar species composition, relative to slopes, high gullies and ridge tops. Sixty-three percent of the species evaluated showed significant positive associations with at least one of the five habitat types. The majority of associ-ations were with low depressions (75 species) and the fewest with ridge tops (8 species). The large number of species–habitat associations and the pronounced con-trast between low (valley) and elevated (ridgetop) habitats in the Korup plot shows that niche differen-tiation with respect to edaphic variables (e.g., soil moisture, nutrients) contributes to local scale tree species distributions and to the maintenance of diver-sity in African forests.
Plant Ecology 01/2011; 212:1363-1374. · 1.83 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: The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m−2 year−1), phosphorus (P as superphosphate, 5 g P m−2 year−1) and potassium (K as KCl, 5 g K m−2 year−1) within 38 ha of old-growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images. We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg−1), low concentrations of Bray-extractable phosphate (PO4 = 2.2 mg kg−1), and modest concentrations of KCl-extractable nitrate (NO3 = 5.0 mg kg−1) and KCl-extractable ammonium (NH4 = 15.5 mg kg−1). Added N increased concentrations of KCl-extractable NO3 and acidified the soil by one pH unit. Added P increased concentrations of Bray-extractable PO4 and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m−2. The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species-rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study.
Austral Ecology 06/2010; 36(4):433 - 445. · 1.82 Impact Factor
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ABSTRACT: To explore the importance of 12 elements in litter production and decomposition, we fertilized 36 1600 m(2)-plots with combinations of N, P, K, or micronutrients (i.e. B, Ca, Cu, Fe, Mg, Mn, Mo, S, Zn) for 6 years in a lowland Panamanian forest. The 90% of litter falling as leaves and twigs failed to increase with fertilization, but reproductive litter (fruits and flowers) increased by 43% with N. K enhanced cellulose decomposition; one or more micronutrients enhanced leaf-litter decomposition; P enhanced both. Our results suggest tropical forests are a non-Liebig world of multiple nutrient limitations, with at least four elements shaping rates of litterfall and decomposition. Multiple metallomic enzymes and cofactors likely create gradients in the break down of leaf litter. Selection favours individuals that make more propagules, and even in an N-rich forest, N is a non-substitutable resource for reproduction.
Ecology Letters 02/2008; 11(1):35-43. · 17.56 Impact Factor
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Ian J Wright,
David D Ackerly,
Frans Bongers, Kyle E Harms,
Guillermo Ibarra-Manriquez,
Miguel Martinez-Ramos,
Susan J Mazer,
Helene C Muller-Landau,
Horacio Paz,
Nigel C A Pitman,
Lourens Poorter,
Miles R Silman,
Corine F Vriesendorp,
Cam O Webb,
Mark Westoby,
S Joseph Wright
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ABSTRACT: When ecologically important plant traits are correlated they may be said to constitute an ecological 'strategy' dimension. Through identifying these dimensions and understanding their inter-relationships we gain insight into why particular trait combinations are favoured over others and into the implications of trait differences among species. Here we investigated relationships among several traits, and thus the strategy dimensions they represented, across 2134 woody species from seven Neotropical forests.
Six traits were studied: specific leaf area (SLA), the average size of leaves, seed and fruit, typical maximum plant height, and wood density (WD). Trait relationships were quantified across species at each individual forest as well as across the dataset as a whole. 'Phylogenetic' analyses were used to test for correlations among evolutionary trait-divergences and to ascertain whether interspecific relationships were biased by strong taxonomic patterning in the traits.
The interspecific and phylogenetic analyses yielded congruent results. Seed and fruit size were expected, and confirmed, to be tightly related. As expected, plant height was correlated with each of seed and fruit size, albeit weakly. Weak support was found for an expected positive relationship between leaf and fruit size. The prediction that SLA and WD would be negatively correlated was not supported. Otherwise the traits were predicted to be largely unrelated, being representatives of putatively independent strategy dimensions. This was indeed the case, although WD was consistently, negatively related to leaf size.
The dimensions represented by SLA, seed/fruit size and leaf size were essentially independent and thus conveyed largely independent information about plant strategies. To a lesser extent the same was true for plant height and WD. Our tentative explanation for negative WD-leaf size relationships, now also known from other habitats, is that the traits are indirectly linked via plant hydraulics.
Annals of Botany 06/2007; 99(5):1003-15. · 4.03 Impact Factor
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ABSTRACT: Edge effects are major drivers of change in many fragmented landscapes, but are often highly variable in space and time. Here we assess variability in edge effects altering Amazon forest dynamics, plant community composition, invading species, and carbon storage, in the world's largest and longest-running experimental study of habitat fragmentation. Despite detailed knowledge of local landscape conditions, spatial variability in edge effects was only partially foreseeable: relatively predictable effects were caused by the differing proximity of plots to forest edge and varying matrix vegetation, but windstorms generated much random variability. Temporal variability in edge phenomena was also only partially predictable: forest dynamics varied somewhat with fragment age, but also fluctuated markedly over time, evidently because of sporadic droughts and windstorms. Given the acute sensitivity of habitat fragments to local landscape and weather dynamics, we predict that fragments within the same landscape will tend to converge in species composition, whereas those in different landscapes will diverge in composition. This 'landscape-divergence hypothesis', if generally valid, will have key implications for biodiversity-conservation strategies and for understanding the dynamics of fragmented ecosystems.
PLoS ONE 02/2007; 2(10):e1017. · 4.09 Impact Factor
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Robert John,
James W Dalling, Kyle E Harms,
Joseph B Yavitt,
Robert F Stallard,
Matthew Mirabello,
Stephen P Hubbell,
Renato Valencia,
Hugo Navarrete,
Martha Vallejo,
Robin B Foster
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ABSTRACT: The importance of niche vs. neutral assembly mechanisms in structuring tropical tree communities remains an important unsettled question in community ecology [Bell G (2005) Ecology 86:1757-1770]. There is ample evidence that species distributions are determined by soils and habitat factors at landscape (<10(4) km(2)) and regional scales. At local scales (<1 km(2)), however, habitat factors and species distributions show comparable spatial aggregation, making it difficult to disentangle the importance of niche and dispersal processes. In this article, we test soil resource-based niche assembly at a local scale, using species and soil nutrient distributions obtained at high spatial resolution in three diverse neotropical forest plots in Colombia (La Planada), Ecuador (Yasuni), and Panama (Barro Colorado Island). Using spatial distribution maps of >0.5 million individual trees of 1,400 species and 10 essential plant nutrients, we used Monte Carlo simulations of species distributions to test plant-soil associations against null expectations based on dispersal assembly. We found that the spatial distributions of 36-51% of tree species at these sites show strong associations to soil nutrient distributions. Neutral dispersal assembly cannot account for these plant-soil associations or the observed niche breadths of these species. These results indicate that belowground resource availability plays an important role in the assembly of tropical tree communities at local scales and provide the basis for future investigations on the mechanisms of resource competition among tropical tree species.
Proceedings of the National Academy of Sciences 01/2007; 104(3):864-9. · 9.68 Impact Factor
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William F Laurance,
Henrique E M Nascimento,
Susan G Laurance,
Ana Andrade,
José E L S Ribeiro,
Juan Pablo Giraldo,
Thomas E Lovejoy,
Richard Condit,
Jerome Chave, Kyle E Harms,
Sammya D'Angelo
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ABSTRACT: Forest fragmentation is considered a greater threat to vertebrates than to tree communities because individual trees are typically long-lived and require only small areas for survival. Here we show that forest fragmentation provokes surprisingly rapid and profound alterations in Amazonian tree-community composition. Results were derived from a 22-year study of exceptionally diverse tree communities in 40 1-ha plots in fragmented and intact forests, which were sampled repeatedly before and after fragment isolation. Within these plots, trajectories of change in abundance were assessed for 267 genera and 1,162 tree species. Abrupt shifts in floristic composition were driven by sharply accelerated tree mortality and recruitment within approximately 100 m of fragment margins, causing rapid species turnover and population declines or local extinctions of many large-seeded, slow-growing, and old-growth taxa; a striking increase in a smaller set of disturbance-adapted and abiotically dispersed species; and significant shifts in tree size distributions. Even among old-growth trees, species composition in fragments is being restructured substantially, with subcanopy species that rely on animal seed-dispersers and have obligate outbreeding being the most strongly disadvantaged. These diverse changes in tree communities are likely to have wide-ranging impacts on forest architecture, canopy-gap dynamics, plant-animal interactions, and forest carbon storage.
Proceedings of the National Academy of Sciences 01/2007; 103(50):19010-4. · 9.68 Impact Factor
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Richard Condit,
Peter Ashton,
Sarayudh Bunyavejchewin,
H S Dattaraja,
Stuart Davies,
Shameema Esufali,
Corneille Ewango,
Robin Foster,
I A U N Gunatilleke,
C V S Gunatilleke, [......],
Sabrina Russo,
Raman Sukumar,
Cristián Samper,
Hebbalalu S Suresh,
Sylvester Tan,
Sean Thomas,
Renato Valencia,
Martha Vallejo,
Gorky Villa,
Tommaso Zillio
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ABSTRACT: Most ecological hypotheses about species coexistence hinge on species differences, but quantifying trait differences across species in diverse communities is often unfeasible. We examined the variation of demographic traits using a global tropical forest data set covering 4500 species in 10 large-scale tree inventories. With a hierarchical Bayesian approach, we quantified the distribution of mortality and growth rates of all tree species at each site. This allowed us to test the prediction that demographic differences facilitate species richness, as suggested by the theory that a tradeoff between high growth and high survival allows species to coexist. Contrary to the prediction, the most diverse forests had the least demographic variation. Although demographic differences may foster coexistence, they do not explain any of the 16-fold variation in tree species richness observed across the tropics.
Science 08/2006; 313(5783):98-101. · 31.20 Impact Factor
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Helene C Muller-Landau,
Richard S Condit, Kyle E Harms,
Christian O Marks,
Sean C Thomas,
Sarayudh Bunyavejchewin,
George Chuyong,
Leonardo Co,
Stuart Davies,
Robin Foster, [......],
Jess K Zimmerman,
Handanakere Shavaramaiah Dattaraja,
Shameema Esufali,
Pamela Hall,
Fangliang He,
Consuelo Hernandez,
Somboon Kiratiprayoon,
Hebbalalu S Suresh,
Christopher Wills,
Peter Ashton
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ABSTRACT: Tropical forests vary substantially in the densities of trees of different sizes and thus in above-ground biomass and carbon stores. However, these tree size distributions show fundamental similarities suggestive of underlying general principles. The theory of metabolic ecology predicts that tree abundances will scale as the -2 power of diameter. Demographic equilibrium theory explains tree abundances in terms of the scaling of growth and mortality. We use demographic equilibrium theory to derive analytic predictions for tree size distributions corresponding to different growth and mortality functions. We test both sets of predictions using data from 14 large-scale tropical forest plots encompassing censuses of 473 ha and > 2 million trees. The data are uniformly inconsistent with the predictions of metabolic ecology. In most forests, size distributions are much closer to the predictions of demographic equilibrium, and thus, intersite variation in size distributions is explained partly by intersite variation in growth and mortality.
Ecology Letters 06/2006; 9(5):589-602. · 17.56 Impact Factor
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Helene C. Muller-Landau,
Richard S. Condit,
Jerome Chave,
Sean C. Thomas,
Stephanie A. Bohlman,
Sarayudh Bunyavejchewin,
Stuart Davies,
Robin Foster,
Savitri Gunatilleke,
Nimal Gunatilleke, [......],
Handanakere Shivaramaiah Dattaraja,
Shameema Esufali,
Pamela Hall,
Consuelo Hernandez,
David Kenfack,
Somboon Kiratiprayoon,
Hebbalalu S. Suresh,
Duncan Thomas,
Martha Isabel Vallejo,
Peter Ashton
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ABSTRACT: The theory of metabolic ecology predicts specific relationships among tree stem diameter, biomass, height, growth and mortality. As demographic rates are important to estimates of carbon fluxes in forests, this theory might offer important insights into the global carbon budget, and deserves careful assessment. We assembled data from 10 old-growth tropical forests encompassing censuses of 367 ha and > 1.7 million trees to test the theory's predictions. We also developed a set of alternative predictions that retained some assumptions of metabolic ecology while also considering how availability of a key limiting resource, light, changes with tree size. Our results show that there are no universal scaling relationships of growth or mortality with size among trees in tropical forests. Observed patterns were consistent with our alternative model in the one site where we had the data necessary to evaluate it, and were inconsistent with the predictions of metabolic ecology in all forests.
Ecology Letters 04/2006; 9(5):575 - 588. · 17.56 Impact Factor
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Biotropica 03/2006; 29(2):234 - 237. · 2.23 Impact Factor
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Christopher Wills, Kyle E Harms,
Richard Condit,
David King,
Jill Thompson,
Fangliang He,
Helene C Muller-Landau,
Peter Ashton,
Elizabeth Losos,
Liza Comita, [......],
Marie Massa,
Cheryl Nath,
Md Nur Supardi Noor,
Abdul Rahman Kassim,
Raman Sukumar,
Hebbalalu Satyanarayana Suresh,
I-Fang Sun,
Sylvester Tan,
Takuo Yamakura,
Jess Zimmerman
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ABSTRACT: An ecological community's species diversity tends to erode through time as a result of stochastic extinction, competitive exclusion, and unstable host-enemy dynamics. This erosion of diversity can be prevented over the short term if recruits are highly diverse as a result of preferential recruitment of rare species or, alternatively, if rare species survive preferentially, which increases diversity as the ages of the individuals increase. Here, we present census data from seven New and Old World tropical forest dynamics plots that all show the latter pattern. Within local areas, the trees that survived were as a group more diverse than those that were recruited or those that died. The larger (and therefore on average older) survivors were more diverse within local areas than the smaller survivors. When species were rare in a local area, they had a higher survival rate than when they were common, resulting in enrichment for rare species and increasing diversity with age and size class in these complex ecosystems.
Science 02/2006; 311(5760):527-31. · 31.20 Impact Factor
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Journal of Ecology 01/2004; 89(6):947 - 959. · 4.69 Impact Factor
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ABSTRACT: We present an analysis of the long-term survival of two cohorts of seedlings of the tropical canopy tree Ocotea whitei (Lauraceae) on a 1-ha plot of mature, lowland moist forest on Barro Colorado Island, Panamá. In 1980, we counted an even-aged cohort of seedlings that germinated in 1979, then measured and tagged survivors in 1981. We also measured and tagged a second, smaller cohort of seedlings that germinated in 1981. We followed the subsequent survival of all seedlings through 1985. Seedling mortality was phenotypically, temporally,
and spatially non-random. Important correlates of nonrandom mortality included: (1) seedling size and age, (2) an El Niño drought, and (3) biotic neighborhood. Larger and older seedlings survived better than smaller and younger seedlings, respectively, and the El Niño-related drought of 1982–1983 was associated with elevated mortality rates. Seedling density, which was strongly correlated with the proximity to the nearest conspecific adult, increased mortality. The observed mortality patterns suggest that processes consistent with the Janzen-Connell hypothesis operate during the recruitment phase of O. whitei population dynamics. However, the processes causing the observed density- and distance-dependent mortality may vary with factors such as total seed number, seedling size, and climatic variation, making it difficult to determine whether time-integrated seedling-to-adult spacing mechanisms other than self-thinning operate on a given plant population. After 6 years in the hectare studied, survivors remained densest and most numerous underneath the adult trees. We conclude that only long-term demographic data, collected at a variety of scales on a variety of species, will ultimately answer the question: do Janzen-Connell effects contribute substantially to structuring tropical forests?
01/2001; 127:509–516.
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Journal of Ecology. 01/2001; 89(6):947-959.
