The assertion that the spatial location of different species is independent of each other is fundamental in major ecological theories such as neutral theory that describes a stochastic geometry of ...biodiversity. However, this assertion has rarely been tested. Here we use techniques of spatial point pattern analysis to conduct a comprehensive test of the independence assertion by analysing data from three large forest plots with different species richness: a species-rich tropical forest at Barro Colorado Island (Panama), a tropical forest in Sinharaja (Sri Lanka), and a temperate forest in Changbaishan (China). We hypothesize that stochastic dilution effects owing to increasing species richness overpower signals of species associations, thereby yielding approximate species independence. Indeed, the proportion of species pairs showing: (i) no significant interspecific association increased with species richness, (ii) segregation decreased with species richness, and (iii) small-scale interspecific interaction decreased with species richness. This suggests that independence may indeed be a good approximation in the limit of very species-rich communities. Our findings are a step towards a better understanding of factors governing species-rich communities and we propose a hypothesis to explain why species placement in species-rich communities approximates independence.
Interactions among neighboring individuals influence plant performance and should create spatial patterns in local community structure. In order to assess the role of large trees in generating ...spatial patterns in local species richness, we used the individual species-area relationship (ISAR) to evaluate the species richness of trees of different size classes (and dead trees) in circular neighborhoods with varying radius around large trees of different focal species. To reveal signals of species interactions, we compared the ISAR function of the individuals of focal species with that of randomly selected nearby locations. We expected that large trees should strongly affect the community structure of smaller trees in their neighborhood, but that these effects should fade away with increasing size class. Unexpectedly, we found that only few focal species showed signals of species interactions with trees of the different size classes and that this was less likely for less abundant focal species. However, the few and relatively weak departures from independence were consistent with expectations of the effect of competition for space and the dispersal syndrome on spatial patterns. A noisy signal of competition for space found for large trees built up gradually with increasing life stage; it was not yet present for large saplings but detectable for intermediates. Additionally, focal species with animal-dispersed seeds showed higher species richness in their neighborhood than those with gravity- and gyration-dispersed seeds. Our analysis across the entire ontogeny from recruits to large trees supports the hypothesis that stochastic effects dilute deterministic species interactions in highly diverse communities. Stochastic dilution is a consequence of the stochastic geometry of biodiversity in species-rich communities where the identities of the nearest neighbors of a given plant are largely unpredictable. While the outcome of local species interactions is governed for each plant by deterministic fitness and niche differences, the large variability of competitors causes also a large variability in the outcomes of interactions and does not allow for strong directed responses at the species level. Collectively, our results highlight the critical effect of the stochastic geometry of biodiversity in structuring local spatial patterns of tropical forest diversity.
Niche and neutral theories emphasize different processes that contribute to the maintenance of species diversity and should leave different spatial structures in species assemblages. In this study we ...used variation partitioning in combination with distance-based Moran's eigenvector maps and habitat variables to determine the relative importance of the effects of pure habitat, pure spatial, and spatially structured habitat processes on the spatial distribution of tree species composition and richness in a 25-ha tropical rain forest of Sinharaja/Sri Lanka. We analyzed the contribution of those components at three spatial scales (10 m, 20 m, and 50 m) for all trees and the three life stages: recruits, juveniles, and adults. At the 10-m scale, 80% of the variation in species composition remained unexplained for recruits and adults, but only 55% for juveniles. With increasingly broader scales these figures were strongly reduced, mainly by an increasing contribution of the spatially structured habitat component, which explained 4-30%, 20-47%, and 8-35% of variation in species composition for recruits, juveniles, and adults, respectively. The pure spatial component was most important at the 20-m scale and reached 20%, 32%, and 23% for recruits, juveniles, and adults, respectively. The spatially structured habitat component described variability at broader scales than the pure spatial component. Our results suggest that stochastic processes and spatially structuring processes of community dynamics, such as dispersal limitation and habitat association, contributed jointly to explain species composition and richness at the Sinharaja forest, but their relative importance changed with scale and life stage. Species assembly at the local scale was more strongly impacted by stochasticity, whereas the signal of habitat was stronger at the 50-m scale where plant-scale stochasticity is averaged out. Recent research points to an emerging consensus on the relative contribution of stochasticity, habitat, and spatial processes in governing community assembly, but how these components change with life stage, and how this is influenced by sample size, remains to be explored.
Studies of forest dynamics plots (FDPs) have revealed a variety of negative density-dependent (NDD) demographic interactions, especially among conspecific trees. These interactions can affect growth ...rate, recruitment and mortality, and they play a central role in the maintenance of species diversity in these complex ecosystems. Here we use an equal area annulus (EAA) point-pattern method to comprehensively analyze data from two tropical FDPs, Barro Colorado Island in Panama and Sinharaja in Sri Lanka. We show that these NDD interactions also influence the continued evolutionary diversification of even distantly related tree species in these FDPs. We examine the details of a wide range of these interactions between individual trees and the trees that surround them. All these interactions, and their cumulative effects, are strongest among conspecific focal and surrounding tree species in both FDPs. They diminish in magnitude with increasing phylogenetic distance between heterospecific focal and surrounding trees, but do not disappear or change the pattern of their dependence on size, density, frequency or physical distance even among the most distantly related trees. The phylogenetic persistence of all these effects provides evidence that interactions between tree species that share an ecosystem may continue to promote adaptive divergence even after the species' gene pools have become separated. Adaptive divergence among taxa would operate in stark contrast to an alternative possibility that has previously been suggested, that distantly related species with dispersal-limited distributions and confronted with unpredictable neighbors will tend to converge on common strategies of resource use. In addition, we have also uncovered a positive density-dependent effect: growth rates of large trees are boosted in the presence of a smaller basal area of surrounding trees. We also show that many of the NDD interactions switch sign rapidly as focal trees grow in size, and that their cumulative effect can strongly influence the distributions and species composition of the trees that surround the focal trees during the focal trees' lifetimes.
The purpose of our study was to examine whether the degree of ectomycorrhizal (EM) colonization was associated with amount of shade for potted seedlings of five rain forest tree species. Seedlings ...were exposed to a range of shade treatments—from the open to that emulating the degree of shade beneath a deep-canopied forest. The experiment was carried out at the field station (580 m.a.s.l) of the Sinharaja World Heritage Site in southwestern Sri Lanka. We selected five species for the study:
Shorea affinis (Thw.) Ashton,
S. congestiflora (Thw.) Ashton,
S. cordifolia (Thw.) Ashton,
S. gardneri (Thw.) Ashton, and
S. zeylanica (Thw.) Ashton, all members of a sympatric clade of endemic canopy trees in the family dipterocarpaceae. Results show that the percentage EM colonization was significantly different among shade treatments, for all five species (
p < 0.05). However, EM root colonization was not significantly different among species. Highest percentages of EM colonization for all species, except for
S. congestiflora, were in treatments providing full open conditions. However, seedling growth of all species were best under partial shade conditions (
p < 0.05), intermediate between the deep shade of a forest understory and no shade of open environments. The non-linear relationship between seedling growth performance and EM colonization is discussed.
Our study tested the potential for establishing shade-tolerant tree species within different canopy removal treatments of an 18-year-old
Pinus caribaea (Caribbean pine) plantation. We investigated ...whether planting within a
Pinus plantation can be a solution to the dispersal, weed competition, and pathogen/insect problems rain forest tree species have during their initial establishment on sites previously cleared of forest. The plantation was originally established on abandoned swidden adjacent to the Sinharaja Man and the Biosphere rain forest reserve in southwestern Sri Lanka. The five species selected for the study were
Caryota urens,
Dipterocarpus zeylanicus,
Pericopsis mooniana,
Shorea stipularis, and
Swietenia macrophylla. Seedlings were monitored over two years for survival, root collar diameter and height growth along transects that were in seven different environmental treatments within the
Pinus plantation. Environmental treatments comprised: (i) under plantings beneath a closed canopy of
Pinus that were unaffected by any row removal; (ii) the centres of clearcut strips of
Pinus that were 6 m width; (iii–vii) and five treatments across clearcut strips within closed canopy
Pinus that were 12 m wide. The five treatments within the 12 m wide clearcut strip included inside edges beneath the
Pinus canopy on the eastern (iii) and western (iv) sides of the strip; outside edges on the eastern (v) and western (vi) sides of the strip; and the centre (vii) of the strip opening. At the end of two years, a sample of the seedlings growing within each environmental treatment were taken and measured for dry masses and leaf areas. Results demonstrate that all species grew poorly and had higher mortality in the
Pinus understorey than the other environmental treatments. The dipterocarp species (
D. zeylanicus,
S. stipularis) were slower growing and more site-sensitive to changes in environmental treatment than the non-dipterocarp species.
S. macrophylla grew tallest in all environmental treatments as compared to the other species. Overall, the best environments for seedling establishment and growth for all species were in the centres of the canopy strips (6–12 m). This study contributes to investigations testing the feasibility of using
Pinus as a nurse for establishing more shade-tolerant species; and as a technique for forest restoration in south and southeast Asia.
1 Responses to the addition of P and Mg are described for eight species of Shorea section Doona (Dipterocarpaceae) which vary in their adult distribution across a topographic/soils gradient at ...Sinharaja Forest Reserve, Sri Lanka. 2 All combinations of the two nutrients resulted in increased dry mass yield, seedling height and leaf number after 24 months for seedlings of these species growing in pots of soil taken from a nearby Pinus plantation compared to a control which did not receive nutrients. In the presence of P, dry mass yield, leaf number and lateral root ratio declined in response to increasing Mg. In the presence of Mg, seedling height and stem mass ratio increased in response to increasing P. 3 After 24 months maximum dry mass yield in response to nutrient addition was greater for the four species which occur on more nutrient-rich soils in the field. Maximum percentage increase in dry mass in response to nutrient addition was negatively correlated with mean dry mass of unfertilized seedlings, but the relationship did not discriminate between species according to adult distribution across the catena. 4 There was no relation between seedling root mass ratio (RMR) or phenotypic plasticity in seedling RMR and adult distribution in relation to nutrient supply. 5 In Shorea section Doona, trade-offs between seedling `responsiveness' and `tolerance' to nutrient supply, in terms of either growth or dry mass allocation, are not powerful determinants of differences in adult distribution in relation to nutrient supply. Differential dry mass yield in response to nutrient addition was determined by differences in seed size and seedling relative growth rates under the experimental conditions, as well as the degree of response to nutrient addition. Response to nutrient addition may be constrained by seedling characteristics relating to variation in irradiance and water availability.
In this study four species of the genus Shorea section Doom were investigated. All occur together as canopy trees in the Sinaraja rainforest of south-west Sri Lanka. Partitioning of the regeneration ...niche can be one explanation for the co-existence of ecologically similar canopy tree species within a forest. Seedlings were planted in plots located in five zones that represent a range of forest groundstorey microenvironments found adjacent to and across canopy openings of three sites – valley, midslope, ridgetop. Experiments were designed to monitor survíval and growth of planted seedlings for two years. At the end of two years percentage survival was calculated, height increment recorded and destructive samples taken to measure dry mass gain of root, stem and leaves. Comparisons were made of establishment and growth performance of seedlings planted in the different plots and sites. Results demonstrated clear differences in survival and growth among species. These differences appeared related to availability of soil moisture and groundstorey radiation regimes. Disturbance patterns that determine species co-existence are suggested.
Resource allocation within trees is a zero-sum game. Unavoidable trade-offs dictate that allocation to growth-promoting functions curtails other functions, generating a gradient of investment in ...growth versus survival along which tree species align, known as the interspecific growth-mortality trade-off. This paradigm is widely accepted but not well established. Using demographic data for 1,111 tree species across ten tropical forests, we tested the generality of the growth-mortality trade-off and evaluated its underlying drivers using two species-specific parameters describing resource allocation strategies: tolerance of resource limitation and responsiveness of allocation to resource access. Globally, a canonical growth-mortality trade-off emerged, but the trade-off was strongly observed only in less disturbance-prone forests, which contained diverse resource allocation strategies. Only half of disturbance-prone forests, which lacked tolerant species, exhibited the trade-off. Supported by a theoretical model, our findings raise questions about whether the growth-mortality trade-off is a universally applicable organizing framework for understanding tropical forest community structure.
Theory predicts that higher biodiversity in the tropics is maintained by specialized interactions among plants and their natural enemies that result in conspecific negative density dependence (CNDD). ...By using more than 3000 species and nearly 2.4 million trees across 24 forest plots worldwide, we show that global patterns in tree species diversity reflect not only stronger CNDD at tropical versus temperate latitudes but also a latitudinal shift in the relationship between CNDD and species abundance. CNDD was stronger for rare species at tropical versus temperate latitudes, potentially causing the persistence of greater numbers of rare species in the tropics. Our study reveals fundamental differences in the nature of local-scale biotic interactions that contribute to the maintenance of species diversity across temperate and tropical communities.
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