Demographic variation and habitat specialization of tree species in a diverse tropical forest of Cameroon

Background: Many tree species in tropical forests have distributions tracking local ridge-slope-valley topography.Previous work in a 50-ha plot in Korup National Park, Cameroon, demonstrated that 272 species, or 63% of those tested, were significantly associated with topography.Methods: We used two censuses of 329,000 trees ≥1 cm dbh to examine demographic variation at this site that would account for those observed habitat preferences. We tested two predictions. First, within a given topographic habitat, species specializing on that habitat(‘residents') should outperform species that are specialists of other habitats(‘foreigners'). Second, across different topographic habitats, species should perform best in the habitat on which they specialize(‘home') compared to other habitats(‘away'). Species' performance was estimated using growth and mortality rates.Results: In hierarchical models with species identity as a random effect, we found no evidence of a demographic advantage to resident species. Indeed, growth rates were most often higher for foreign species. Similarly, comparisons of species on their home vs. away habitats revealed no sign of a performance advantage on the home habitat.Conclusions: We reject the hypothesis that species distributions along a ridge-valley catena at Korup are caused by species differences in trees ≥1 cm dbh. Since there must be a demographic cause for habitat specialization, we offer three alternatives. First, the demographic advantage specialists have at home occurs at the reproductive or seedling stage, in sizes smaller than we census in the forest plot. Second, species may have higher performance on their preferred habitat when density is low, but when population builds up, there are negative density-dependent feedbacks that reduce performance. Third, demographic filtering may be produced by extreme environmental conditions that we did not observe during the census interval.

Effect of wood density and water permeability on wood decomposition rates of 32 bornean rainforest trees

Aims a better understanding of wood litter decomposition is essential for predicting responses of forest ecosystems to global climate change.recent studies suggest that chemical properties of wood litters,rather than physical ones such as wood density,are more important for inter-specific differences in wood decomposition rates.However,empirical data are still limited,especially for tropical trees.In addition,decom-position rate of wood litter often varies with time,which makes inter-specific comparison difficult.We studied the wood decomposition of 32 rainforest trees to elucidate(i)the degree of interspecific variation in wood decomposition rate of a given size and configuration and(ii)if initial wood density and water permeability are consistent predictors of the overall decomposition rate and its pattern over time.Methods a common garden decomposition experiment was conducted in a tropical rainforest in malaysian borneo for 32 native tree species.small wood sticks were set on the forest floor and the weight loss was monitored monthly for 2.7 years.Important Findings We found large variation in the wood decomposition rate(a 49-fold range),suggesting that we need to consider this variation when cal-culating community-level carbon dynamics of tropical rain forests.the physical traits of wood,i.e.wood density and water permeability,were related to wood decomposition rate and its pattern over time.Decomposition half-time related positively and negatively to initial wood density and water permeability,respectively.the time-dependentrate model fitted better for 18 species(56%of the study species)that had higher water permeabilities than the others,suggesting that micelle porosity in wood relates to temporal changes in decomposition rate.

Demographic trade-offs determine species abundance and diversity

Aims Much recent theory has focused on the role of neutral processes in assembling communities,but the basic assumption that all species are demographically identical has found little empirical support.Here,we show that the framework of the current neutral theory can easily be generalized to incorporate species differences so long as fitness equivalence among individuals is maintained through trade-offs between birth and death.Methods Our theory development is based on a careful reformulation of the Moran model of metacommunity dynamics in terms of a non-linear one-step stochastic process,which is described by a master equation.Important Findings We demonstrate how fitness equalization through demographic trade-offs can generate significant macroecological diversity patterns,leading to a very different interpretation of the relation between Fisher’s a and Hubbell’s fundamental biodiversity number.Our model shows that equal fitness(not equal demographics)significantly promotes species diversity through strong selective sieving of community membership against high-mortality species,resulting in a positive association between species abundance and per capita death rate.An important implication of demographic trade-off is that it can partly explain the excessively high speciation rates predicted by the neutral theory of the stronger symmetry.Fitness equalization through demographic trade-offs generalizes neutral theory by considering heterospecific demographic difference,thus representing a significant step toward integrating the neutral and niche paradigms of biodiversity.

Age structure in neutral theory resolves inconsistencies related to reproductive-size threshold

Aims Neutral theory consists of a suite of models that assume ecological equivalence among individual organisms.They have been most commonly applied to tropical forest tree communities either as null models or as approximations.Neutral models typically only include reproductive adults;therefore,fitting to empirical tree community data requires defining a reproductive-size threshold,which for trees is usually set arbitrarily to a diameter at breast height(DBH)of 100 mm.The inevitable exclusion of some reproductive adults and inclusion of some saplings cause a non-random sampling bias in neutral model fits.Here,we investigate this problem and resolve it by introducing simple age structure into a neutral model.Methods We compared the performance and sensitivity of DBH threshold of three variants of a spatially explicit neutral model:the traditional model,a model incorporating random sampling and a model with two distinct age classes—reproductive adults and saplings.In the age-structured model,saplings are offspring from adults that disperse according to a Gaussian dispersal kernel around the adults.The only extra parameter is the ratio of adults to saplings,which is not a free parameter but directly measurable.We used species–area relationships(SARs)to explore the predicted effect of saplings on the species richness at different scales in our model.We then evaluated the three model variations to find the parameters required to maintain the observed level of species richness in the 50-ha plot on Barro Colorado Island(BCI).We repeated our analysis filtering the data at different minimum tree-size thresholds in order to find the effect this threshold has on our results.Lastly,we used empirical species–individual relationships(SIRs)to test the pre-existing hypothesis that environmental filtering is the primary cause of differences between the assemblage of saplings and that of adults on BCI.Important Findings Our age-structured neutral model was characterized by SARs that were insensitive to the presence of saplings at large scales and highly sensitive to them at small scales.Both models without age structure were highly sensitive to the DBH threshold chosen in a way that could not be explained based on random samplings alone.The age-structured neutral model,which allowed for non-random sampling based on life stage,was consistent with species richness observations.Our analysis of empirical SIRs did not support environmental filtering as a dominant force,but it did show evidence for other differences between age classes.Age can now be easily incorporated into future studies of neutral models whenever there is a concern that a sample is not entirely composed of reproductive adult individuals.More generally,we suggest that modeling studies using tree data subject to a minimum size threshold should consider the sensitivity of their results to that threshold.