Spatial distribution - phylogeny or environment?

Submitted by editor on 5 June 2018.Get the paper!

A number of studies has shown the individual influence of dispersal mechanism, species height, sexual system, and wood density on the spatial distribution pattern of tropical tree populations at small spatial scales (i.e. < 50 m). These traits are usually conserved among closely related species, thus populations of these species should have more similar spatial distribution patterns than populations of phylogenetically distant species. Additionally, variation in the abiotic and biotic environment might result in distinct spatial distribution patterns of local populations of the same species. For the fist time, our study accounted for both individual and joint effects of traits, phylogenetic relationships among species, and site characteristics on the degree of overdispersion or aggregation of tree populations at different spatial scales within fourteen 1-ha plots of the Atlantic Rainforest in southeastern Brazil.

Mountain range of “Serra do Mar” in southeastern Brazil. Fourteen permanent 1-ha plots were established along the altitudinal range in 2005. Since then, four forest inventories have been conducted inside the plots. Picture by Professor Nivea Dias.

To do so, we first quantified the degree of overdispersion or aggregation of the populations with a newly developed standardized index err(r), which is based on standardized effect sizes of the pair correlation function. Then we employed variation partitioning to determine the contribution of our three variable sets (traits, phylogenetic relationships among species, and site characteristics) to err(r) calculated at different spatial scales (1, 10, 20, 30, 40, and 50 m).

Variation in err(r) was mostly explained by phylogenetic relationships among species, indicating that traits not included in our analysis are important drivers of the spatial distribution pattern of populations. Site characteristics explained a smaller part of the variation in err(r), which suggests that the spatial distribution patterns of local populations of the same species are context-dependent. Finally, the traits studied here provided the smallest explanation of the variation and indicated a minor role of seed dispersal. Residual variation in err(r) was low but yet suggests that stochasticity and/or variables not included in the models (e.g., direct measures of post-dispersal processes) also influence the spatial distribution pattern of the populations.

Variation partitioning of the degree of overdispersion or aggregation of tree populations of the Atlantic Rainforest in southeastern Brazil, as measured by the standardized pair correlation function index err(r), including variable sets representing traits, phylogenetic relationships, and site characteristics. The standardized index err(r) was calculated at distances of (a) 1 m, (b) 10 m, (c) 20 m, (d) 30 m, (e) 40 m, and (f) 50 m from focal trees. “Traits”, “Phylo”, and “Site” represent the whole variation explained by the trait, phylogenetic relationship, and site components respectively; the portion inside the circles represents the pure variation explained by traits, phylogenetic relationships, and site where populations were sampled. “***p < 0.001”; “**p < 0.01”; “*p < 0.05”.

 

Our results suggest that many ecological processes act in concert at the Brazilian Atlantic Rainforest. At all spatial scales evaluated, the most important process by far is the interaction of trees with their biotic and abiotic environment mediated by as yet unknown traits, followed by stochasticity and/or post-dispersal processes, and to a lesser extent tree responses to environmental characteristics and seed dispersal. More importantly, the relative importance of these processes differs from that previously described for other tropical forests. Determining why a given ecological process is more important in some tropical tree communities than in others and which species traits mediate these processes offer promising avenues for further research.

The authors through Valeria Martins

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