Abstract
How tropical tree species respond to disturbance is a central issue of forest ecology, conservation and resource management. We define a hierarchical model to investigate how functional traits measured in control plots relate to the population change rate and to demographic rates for recruitment and mortality after disturbance by logging operations. Population change and demographic rates were quantified on a 12-year period after disturbance and related to seven functional traits measured in control plots. The model was calibrated using a Bayesian Network approach on 53 species surveyed in permanent forest plots (37.5 ha) at Paracou in French Guiana. The network analysis allowed us to highlight both direct and indirect relationships among predictive variables. Overall, 89% of interspecific variability in the population change rate after disturbance were explained by the two demographic rates, the recruitment rate being the most explicative variable. Three direct drivers explained 45% of the variability in recruitment rates, including leaf phosphorus concentration, with a positive effect, and seed size and wood density with negative effects. Mortality rates were explained by interspecific variability in maximum diameter only (25%). Wood density, leaf nitrogen concentration, maximum diameter and seed size were not explained by variables in the analysis and thus appear as independent drivers of post-disturbance demography. Relationships between functional traits and demographic parameters were consistent with results found in undisturbed forests. Functional traits measured in control conditions can thus help predict the fate of tropical tree species after disturbance. Indirect relationships also suggest how different processes interact to mediate species demographic response.
Highlights
Functional ecology assumes that biological functions scale up from the organismic level to higher organization levels [1,2,3,4]
Population change rate over the study period varied from -0.01 (Pogonophora schomburgkiana Euphorbiaceae) to 0.25 (Schefflera decaphylla), while recruitment and mortality rates respectively varied from 0.4% (Tapura capitulifera Dichapetalaceae) to 15.1% (Schefflera decaphylla) and from 0 (Platonia insignis Clusiaceae) to 2.4% (Pogonophora schomburgkiana, Table S1 in Appendix S1)
The recruitment rate rREC was directly related to three functional traits that together explained 45% of its variance (Figure 2): Leaf phosphorus concentration (LPC) had a positive effect, whereas wood density (WD), and seed volume (SV) had negative effects on rREC
Summary
Functional ecology assumes that biological functions scale up from the organismic level to higher organization levels [1,2,3,4]. Biological functions can be investigated by means of functional traits which are morphological, physiological or phenological characteristics that influence individual fitness through growth, fecundity and survival [4]. The relationships between the population change rate and functional traits are not direct but indirect through demographic parameters. Dependencies among population change, demographic processes and functional traits are widely acknowledged [3,5,6], but have rarely been integrated into a consistent model. We evaluate an ecological model [7] connecting these variables in a three-level hierarchy (Figure 1), in order to identify key traits indicative of species response to disturbance in tropical forests. The model is general, key issues concern the nature of relevant traits and how they relate to demographic parameters
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