Trait-mediated assembly processes predict successional changes in community diversity of tropical forests

Abstract

Interspecific differences in relative fitness can cause local dominance by a single species. However, stabilizing interspecific niche differences can promote local diversity. Understanding these mechanisms requires that we simultaneously quantify their effects on demography and link these effects to community dynamics. Successional forests are ideal systems for testing assembly theory because they exhibit rapid community assembly. Here, we leverage functional trait and long-term demographic data to build spatially explicit models of successional community dynamics of lowland rainforests in Costa Rica. First, we ask what the effects and relative importance of four trait-mediated community assembly processes are on tree survival, a major component of fitness. We model trait correlations with relative fitness differences that are both density-independent and -dependent in addition to trait correlations with stabilizing niche differences. Second, we ask how the relative importance of these trait-mediated processes relates to successional changes in functional diversity. Tree dynamics were more strongly influenced by trait-related interspecific variation in average survival than trait-related responses to neighbors, with wood specific gravity (WSG) positively correlated with greater survival. Our findings also suggest that competition was mediated by stabilizing niche differences associated with specific leaf area (SLA) and leaf dry matter content (LDMC). These drivers of individual-level survival were reflected in successional shifts to higher SLA and LDMC diversity but lower WSG diversity. Our study makes significant advances to identifying the links between individual tree performance, species functional traits, and mechanisms of tropical forest succession.community turnover | hierarchical Bayes | individual variation | secondary forest | species interactions Q uantifying the relative importance of mechanisms that drive community assembly remains a central challenge in ecology. Interspecific variation in ecological strategies is considered a major driver of community assembly and has been classified by Chesson (1) into relative fitness (i.e., per capita population growth rate) differences and stabilizing niche differences (2). These processes act concurrently, and their importance may vary over space and time (3). However, the simultaneous effects of different assembly mechanisms on community dynamics have not been wellcharacterized, particularly in diverse communities such as tropical forests. High dynamism of vegetation composition during tropical forest succession creates an ideal opportunity to investigate the drivers of community assembly.Interspecific niche and fitness differences can be characterized through the lens of interspecific functional trait variation, an approach that can shed light on drivers of community functional composition and dynamics (2, 4, 5). Traits may mediate niche and fitness differences in at least four ways (Fig. 1). First, interspecific trait variation can be correlated with fitnes...

Plant functional traits have globally consistent effects on competition

“…To include the effects of traits on the parameters of the growth model, we build on previous studies that explored the role of traits for tree performances and tree competition 8,9,11 . We modelled the effect of traits, one trait at a time.…”

“…Here we extended the approach of this most recent study 11 by splitting α h (t f − t c ) into α t t f + α e t c (which is equivalent to the hierarchical distance if α t = − α e ) and including two α 0 , one for intraspecific and one for interspecific competition.…”

“…Data on species functional traits were extracted from existing sources. We focused on wood density, species SLA and maximum height, because these traits have previously been related to competitive interactions and are available for large numbers of species 8,9,11,15,30 (see Extended Data Tables 3 and 4 for trait coverage and their correlations). Where available, we used data collected locally (references for the local trait data used in this analysis include refs 15, 51, 60-62); otherwise we sourced data from the TRY trait data base 18 (references for the data extracted from the TRY database used in this analysis include refs 2, 3, 15, 63-130).…”

“…A long-standing hypothesis is that the intensity of competition decreases as two species diverge in trait values 7 (trait dissimilarity). The few studies [8][9][10][11][12][13] that have explored links between traits and competition have shown that linkages were more complex than this, as particular trait values may also confer competitive advantage independently from trait dissimilarity 9,13,14 . This distinction is fundamental for species coexistence and the local mixture of traits.…”

Do community-weighted mean functional traits reflect optimal strategies?

“…At a larger scale, more than one-third of the total global variation in SLA occurs within areas of similar abiotic conditions, as opposed to among sites that vary with respect to climate variables [5]. Together, shifts of CWM trait values along abiotic gradients and the substantial proportion of local trait variation represent a major challenge for prediction in community ecology (and for the CWM-optimality hypothesis in particular), because they imply the combined action of mechanisms that constrain and maintain local functional diversity [18][19][20]. Reconciling these contrasting patterns is a key to advancing a predictive theory of functional community ecology.…”