Plant defense syndromes in a tropical moist forest canopy community

Background/Question/Methods

Plant-herbivore interactions influence tropical rainforest biodiversity, where stable abiotic conditions facilitate a greater role for predatory-prey interactions in driving evolution.  Further, niche theory suggests that herbivores shape communities by altering competitive dynamics between plant species. Recent studies have shown that multivariate plant defense traits among co-occurring species collapse into distinct strategies for mitigating fitness losses, or defense syndromes.  In a diverse tropical rainforest community, the existence of plant defense syndromes indicates a non-trivial role for plant-herbivore interactions in shaping community assembly—if defense is independent of growth/longevity traits and is not inherently phylogenetically conserved. We combined functional and phylogenetic analyses to evaluate the existence and independence of plant defense syndromes within a canopy tree community in a moist tropical forest on Barro Colorado Island, Panama.  We utilized a previously published DNA-barcode community phylogeny and collected outer-canopy foliage for 345 individuals across 84 species.  We measured four defense traits (toughness, total phenols, condensed tannins, and hydrolysable tannins), two growth traits (nitrogen and phosphorus), and two structural/longevity traits (thickness, LMA).  We evaluated trait covariance using regression and PCA, and identified defense syndromes using iterative K-means clustering.  We examined trait phylogenetic dependence using Blomberg’s K statistic, phylogenetic autocorrelograms, and phylogenetically independent contrasts.

Results/Conclusions

We found two lines of evidence for the existence and independence of plant defense syndromes within this tropical canopy tree community.  First, defense traits do not follow simple patterns of bivariate trade off, nor do they contrast uniformly with growth or longevity traits.  Rather, patterns of allocation to growth, chemical defense, or physical resistance/longevity collapse into three distinct defense syndromes, which are well-aligned with the plant defense syndrome triangle described by Agrawal and Fishbein (2006).  Secondly, defense traits show markedly less phylogenetic conservatism than traits related to growth and longevity. Phylogenetic signal, expressed by Blomberg’s K, was lower and generally less significant for defense traits compared to traits related to growth and longevity. Phylogenetic autocorrelograms revealed the recent divergence of all defense traits, in sharp contrast with an increase in similarity for growth and longevity traits among closely related species.  These findings suggest that plant defense is a functionally and evolutionarily important source of phylogenetic variation among co-existing individuals, and support the role of plant-herbivore interactions in shaping community assembly.