Linking leaf toughness to performance traits and life history characteristics of tree species in a moist forest in Bolivia

Background/Question/Methods Recent comparative studies within and across plant communities elucidate the central role of leaf functional traits in predicting performance and life history traits of plant species. In particular, species with high leaf mass per unit area (LMA) and/or stronger leaves generally exhibit lower herbivory rates, longer leaf life span, better shade survival, but they grow more slowly. However, it remains little explored how components of LMA and overall leaf strength, i.e., tissue-level traits (density, toughness) and morphology (thickness, leaf size), contribute to species difference in leaf and whole-plant level performance. Also unknown is how these leaf functional traits change between ontogenetic stages of plants. Here, we used both shearing and punching tests to determine biomechanical strength of seedling, sapling and adult leaves for 19 tree species in a tropical moist forest in Bolivia, ranging widely in juvenile light requirements, to address three questions. 1) How do components of overall leaf strength correlate with each other? 2) How do biomechanical and morphological traits change between ontogenetic stages? 3) How do leaf biomechanical and morphological traits of juvenile leaves correlate with leaf and whole-plant level performance (herbivory, leaf life span, growth, survival) as well as regeneration light requirements? Results/Conclusions A principle component analysis revealed 3 major PCA axes of leaf functional trait variations. The first axis represents tissue-level traits that contribute to overall strength of leaves (toughness and tissue density for lamina and vein), the second axis represents lamina thickness, and the third axis the total leaf area. Combined, these axes explained 92% of variation in measured traits. Traits contributing to PCA1, i.e., tissue toughness and density, were correlated strongly with both leaf- and whole-plant level performance of the species, such that tougher leaves were linked to lower palatability to a generalist herbivore (snail), longer leaf life span, better sapling survival, and slower growth rate. In contrast, thickness and area of leaves differed independently of the tissue-level traits and performance traits. Tissue-level traits contributing to biomechanical strength were strongly concordant across ontogenetic stages whereas leaf thickness and leaf size exhibited unexpected trends among ontogenetic stages. Conclusion: Tissue density and biomechanical strength expressed per unit fracture area were two key functional traits; they were strongly associated with each other and concordant across organs (leaves and stems) and ontogenetic stages of trees, and varied among species in relation to life history strategies and ecological regeneration light requirements.