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Interspecific and size-dependent variation in carbon concentration and wood chemical traits of tropical trees

Interspecific and size-dependent variation in carbon concentration and wood chemical traits of tropical trees
Adam Robert Martin


Faculty of Forestry, University of Toronto, Toronto, ON M5R 0A3, CANADA.


Tropical forests play a major role in global carbon (C) dynamics and maintain some of the highest biological complexity on Earth; however, little is known about how variation in wood chemical traits contributes to tropical forest structure and function. This research examines inter- and intraspecific patterns in wood chemical traits in order to understand 1) the role wood chemical traits play in tropical forest C dynamics, and 2) the adaptive significance of wood chemical traits in tropical trees. I found wood C concentration varies widely among co-occurring tropical tree species, with average C concentration (47.4 ± 0.33% w/w (S.E.)) being significantly lower than values assumed in prominent forest C accounting protocols. Failing to account for this variation leads to overestimates of ~3.3 – 5.3% in tropical forest C accounting, an error that compounds significantly at larger spatial scales. I also show that oven drying samples prior to elemental analysis underestimates wood C concentration by 2.5 ± 0.17%, due to the loss of the "volatile C fraction". Counter to expectations, I found wood C concentration is not phylogenetically conserved nor correlated to species demography or life history traits. Wood chemical traits showed consistent size-dependent patterns: wood C (in 16 of 24 species) and lignin (in 15 of 16 species) was higher in saplings vs. conspecific canopy trees. These patterns, complimented by phylogenetic analyses, suggest saplings require wood chemical traits that confer greater pathogen defense. When analyzed across a continuous size spectrum, I found wood C concentration (and leaf structural traits) increases linearly, while wood starch concentration (and leaf traits associated with C gain) shows "hump-shaped" patterns with peak values closely preceding reproductive onset; the latter result suggests C may limit growth in larger trees. Overall, my dissertation provides one of the first comprehensive examinations of wood chemical trait variation in tropical trees. In doing so it provides novel, timely, and critical insights into how wood chemical traits contribute to tropical forest structure and function.