Within-Species Variation in Allocation and Tissue Traits Across Large Aridity Gradients

It is critical to understand the causes of variation in plant functional traits in order to successfully incorporating them into mechanistic models of plant and ecosystem function. Trait variation within a species is particularly useful in this endeavor because other critical confounding factors (e.g. variation in life history strategies) are largely controlled, and patterns that are replicable across species are likely to prove generalizable. We measured variation in leaf and branch traits across the aridity range of seven widespread Eucalpyts in southwestern Australia and Tasmania. While the total amount and dominant scale (e.g. within canopy versus between individuals versus between populations) variation differed considerably by trait and by species, we found consistent and systematic climate-related trait variation in almost all species. Wood density, leaf dry matter content, and leaf mass per area increased and leaf area to sapwood area ratio decreased near the arid range edge of both mesic and xeric species, suggesting that 1) increased drought tolerance in eucalypts is accomplished through coordinated plastic or ecotypic changes in tissue robustness (wood density, leaf dry matter content), and hydraulic support (leaf area:sapwood area) and 2) drought stress is relative to species physiology (i.e. all species were drought stressed at their dry range margin regardless of how wet that margin was).