Leaf trait coordination and variation of blue oak across topo-environmental scales

Abstract Trees are arguably the most diverse and complex macro-organisms on Earth. The equally diverse functions of trees directly impact fluxes of carbon, water, and energy from the land surface. A number of recent studies have shed light on the substantial within-species variability across plant traits including aspects of leaf morphology and plant allocation of photosynthates to leaf biomass. Yet within-tree variability in leaf traits due to microclimatic variations, leaf hydraulic coordination across traits at different physiological scales, and variations in leaf traits over a growing season remain poorly studied. This knowledge gap is stymieing fundamental understanding of what drives trait variation and covariation from tissues to trees to landscapes. Here we present an extensive dataset measuring within-tree heterogeneity in leaf traits in California blue oak (Quercus douglasii) across an edaphic gradient and over the course of a growing season at an oak-grass savanna in southern California, USA. We found a high level of within-tree crown AL:AS variation not attributable to sample height or aspect. We also found a higher level of trait integration at the tree- rather than branch-level, suggesting that trees optimize water use at the organismal-level. Despite the large variance in traits within a tree crown and across trees, we did not find strong evidence for adaptive plasticity or acclimation in leaf morphological traits (e.g., changes to phenotype which increased fitness) across temporal and spatial water availability gradients. Collectively, our results highlight strong variation in drought-related physiology, but limited evidence for adaptive trait plasticity over shorter time scales.