Recent debate has highlighted an important knowledge gap in the use of plant functional traits to explain ecological dynamics: what is the connection between leaf traits related to carbon economics (i.e. the leaf economic spectrum) and those related to water-use? The coupling or decoupling of economic and water-use traits is often explained through leaf anatomy, yet there has been little research into how suites of economic and water-usage traits interact through leaf morphology traits. Our study explores the interaction of these three suites of traits in multiple species of different evolutionary origins growing in similar environments.
The Los Angeles Metropolitan Area provides a novel and useful environment for addressing this objective as it contains >100 tree species from multiple biogeographic regions and spans a climate gradient of increasing temperature and aridity. We selected 10 tree species along a spectrum of phylogenetic relatedness and biogeographical origin. We then characterized each species by its suites of functional traits based on carbon economics (specific leaf area, % leaf nitrogen), water-use traits (stomatal density & length, wood density), and leaf morphology (leaf thickness, leaf dissection). Trait values were compared through principle component analysis (PCA), and metrics of inter- and intra-specific variability.
We found a strong decoupling of economics and water-usage across all species along the entire LA climate gradient. Using PCA, we found primary economic traits and water-use traits are orthogonal to each other. When explored for species effects, we found certain tree species (Jacaranda mimisoflia, Tipuan tipu) to both rank high in “fast” economic traits and “conservative” water-use traits. Pairwise comparisons between trait values show leaf morphology relates more strongly to carbon economics in the milder climate of the coast versus a stronger correlation to water-usage in the extreme climate of the desert. These results suggest an overall decoupling of economics from water usage. Furthermore, the lability of leaf morphology may allow species to acclimate to heat and atmospheric drought by linking more with water-usage than economics. In the context of the urban ecosystem these results are useful for understanding how the urban forest may respond to large regional shifts in climate regimes as current models project coastal climates of Southern California to resemble the climates of desert regions in the coming future.