Traits may drive strong climate associations of species within widespread genera across communities ranging in climatic aridity. The genus Quercus is widely distributed across California dominating communities with wide variation in precipitation and temperature and is thus an excellent system to test hypotheses for the trait-based mechanism of drought tolerance and its evolution. Trait-climate relationships can be moderated by species-relatedness, and/or can be associated with convergent adaptation across a phylogeny. We measured functional traits and tested their associations with the climate of the native range of distribution for adult trees of 15 species across the three sections of the genus Quercus in California (Lobatae, Protobalanus and Quercus) grown in a common garden. We focused on traits that contribute to drought tolerance including wood density, turgor loss point, vein length per area, leaf area, leaf thickness, and leaf mass per area. We also compiled climate envelopes for all species based on observations from distribution data: aridity index (AI), mean and maximum annual temperatures (MAT and TmaxWM), and mean annual precipitation (MAP). We tested trait differences among Quercus species from different sections of the genus, and test the relationships among traits and between traits and climate within an explicitly evolutionary framework.
Results/Conclusions
The measured drought tolerance traits did not differ on average across the three sections of the genus Quercus in California. We found strong evolutionary trait-climate relationships, with leaf area, wood density, and turgor loss point. Leaf area and wood density were negatively related to aridity (i.e., positively with AI and/or MAP; |r| = 0.55-0.63; p<0.05), wood density positively with MAT and TmaxWM (r = 0.53 and 0.64, respectively; p<0.05), and lower turgor loss point with higher TmaxWM (r = -0.62; p<0.05). Additionally, we found strong evolutionary coordination among drought tolerance traits; species with larger leaves had lower leaf mass per area, wood density, vein length per area (r = -0.56 to -0.75; p<0.05), and less negative turgor loss point (r = 0.78; p<0.05). Our results suggest strong adaptation of drought tolerance across the California native Quercus species, involving repeated convergent adaptation of key traits. Future work will expand our analysis to include stomatal trait diversity, nutrient concentrations and species’ photosynthetic capacity.