Evolution of Drought Tolerance Across California Native Oaks

Abstract:

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 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 (WD), turgor loss point (π_TLP), vein lengths per area (VLA_major, VLA_minor), leaf area (LA), leaf mass per area (LMA), anatomical maximum stomatal conductance (g_max) and time integrated photosynthetic rate (A_mass). We also compiled climate envelopes for all species based on observations from distribution data: aridity index (AI), mean and maximum annual temperatures (MAT and Tmax_WM), and mean annual precipitation (MAP). We tested trait differences among Quercus species from different sections, the relationships among traits and between traits and climate within an explicitly evolutionary framework. The 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 LA, WD,π_TLP, VLA_minor and A_mass. LA and A_mass were negatively related to aridity (r = 0.57-0.71; p<0.05), WD positively with MAT and Tmax_WM (r = 0.53 and 0.64; p<0.05), A_mass and VLA_minor negatively with MAT and Tmax_WM (r = -0.58 and -0.54; p<0.05) and lower π_TLP with higher Tmax_WM (r = -0.62; p<0.05). Additionally, we found strong evolutionary coordination among drought tolerance traits; species with more negative π_TLP had larger LMA, WD, VLA_major (r = -0.54 to -0.58; p<0.05), lower LA and A_mass (r = -0.78 and 0.76; p<0.05). Our results suggest strong adaptation of drought tolerance across the California native Quercus species, involving repeated convergent adaptation of key traits.