Drought resistance may strongly affect the distributions of long-lived woody species. With climate change, drought resistance may be critical in reinforcing or shifting species ranges. However, common measures of leaf-level drought tolerance may be complicated by phenological patterns. Within the species rich oak genus, leaf phenology varies from evergreen to winter or drought deciduous. We established a greenhouse common garden of warm region American oak saplings to investigate stem xylem vulnerability and leaf stomatal response to drought. We monitored stomatal closure (20 species, N=88) during a dry-down experiment and determined the leaf water potential at stomatal closure (Psi[crit]); lower Psi[crit] indicates higher drought tolerance. We also measured stem xylem vulnerability (9 species, N=47) using a pneumatic air vacuum method and determined the water potential at 50% embolism discharge (P50); lower P50 indicates greater xylem resistance to embolism. All measurements include species spanning the same aridity gradient. We asked 1) whether there was a positive relationship between P50 and Psi[crit] indicating that more resistant stems have more tolerant leaves, 2) whether aridity in the climate of origin predicted stem and leaf drought resistance, and 3) whether leaf phenological patterns explain P50 and Psi[crit] results.
Aridity index (AI=MAP/MAE, low AI=high aridity) predicts species mean Psi[crit] values, but only after removing tropical oaks. Psi[crit] values are most negative in drought deciduous species. Species xylem resistance is greatest in species from climates of high aridity and high seasonality, but is not significantly correlated with Psi[crit]. Psi[crit] is positively correlated with AI, yet the relationship is only significant (p=0.04, R2 = 0.29) among oaks at AI<1. Psi[crit] is on average lowest (highest drought tolerance) among drought deciduous species, and tends to be intermediate in evergreen species and higher in winter deciduous species. From our observations, Psi[crit] may not reflect stomatal closure in some species, but rather the desiccation of the leaf (in tropical and some drought deciduous species). However, AI positively predicts P50 across all species (p = 0.002,R2=0.78), as does diurnal temperature variation (p=0.01,R2=0.62). P50 is not significantly correlated with Psi[crit], but shows a similar trend of highest resistance among drought-deciduous species and decreasing resistance among evergreen and winter deciduous species. Among these oaks, leaf loss habits are likely more critical for climate adaptation than stomatal closure values. However, xylem resistance to embolism is strongly associated with climate, and is greatest among arid and semi-arid species, regardless of leaf phenology.