Stomatal sensitivity to heat and drought depends on coordination between leaf phenology, functional and physiological traits in Sonoran Desert tree species

In the Sonoran Desert, due to high air temperature (T_air) and vapor pressure deficit (VPD), trees face extreme water and thermal stress even during the rainy season. In response to T_air and VPD, trees should finely regulate their stomatal conductance (g_s), but we would expect evergreen species to generally show less sensitivity to these changes than deciduous species because of their greater tolerance to drought stress. We measured g_s in six deciduous and six evergreen tree species in field conditions, using environmental data from a nearby climate station. We evaluated stomatal sensitivity using five indices and measured leaf water potential at the turgor loss point (Ψ_tlp), along with leaf traits such as leaf dry-matter content (LDMC), stomatal size, and density, which are related to drought response. Our findings showed that high T_air and VPD significantly influence g_s, often causing stomatal closure at higher leaf temperatures. Evergreens exhibited greater drought tolerance, less sensitivity to VPD and temperatures, and higher LDMC (P = 0.004), smaller specific leaf areas (SLA, P < 0.001), and lower Ψ_tlp (P < 0.001). In contrast, deciduous species regulate drought stress through isohydric stomatal control and may drop leaves under extreme conditions, while evergreens withstand higher stress levels despite potential hydraulic dysfunction.