The effects of environmental stress on Quercus morphology, physiology, and distribution in the Chiricahua Mountains

Patterns of plant morphology, physiology and composition along environmental gradients may inform predictions regarding their sensitivity to climate fluctuations. Our research examined distribution of foundation tree species and their responses to environmental variation in a montane ecosystem. We measured leaf thickness, stomatal density, carbon isotopic composition (¹³C as an indicator of water-use efficiency), and diversity of evergreen oaks along a gradient of elevation and environmental stress in the Chiricahua Mountains of southeastern Arizona. We studied three species of Quercus (Q. hypoleucoides, Q. arizonica, and Q. palmeri) within a Madrean pine-oak woodland between approximately 1,500 and 2,200 meters. These species have not previously been examined with regard to leaf-level physiology and morphology, and their local distribution patterns have not been described. Our results suggest an increase in leaf thickness and a decrease in stomatal density for all species with increasing elevation. Differences in these traits among the three species may relate to observed differences in distribution. We observed a correlation between leaf thickness and relative abundance where the thin-leaved Q. palmeri was dominant at lowest elevations, thick-leaved Q. hypoleucoides occupied the highest elevations, and Q. arizonica was most abundant in the intermediate range. Our results suggest coupled shifts in species distribution and morphology with predicted increases in regional average temperatures. These results may be useful in predicting distribution and trait variation of foundational species in climate change scenarios.