Functional differentiation among seven ecologically-stratified oak (Quercus) species in southeastern Arizona

Background/Question/Methods

Physiological limitations may drive plant elevational range limits when temperature and precipitation gradients are steep. Climatic gradients are especially strong in the mountains of semi-arid southeastern Arizona. Differing drought and freezing tolerances may contribute to habitat partitioning for the many sympatric Quercus species in these mountains. We used measurements of specific leaf area (SLA) and leaf thickness (LT) of species across elevational gradients to test for functional differences among species that might indicate contrasting physiological tolerances. Leaf samples were gathered from multiple populations of seven common oak species in two mountain ranges. Drought tolerant species were expected to have lower SLA to reduce surface area and greater LT to maximize water use efficiency. Freezing tolerant species were predicted to demonstrate low SLA to reduce heat loss and higher LT to maximize mass-based photosynthetic rates and water use efficiency. We then expected similar LT and SLA values at the extremes of the elevation gradient and lower LT and higher SLA in the species sampled in the more moderate freezing- and drought-prone zones of the middle elevations. 

Results/Conclusions

Species was a significant factor in predicting SLA and LT (p<0.01 for SLA, p<0.1 for LT) across elevation. Pair-wise comparisons between most species were significant (p<0.05) for SLA and LT (with most comparisons p<0.01). Oak species that occupy similarly ranked habitats, determined by an ecological index that reflects elevation and water availability, were more likely to have non-significant differences in SLA and LT. Higher elevation species as a group had lower SLA values and higher LT values than low elevation species. The single winter-deciduous species (all others subevergreen) had the highest mean SLA and one of the lowest mean LT values and was significantly different (p<0.01) from all other oak species. These data suggest that oak species functional attributes do not shift smoothly with the climate gradient along the slope, nor do similar trait values converge among species living at elevation extremes. Drought and freezing pressures may have different influences on the functional traits of these oak species. These differences may be reflected in other leaf anatomy modifications or different life history strategies. Further measurements are planned to examine physiological tolerances of species to drought and seasonality, including leaf hydraulic conductance, stem and leaf anatomy, and tree phenology.