As global temperatures rise, many plant populations in mountains are migrating away from established elevation limits. The extent to which these shifts are caused by plant vulnerabilities to variations in temperature and precipitation is still not fully understood. We used a system of montane oak species that occur sympatrically in SE Arizona to ask whether the stratified elevation ranges were due to either drought tolerances at the lower range edges or freezing tolerances at the upper range edges. We examined species habitats, drought avoidance (leaf abscission during drought), drought resistance (wilting point), and stem tolerance to freezing (percent injury measured as electrolyte leakage from tree branches measured during multiple seasons) in six oak species (N=138).
Results/Conclusions
Plant vulnerabilities to drought conditions appear to be the strongest influence on the elevation ranges of these montane oaks. Species and collection season were significant predictive factors in stem freezing injury (p<0.001 at -15oC). Between-species differences in acclimated stem injury were significant (p<.001), but the results were not associated with elevation differences. Elevation was a factor in summer-collected stems: elevation by species interactions were a significant predictor of non-acclimated stem freezing injury (p=.05, R2=.23), but between-species differences had the strongest effect. All species demonstrated acclimation abilities, and most can to acclimate to avoid injury at temperatures lower than those they commonly experience.
Species differed in access to water during the drought season, as indicated by declining pre-dawn water potential with decreasing elevation (R2=0.79, p<0.001); lower elevations experienced much higher temperatures during drought months, increasing leaf-to-air vapor pressure deficits (VPD). Although wilting points (water potential at turgor loss) in leaves are often associated with climatic conditions, no differences existed between species, and elevation had a weak positive effect on wilting point in all study individuals (R2=0.04, p=0.03). There was a stronger positive association between leaf capacitance at full turgor, or leaf water storage, and elevation (R2=0.13, p<0.001), demonstrating greater hydraulic safety margins in leaves of species that occur in climates of lower leaf-to-air VPD. Water storage aligns with leaf phenology: lower elevation species were more likely to lose abscise leaves and enter dormancy during drought, while higher elevation species were not able to lose leaves but perhaps avoided wilting by maintaining a greater hydrated safety margin in their leaves. Our work indicates that avoiding drought through leaf abscission strategies, rather than variation in leaf wilting points, restricts elevation ranges in this dry mountain system.