Current evidence suggests that plants often display functional traits that are highly adapted to local environmental conditions. We evaluated potential patterns of local adaptation in traits related to long-distance water transport in a native (Populus fremontii), and a recently introduced (Tamarix spp) riparian tree species. We established Populus and Tamarix common garden to address whether xylem traits varied in genotypes sourced across a broad thermal distribution gradient in the southwestern United States. Similar to a leaf economic spectrum, we hypothesized that these species would display a similar heat-tolerance / frost-tolerance spectrum of leaf and wood traits. Specifically, heat-adapted genotypes would have large vessels and high specific leaf area (SLA) to support high transpiration rates required for high canopy evaporative cooling, and cold-adapted genotypes have small vessels to limit freeze-thaw cavitation, and low SLA. As an indirect consequence, we also hypothesized that hydraulic efficiency and vulnerability to drought-induced xylem cavitation is greater in low elevation populations. We compared specific leaf and petiole area (SLA and SPA , cm2 g-1), and petiole xylem size, across different source populations. Wood hydraulic and anatomical traits were measured in one-year old branches and included mean vessel area, hydraulic mean diameter, maximum hydraulic conductivity, cavitation vulnerability (using the double-ended pressure chamber method), and sapwood density.
Results/Conclusions
Local adaptation in xylem traits was evident in both species from this study. Results for P. fremontii indicate that SLA (R2=0.64, p=0.05) and SPA (R2=0.39, p=0.09) decreased with increasing elevation, having thinner and smaller leaves to reduce radiative heat load and thermal capacitance. Petiole xylem size did not differ across the gradient. Wood vessel area in both species decreased with elevation (R2=0.33, p=0.02). The mean (±se) area of individual vessels for P. fremontii was 888 (47) µm2 and 667 (50) µm2 for the low and high elevation sites, respectively, while for Tamarix it was 1325 (84) µm2 and 775 (36) µm2. This indicates that along an increasing frost event continuum with individual and total xylem area decreasing, the maximum stem specific hydraulic conductivity should also decrease. If cavitation vulnerability deceases with elevation, then there is a potential link between vessel diameter and pit-pore size for these species. If otherwise, then both the frequency and intensity of frost events that induces freeze-thaw cavitation may be what regulates vessel size. Rapid local adaptation could be a major factor that has allowed for the widespread distribution of both species.