Current evidence suggests that plants recently introduced to novel habitats often display functional traits that are highly adapted to local environmental conditions. Rapid local adaptation may be advantageous for establishing functional niche space across wide ranging habitats, but may leave recently introduced, invasive plants susceptible to the effects of climate change and other anthropogenic forces. In this study, we evaluated potential patterns of local adaptation in traits related to long-distance water transport in a recently introduced shrub/tree that is now widespread in riparian areas. We addressed whether xylem traits varied in Tamarix ramosissima x chinensis genotypes sourced across a broad thermal distribution gradient in the southwestern United States. We hypothesized that vessel area is largest at the lowest elevation sites, and decreases as populations occur along an increasing frost frequency gradient. Additionally, we hypothesized that hydraulic efficiency and vulnerability to drought-induced xylem cavitation is greater in low elevation populations as an indirect consequence of greater vessel diameter. A Tamarix common garden was established in 2015 from 18 source populations ranging in elevation from 15 to 1900 m. One-year old branches were analyzed for mean vessel area, maximum hydraulic conductivity, cavitation vulnerability (using the traditional centrifuge method), and sapwood density.
Results/Conclusions
Local adaptation in Tamarix xylem traits is evident from this study. Results indicate that vessel area is largest at the lowest elevation populations and decreases with elevation (R2=0.60, p=0.0007). The mean area (±se) of individual vessels for the low elevation sites ranged from 1067 (99) to 1562 (82) µm2, while the high elevation sites ranged from 624 (40) to 932 (31) µm2. The total percent vessel area is also greater for the lower relative to the higher elevation populations (8.9 ± 0.7% and 5.4 ± 0.7%, respectively; R2=0.67, p=0.0135). A significant but weak relationship with elevation also occurred for stem density (R2=0.23, p<0.0001). This indicates that along sites with increasing frost events, the maximum stem specific hydraulic conductivity should be greater for the low elevation populations with both greater individual and total vessel area than the higher elevation ones. If cavitation vulnerability deceases with elevation, then there is a potential link between vessel diameter and pit-pore size for this 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 this invasive species.