Adaptive versus non-adaptive responses to drought in a non-native riparian tree/shrub, Tamarix spp.
Rapidly colonizing species often thrive in a wide-range of conditions due to a high degree of phenotypic plasticity that results in populations of "general purpose" genotypes. Alternatively, species with high genetic variation could rapidly respond to forces of selection such that a local population evolves traits that provide an advantage in its local environment. We tested this generalist versus local adaptation paradigm in a widely distributed, recently introduced riparian tree/shrub, Tamarix spp. Using an eighty-day, mid-summer common garden drought experiment, we tested three inter-related hypotheses: (1) stomatal sensitivity to soil water depletion is lower in genotypes from a low-elevation, ephemerally flowing river than genotypes from a low-elevation, perennially flowing river, indicative of local adaptation to hydrological conditions, (2) stomatal sensitivity to soil water depletion is lower in genotypes from high-elevations with regular freeze-thaw exposure than genotypes sourced from locations with no freeze-thaw exposure, indicative of local adaptation to temperature conditions, and (3) differences among genotypes in drought sensitivity are correlated with differences in fine root area to leaf area ratios (Ar:Al). For the most part, results did not support hypothesis 1, but largely supported hypotheses 2 and 3. Specifically, sap-flux scaled canopy transpiration (El) declined 14 days earlier in the low-elevation populations after drought initiation compared to El of the highest elevation population with the highest freeze-thaw exposure. Root area to leaf area ratios of the two low-elevation populations were also only 42% and 55% of Ar:Al in the highest elevation population. Results indicate that the high degree of reported Tamarix hybridization since its introduction to North America has largely produced a swarm of "generalist" genotypes in terms of drought sensitivity. Nevertheless, rapid changes in ecohydrologic conditions may result in some Tamarix populations becoming maladapted sooner to reductions in available water than others in the western US.