Tamarix ramosissima is a non-native tree species that is widely distributed throughout riparian ecosystems in the western US. Its presence has resulted in significant modification of riparian ecosystem structure, biogeochemistry and species biodiversity, and has been the focus of significant research linked to its water use requirements and associated impacts on ecohydrological processes. More recently, it has been the focus of functional trait-based research linking resource allocation to the capacity of Tamarix to cope with episodic disturbance from herbivory, fire and drought. To better understand patterns of local adaptation in resource allocation and associated drought tolerance, Tamarix cuttings sourced from populations across a broad climate gradient were planted in an experimental common garden. We hypothesized that populations from warm climates with little or no freezing limitations would display a greater reduction in growth, leaf area and whole-canopy gas exchange compared to populations from colder climates as a function of cold-adapted plants having a greater overall root area to leaf area ratio. We measured sap flux density, leaf water potential, d13C of recent photosynthates, and radial growth (n = 20 two-year old trees from cold and hot populations) prior to, and during a three-month drought treatment.
We found significant differences in sap flux density, water use per leaf area and canopy stomatal conductance across populations. Sap flux density was as much as 30% higher in populations from high elevation sites compared to low elevation sites. Water use per unit leaf area was highest in low elevation source populations prior to the onset of drought, although showed a much greater decline compared to high elevation source populations, with 65% decline compared to 35% decline respectively relative to pre-drought conditions. Mean canopy stomatal conductance was on average 40% greater in the highest elevation source population compared to the lowest elevation source population over the course of the measurement period. In addition, we found significant differences in growth rates across populations. High elevation source populations showed 60% greater radial growth compared to low elevation source populations over the same time period. These results suggest that Tamarix populations from low elevation regions are more sensitive to drought compared to high elevation regions. These results further suggest that effectiveness of restoration and management practices may vary depending on geographic location.