As riparian habitats of Arizona rapidly change, many once permanent streams are converting to temporary flows. This drives the need to identify localized performance of riparian tree populations with respect to water availability (hydrologic niche) and temperature. Towards understanding the localized physiological behavior of a population from a given hydrologic niche, we examined how stream type, depth to water source, and temperature influence plant performance during establishment. We collected Fremont cottonwoods (Populus fremontii) cuttings from paired ephemeral and perennial streams. We grew 48 individuals for 3.5 months in rhizotrons (cylindrical tubes filled with soil) of different lengths (0.9m, 1.7m, and 2.7m) to simulate stressors related to water source depth, and in 2 locations (Phoenix and Flagstaff, AZ) that differ in mean annual temperature by >15oC. We measured growth, leaf water potential, photosynthesis, hydraulic conductivity, leaf turgor loss point (TLP), and root area to leaf area ratios (AR:AL) to address three hypotheses: H1) populations from different hydrologic niches differ in their hydraulic traits (e.g., xylem cavitation resistance, TLP, AR:AL); H2) compared to perennial populations, ephemeral populations display increased rates of root development; and H3) perennial populations show limited regulation of transpiration and have higher rates of photosynthesis.
Results/Conclusions
Our results revealed three patterns relevant to our hypotheses. First, related to H1, ephemeral populations supported a AR:AL that was 1.15 times greater than perennial populations (p<0.01, n=39), and perennial populations experienced turgor loss at lower levels of water stress compared to ephemeral populations (TLP = -1.92 versus -2.31 MPa). Second, in support of H3, ephemeral populations were generally more efficient at photosynthesis, with a 20% increase in the rate of CO2 uptake (Vcmax), and a 15% increase in the velocity of the electron transport chain (Jmax). Our results indicate that there is conserved physiological performance in populations of P. fremontii from different hydrologic niches, and these differences could be used to inform forest and wetland management strategies aimed at mitigating the effects of climate change, which is leading towards a shift from perennial to ephemeral streams in the western US. This has major implications for conservation and restoration efforts that are crucial to maintaining riparian species that support rich communities of organisms in arid environments.