Topography can influence tree species presence and/or absence by influencing the microclimate, including soil moisture, temperature, and relative humidity. However, many tree species can alter both physiological and morphological traits in order to grow within a range of hydroclimate gradients. In this study, we examined the topographic influence on inter- and intraspecific plant hydraulic traits of three conifers of the Northern Rockies: Pinus ponderosa, Picea engelmannii, and Pseudotsuga menziesii to understand the mechanisms that allow different plant species to co-exist across a watershed. We quantified differences in plant water potential (Ψ), xylem tissue vulnerability to cavitation (P50, or 50% loss of hydraulic conductivity), and safety margins for co-occurring trees from low and high elevations. We then investigated xylem source water across species and elevation to link rooting depth to xylem vulnerability.
Results/Conclusions
We found that xylem vulnerability to cavitation, as measured using P50, did not correspond to where tree species were typically found. For example, P. ponderosa were typically found in the most xeric locations while P. engelmannii were typically found in the most mesic locations, and yet P. engelmannii had more negative P50 values. P. menziesii had the lowest P50 value, but displayed little variation in vulnerability to cavitation across the elevational gradient. These patterns were reflected in the safety margins as well; P. menziesii had the widest safety margin, P. engelmannii was in the middle, and P. ponderosa had the narrowest safety margin. All three species were also using water sources deeper than 30cm in depth, allowing them to persist throughout the mid-summer drought. Overall, species specific hydraulic traits did not necessarily follow a predictable response to the environment; instead, a combination of physiological and morphological traits likely allow trees to persist across a broader hydroclimate gradient than would be otherwise expected from hydraulic trait measurements alone.