Hydraulic traits of co-existing conifers do not correlate with local hydroclimate condition: a case study in the northern Rocky Mountains, USA

In this study, we examined the inter- and intra-specific variation of hydraulic traits of three conifers of the Northern Rockies: Pinus ponderosa, Picea engelmannii, andPseudotsuga menziesii to understand the mechanisms that allow different plant species to co-exist across a watershed. We quantified differences in plant xylem water potential (psi(x)), xylem tissue vulnerability to cavitation (P-50, or psi causing 50% loss of hydraulic conductivity), and safety margins for co-occurring trees from low and high elevations. We then investigated xylem vulnerability to cavitation with rooting depth. We found that xylem vulnerability to cavitation did not correspond to where tree species were found in the landscape. For example, P. ponderosa grew in more xeric locations, while P. engelmanniiwere largely confined to more mesic locations, yetP. engelmannii had more negativeP(50) values. P. menziesii had the lowestP(50) value, but displayed little variation in vulnerability to cavitation across the hydroclimatic gradient. These patterns were also reflected in the safety margins; P. menziesii had the widest safety margin, P. engelmannii was intermediate, and P. ponderosa displayed the narrowest safety margin. All three species were also using water sources deeper than 30 cm 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 hydroclimatic gradient than would be otherwise expected from hydraulic trait measurements alone.

Automated summary

This study examined the hydraulic traits of three conifers in the Northern Rockies to understand the mechanisms allowing different plant species to co-exist across a watershed. The study found that the hydraulic traits of trees did not necessarily correspond to their specific locations in the landscape, and that a combination of physiological and morphological traits likely allowed trees to persist across a broader hydroclimatic gradient.