Integrating Tree Hydraulic Trait, Forest Stand Structure, and Topographic Controls on Ecohydrologic Function in a Rocky Mountain Subalpine Watershed
Authors
Lara Kueppers1,2* (lmkueppers@berkeley.edu), H. Marshall Worsham1, Thomas Powell3, Max Berkelhammer4, James Dennedy-Frank5, Chris Still6,7, Ian Breckheimer7, Ben Blonder1, Erica Siirila-Woodburn2
Institutions
1University of California–Berkeley, CA; 2Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA; 3University of the South, Sewanee, TN; 4University of Illinois, Chicago, Chicago, IL; 5Northeastern University–Boston, MA; 6Oregon State University, Corvallis, OR; 7Rocky Mountain Biological Laboratory, Crested Butte, CO
Abstract
Predicting forest responses to climate change, requires understanding interactions among species differences in drought sensitivity, forest structure and landscape heterogeneity. Researchers are integrating these elements within a high-elevation watershed, using (1) field measurements of tree hydraulic traits, transpiration, canopy water content, tree ring-width variation, and soil moisture; (2) airborne observations of canopy temperature and water content; and (3) watershed-scale process modeling. Emerging results relate to a subset of the hypotheses:
- Rooting depth and stem capacitance explain differences in transpiration patterns across the watershed’s dominant tree species.
Preliminary results indicate significantly less negative xylem-P50 (xylem pressure at 50% loss of conductivity) for aspen versus the conifers. Spruce and pine, which have low sapflow, tightly regulate conductance, even at relatively high water potentials. Fir, which has high peak sapflow, releases more water from stem tissue than the other species. Aspen, which also has high transpiration, maintains flow with a low safety margin between stomatal-P50 and its xylem-P50. - Across forest stands, radial growth covaries with topographic
Preliminary results show that annual growth declines linearly with elevation and topographic position index for both spruce and fir, with lower growth at ridge positions. Growth is positively associated with relative moisture-retaining capacity of a site. - Interannual differences in growth are smallest in convergent, high-elevation topographic positions with low incident solar radiation where the residence time of soil moisture is
Spruce growth is more sensitive to interannual precipitation variation and positively correlated with summer precipitation, while fir growth is inversely correlated with current and prior-year winter precipitation. This signal was strongest at high-elevation and convergent positions, suggesting that more and longer snow duration imposes energy rather than moisture limits to fir growth.
The team also demonstrate new simulation capabilities, using ParFlow-ELM-FATES to isolate the impact of subsurface lateral flow on evapotranspiration fluxes in complex terrain.