Integrating Tree Hydraulic Trait, Forest Stand Structure, and Topographic Controls on Ecohydrologic Function in a Rocky Mountain Subalpine Watershed

Authors

Lara Kueppers1,2* ([email protected]), 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; 3The University of the South, Sewanee, TN; 4University of Illinois–Chicago, IL; 5Northeastern University, Boston, MA; 6Oregon State University–Corvallis, OR; 7Rocky Mountain Biological Laboratory, Crested Butte, CO

Abstract

Large uncertainties persist regarding forest responses to chronic climate change, especially with episodic drought disturbance. This is due, in part, to a lack of understanding of how forest composition and structure interact with physical landscape heterogeneity to influence ecosystem function and vulnerability. Tree species differences in hydraulic traits influence sensitivity of transpiration to moisture deficits, as well as sensitivity of growth, mortality, and fire risk during drought. Understanding species differences is critical for projecting future changes in forest structure and hydrological fluxes. Also, in mountainous regions, exposure to climate variability is mediated by microclimatic conditions that vary by topographic position. Forests occurring in high elevation, lower radiation, or convergent topo-positions are expected to be buffered from historical and future droughts. Alternatively, they may be more sensitive to future droughts compounded by warming due to maladapted stand structure and composition. Similarly, where stands are already exposed to high radiation and low moisture, they may be less sensitive to additional drought and warming, especially if dominated by drought-tolerant species, or, alternatively, warmer droughts could push the stand over a threshold to a nonforest state. These divergent possibilities have implications for watershed function.

The project is pursuing four hypotheses in a high-elevation, low diversity Rocky Mountain watershed:

  1. Rooting depth and stem capacitance, more so than other hydraulic traits, explain the differences in diurnal and seasonal transpiration patterns and in growth sensitivity to climate across the watershed’s four dominant tree species.
  2. Across forest stands, soil moisture, transpiration, canopy water content, and radial growth covary with stand density, leaf area, species dominance, and topographic position.
  3. Interannual differences in stand-scale soil moisture, canopy water content, transpiration, and growth are smallest in convergent, high-elevation topographic positions with low incident solar radiation where the residence time of soil moisture is longest. Interannual differences are expected to be largest in convergent zones with high incident radiation, where maximum transpiration and the potential range of fluxes is very high.
  4. Forest structure and composition have little influence on seasonal soil moisture and transpiration dynamics at landscape positions with convergent topography or where subsurface lateral flow contributions are high but have stronger influence on these dynamics in divergent and neutral positions, particularly during drought.

The approach integrates: (1) field measurements of tree hydraulic traits, transpiration, canopy water content, tree ring–width variation, and soil moisture; (2) airborne observations of transpiration and canopy water content; and (3) coupled 3D hydrologic-vegetation demographic modeling using the ParFlow-FATES-Hydro model.