Contribution of competition for light to within‐species variability in stomatal conductance

Sap flux ( J S ) measurements were collected across two stands dominated by either trembling aspen or sugar maple in northern Wisconsin. Observed canopy transpiration ( E C ‐ obs ) values derived from J S were used to parameterize the Terrestrial Regional Ecosystem Exchange Simulator ecosystem model. Modeled values of stomatal conductance ( G S ) were used to determine reference stomatal conductance ( G Sref ), a proxy for G S that removes the effects of temporal responses to vapor pressure deficit ( D ) on spatial patterns of G S . Values of G Sref were compared to observations of soil moisture, several physiological variables, and a competition index ( C I ) derived from a stand inventory, to determine the underlying cause of observed variability. Considerable variability in G Sref between individual trees was found, with values ranging from 20 to 200 mmol m −2 s −1 and 20 to 100 mmol m −2 s −1 at the aspen and maple stands, respectively. Model‐derived values of G Sref and a sensitivity to D parameter ( m ) showed good agreement with a known empirical relationship for both stands. At both sites, G Sref did not vary with topographic position, as indicated by surface soil moisture. No relationships were observed between G Sref and tree height ( H T ), and a weak correlation with sapwood area ( A S ) was only significant for aspen. Significant nonlinear inverse relationships between G Sref and C I were observed at both stands. Simulations with uniform reductions in incident photosynthetically active radiation ( Q 0 ) resulted in better agreement between observed and simulated E C . Our results suggest a link between photosynthesis and plant hydraulics whereby individual trees subject to photosynthetic limitation as a result of competitive shading exhibit a dynamic stomatal response resulting in a more conservative strategy for managing hydrologic resources.