Seasonal trends in leaf-level photosynthetic capacity and water use efficiency in a North American Eastern deciduous forest and their impact on canopy-scale gas exchange

• Vegetative transpiration (E) and photosynthetic carbon assimilation (A) are known to be seasonally dynamic, with changes in their ratio determining the marginal water use efficiency (WUE). Despite an understanding that stomata play a mechanistic role in regulating WUE, it is still unclear how stomatal and nonstomatal processes influence change in WUE over the course of the growing season. As a result, limited understanding of the primary physiological drivers of seasonal dynamics of canopy WUE remains one of the largest uncertainties in earth system model projections of carbon and water exchange in temperate deciduous forest ecosystems.
• We investigated seasonal patterns in leaf-level physiological, hydraulic, and anatomical properties, including the seasonal progress of the stomatal slope parameter (g₁; inversely proportional to WUE) and the maximum carboxylation rate (V_cmax).
• V_cmax and g₁ were seasonally variable; however, their patterns were not temporally synchronized. g₁ generally showed an increasing trend until late in the season, while V_cmax peaked during the midsummer months. Seasonal progression of V_cmax was primarily driven by changes in leaf structural, and anatomical characteristics, while seasonal changes in g₁ were most strongly related to changes in V_cmax and leaf hydraulics.
• Using a seasonally variable V_cmax and g₁ to parameterize a canopy-scale gas exchange model increased seasonally aggregated A and E by 3% and 16%, respectively.