Seasonal trends in photosynthetic parameters and stomatal conductance of blue oak (Quercus douglasii) under prolonged summer drought and high temperature

Understanding seasonal changes in photosynthetic parameters and stomatal conductance is crucial for modeling long-term carbon uptake and energy fluxes of ecosystems. Gas exchange measurements Of CO(2) and light response curves on blue oak leaves (Quercus douglasii H. & A.) were conducted weekly throughout the growing season to study the seasonality of photosynthetic capacity (V(cmax)) and Ball-Berry slope (m) under prolonged summer drought and high temperature. A leaf photosynthetic model was used to determine V(cmax). There was a pronounced seasonal pattern in V(cmax). The maximum value of V(cmax), 127 mumol m(-2) s(-1), was reached shortly after leaf expansion in early summer, when air temperature was moderate and soil water availability was high. Thereafter, V(cmax) declined as the soil water profile became depleted and the trees experienced extreme air temperatures, exceeding 40 degreesC. The decline in V(cmax) was gradual in midsummer, however, despite extremely low predawn leaf water potentials (Psi(pd), similar to -4.0 MPa). Overall, temporal changes in V(cmax) were well correlated with changes in leaf nitrogen content. During spring leaf development, high rates of leaf dark respiration (R(d), 5-6 mumol m(-2) s(-1)) were observed. Once a leaf reached maturity, Rd remained low, around 0.5 mumol m(-2) s(-1). In contrast to the strong seasonality of Vc, m and marginal water cost per unit carbon gain (partial derivativeE/partial derivativeA) were relatively constant over the season, even when leaf Psi(pd) dropped to -6.8 MPa. The constancy of partial derivativeE/partial derivativeA suggests that stomata behaved optimally under severe water-stress conditions. We discuss the implications of our findings in the context of modeling carbon and water vapor exchange between ecosystems and the atmosphere.