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 CO2 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 (Vcmax) and Ball-Berry slope (m) under prolonged summer drought and high temperature. A leaf photosynthetic model was used to determine Vcmax.
There was a pronounced seasonal pattern in Vcmax. The maximum value of Vcmax, 127 μmol 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, Vcmax declined as the soil water profile became depleted and the trees experienced extreme air temperatures, exceeding 40 °C. The decline in Vcmax was gradual in midsummer, however, despite extremely low predawn leaf water potentials (Ψpd, ∼ −4.0 MPa). Overall, temporal changes in Vcmax were well correlated with changes in leaf nitrogen content. During spring leaf development, high rates of leaf dark respiration (Rd, 5−6 μmol m−2 s−1) were observed. Once a leaf reached maturity, Rd remained low, around 0.5 μmol m−2 s−1. In contrast to the strong seasonality of Vcmax, m and marginal water cost per unit carbon gain (∂E/∂A) were relatively constant over the season, even when leaf Ψpd dropped to −6.8 MPa. The constancy of ∂E/∂A 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.