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

Liukang Xu (1) and Dennis D. Baldocchi (1, 2)

1. Ecosystem Science Division, Department of Environmental Science, Policy and Management, 151 Hilgard Hall, University of California at Berkeley, CA 94720, USA / 2. Author to whom correspondence should be addressed (baldocchi@nature.berkeley.edu) / Received October 21, 2002; accepted January 24, 2003; published online August 1, 2003

Summary

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 (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 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, V_cmax declined as the soil water profile became depleted and the trees experienced extreme air temperatures, exceeding 40 °C. The decline in V_cmax was gradual in midsummer, however, despite extremely low predawn leaf water potentials (Ψ_pd, ~ –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 µmol m^–2 s^–1) were observed. Once a leaf reached maturity, R_d remained low, around 0.5 µmol m^–2 s^–1. In contrast to the strong seasonality of V_cmax, m and marginal water cost per unit carbon gain (∂E/∂A) were relatively constant over the season, even when leaf Ψ_pd 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.

Keywords: Ball-Berry slope, dark respiration, marginal water cost per unit carbon gain, maximum carboxylation rate, maximum electron transport capacity.

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ISSN 0829-318X Copyright © 2002–2008 Heron Publishing