Photosynthetic Productivity of Oak Stand under Variable Environmental Conditions and Water Availability

An especial method was applied for calculating the photosynthetic productivity of oak (Quercus robur L.) stand. The methodological basis is that the efficiency of photosynthetic utilization of intercepted PAR over the daylight period depends on environmental conditions and distribution of PAR within the canopy. This method was also used to determine the photosynthetic productivity of individual trees within the stand and to estimate the contribution of trees with various extents of growth retardment to the total photosynthetic productivity. For the oak stand in the forest-steppe region, the photosynthetic efficiency was expressed as a function of climatic variables such as solar irradiance day sum, day average temperature, air humidity, and water availability (quantified as the predawn leaf water potential, PLWP). This functional dependence differed for sun-exposed and shade leaves. Under optimum water availability (PLWP = −0.5 MPa), the photosynthetic productivity of a 60-year-old oak stand amounted 44.5 t C/(ha year) but it declined with the decrease in water availability. Under severe water deficit, when PLWP dropped to −2.1 MPa, the photosynthetic productivity decreased 17-fold and equaled 2.6 t C/(ha year). The strongest decline in photosynthetic productivity occurred on sunny days under moisture deficit in dominant and codominant trees belonging to growth classes I and II.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 522–531.Original Russian Text Copyright © 2005 by Molchanov.