Measuring and modelling seasonal variation of carbon dioxide and water vapour exchange of a Pinus ponderosa forest subject to soil water deficit

We conducted ecosystem carbon and water vapour exchange studies in an old-g rowth Pinus ponderosa forest in the Pacific North-west region of the United States. The canopy is heterogeneous, with tall multiaged trees and an open , clumped canopy with low leaf area. Carbon assimilation can occur througho ut relatively mild winters, although night frosts can temporarily halt the process and physiological factors limit its efficiency. In contrast, carbon assimilation is often limited in the 'growing season' by stomatal closure associated with high evaporative demand (D) and soil water deficits. All of these factors present a challenge to effectively modelling ecosystem proce sses. Our objective was to generate an understanding of the controls on eco system processes across seasonal and annual cycles from a combination of fi ne-scale process modelling, ecophysiological measurements, and carbon and w ater vapour fluxes measured by the eddy covariance method. Flux measurement s showed that 50% and 70% of the annual carbon uptake occurred outside the 'growing season' (defined as bud break to senescence, similar to days 125-2 75) in 1996 and 1997. On a daily basis in summer, net ecosystem productivit y (NEP) was low when D and soil water deficits were large. Whole ecosystem water vapour fluxes (LE) increased from spring to summer (1.0-1.9 mm d(-1)) as conducting leaf area increased by 30% and as evaporative demand increas ed, while evaporation from the soil surface became a smaller portion of tot al LE as soil water deficits increased. The models underestimated soil evap oration, particularly following rain. In the SPA model, varying the tempera ture optimum for photosynthesis seasonally resulted in overestimation of ca rbon uptake in winter and spring, showing that in coniferous forests, assum ptions about temperature optima are clearly important. Daily estimates of s oil surface CO2 flux from measurements and site meteorological data demonst rated that modelling of soil CO2 flux based on an Arrhenius-type equation i n CANPOND overestimated CO2 respired from the soil during drought and when temperatures were low.