THE TEMPORAL RESPONSE OF FOREST ECOSYSTEMS TO DOUBLED ATMOSPHERIC CO2CONCENTRATION

Vegetation responses to high [CO2] include both direct photosynthetic effects and indirect effects associated with various plant and soil fe edbacks. Synthesis of these direct and indirect effects requires ecosy stem process models describing the cycling of carbon and essential min eral nutrients through plants and soils. Here we use the ecosystem mod el G'DAY to investigate responses to an instantaneous doubling of [CO2 ]. The analysis indicates that the magnitude and even direction of the growth response to high [CO2] can vary widely on different timescales , because responses on different timescales are determined by differen t ecosystem-level feedbacks and hence by different sets of key model p arameters. Of particular importance are parameters describing the flex ibility of plant and soil nitrogen to carbon (N:C) ratios; large respo nses occur if N:C ratios decline significantly at high [CO2], with lit tle or no response if N:C ratios are inflexible. According to G'DAY, t he CO2-response changes over time because responses on longer timescal es are dictated by the N:C ratios of less rapidly cycled organic matte r.