Contrasting responses of a simple terrestrial ecosystem model to global change

A simple parameter-scarce model of vegetation dynamics is introduced which describes a single dominant vegetation type using three equations for veget ation carbon, fractional coverage by the vegetation and soil carbon. The mo del is driven by net primary productivity, as calculated by a submodel of p hotosynthesis and plant respiration. Three categories of response to prescr ibed increases in atmospheric CO2 concentration and temperature are identif ied: (1) The emergence of a new dominant vegetation type in a cold environm ent. When the vegetation is establishing, there is a long period (dependent upon the 'seeding' fraction) of slow vegetation spread. This is followed b y a rapid increase in fractional cover as the vegetation moves to being in near equilibrium with the perturbed climate, causing a pulse of positive Ne t Ecosystem Productivity (NEP). (2) Robust behaviour of an established vege tation in a warm environment. Extra carbon assimilated is mostly allocated to spreading, but because the fractional cover is nearly complete, the carb on is further diverted into extra litter fall ('self shading'). The soil ca rbon reservoir grows and NEP is initially weakly positive. However, soil re spiration increases more rapidly with warming than net primary production, causing a gradual switch to weakly negative NEP. (3) 'Die-back' can occur a t high temperatures. Net primary productivity starts to decrease, causing a decline in litter supply and shrinkage of the soil carbon reservoir. Event ually, there is not enough incoming carbon to match natural disturbance rat es and the vegetation rapidly decreases in fractional coverage until it dis appears. This causes a large pulse of negative NEP. The timing of such 'die -back' is strongly linked to the chosen description of how dark and plant r espiration depend upon temperature. (C) 2000 Elsevier Science B.V. All righ ts reserved.