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 vegetation carbon, fractional coverage by the vegetation and soil carbon. The model is driven by net primary productivity, as calculated by a submodel of photosynthesis and plant respiration. Three categories of response to prescribed increases in atmospheric CO2 concentration and temperature are identified: (1) The emergence of a new dominant vegetation type in a cold environment. When the vegetation is establishing, there is a long period (dependent upon the ‘seeding’ fraction) of slow vegetation spread. This is followed by a rapid increase in fractional cover as the vegetation moves to being in near equilibrium with the perturbed climate, causing a pulse of positive Net Ecosystem Productivity (NEP). (2) Robust behaviour of an established vegetation in a warm environment. Extra carbon assimilated is mostly allocated to spreading, but because the fractional cover is nearly complete, the carbon is further diverted into extra litter fall (‘self shading’). The soil carbon reservoir grows and NEP is initially weakly positive. However, soil respiration increases more rapidly with warming than net primary production, causing a gradual switch to weakly negative NEP. (3) ‘Die-back’ can occur at high temperatures. Net primary productivity starts to decrease, causing a decline in litter supply and shrinkage of the soil carbon reservoir. Eventually, there is not enough incoming carbon to match natural disturbance rates and the vegetation rapidly decreases in fractional coverage until it disappears. 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 respiration depend upon temperature.