Temperate Grassland Responses to Climate Change: an Analysis using the Hurley Pasture Model

The Hurley Pasture Model is process-based and couples the carbon, nitrogen and water cycles in the soil-grass-animal system. It was used to examine the responses of grasslands in southern, lowland and northern, upland climates in Britain. Short-term response to step-wise increases in CO # concentration (350 to 700 μmol mol−") and temperature (5 °C) were contrasted with long-term equilibrium (the term ‘equilibrium’ is equivalent to ‘steady state ’ throughout this paper) responses and with responses to gradually increasing [CO # ] and temperature. Equilibrium responses to a range of climate variables were also examined. Three conclusions were drawn regarding the interpretation of experiments : (1) initial ecosystem responses to stepwise changes can be different in both magnitude and sign to equilibrium responses, and this can continue for many years ; (2) grazing can drastically alter the magnitude and sign of the response of grasslands to climate change, especially rising temperatures ; and (3) effects of changes in climate, especially temperature and rainfall, are likely to be highly site-specific. It was concluded that experiments should try to lessen uncertainty about processes within models rather than try to predict ecosystem responses directly. Three conclusions were also drawn about the operation of grasslands as carbon sinks: (1) increasing [CO # ] alone will produce a carbon sink, as long as it continues to accelerate photosynthesis and increase net primary productivity ; (2) by contrast, increasing temperatures alone are likely to produce a carbon source, because soil respiration is accelerated more than net primary productivity, even when assuming the same temperature function for most soil and plant biochemical processes ; and (3) the net effect of projected increases in [CO # ] and temperature is likely to be a carbon sink of 5–15 g C m−# yr−" in humid, temperate grasslands for several decades, which is consistent with the magnitude of the hypothesized current global terrestrial carbon sink. # 1997 Annals of Botany Company