UK CONIFER FORESTS MAY BE GROWING FASTER IN RESPONSE TO INCREASED N DEPOSITION, ATMOSPHERIC CO2 AND TEMPERATURE

Site studies have shown that conifer plantations in northern Britain h ave increased in General Yield Class (GYC) by 1 m(3) ha(-1) a(-1) per decade or more (20-40 per cent) since the 1930s. Large increases in fo rest productivity have also occurred in many other regions of Europe. Are these increases due to improved silvicultural practices or to incr eases in N deposition, CO2 and temperature? Two process-based mathemat ical models of forest growth were used to simulate the responses of co nifer forests growing in the Scottish southern uplands to increases in atmospheric N deposition, CO2 concentration and temperature, during t his century and next century. The models differed substantially in the ways in which underlying processes were represented: one simulated a managed plantation, the other a natural forest. Nevertheless, both sho wed that: (1) increases in N deposition, CO2 and temperature together might account for up to half of the observed increase in GYC this cent ury; (2) increased N deposition and CO2, considered separately, probab ly increased forest productivity by a modest amount (7-14 per cent), b ut their combined effect has been approximately additive; (3) increase d temperature, even when combined with increasing CO2 concentrations, promoted growth less than expected from site studies relating GYC to t emperature; and (4) substantial further increases in productivity, GYC , leaf area index and standing biomass are forecast during the next ce ntury as a result of increasing CO2 concentrations and continued N dep osition, with or without climatic warming. The predicted increases in GYC could be large enough to have profound effects on the forest indus try.