A process model analysis of environmental limitations on the growth of Sitka spruce plantations in Great Britain

Abstract

The extent to which growth is limited by climatic factors or soil fertility is of significance to foresters concerned with the management of Sitka spruce plantations throughout Great Britain. A simplified physiologically based model, driven with monthly weather data, provided a means to assess growth limitations imposed by solar radiation, subfreezing and suboptimal temperatures, soil drought and atmospheric vapour pressure deficits, wind and soil fertility at sites differing in maximum wood volume production by fourfold. To drive the model, conventional weather station data were extrapolated to provide estimates of precipitation, solar radiation, vapour pressure deficits, suboptimal temperatures and frequency of subfreezing conditions. Variation in solar radiation associated with topography accounted for nearly half the differences observed in plantation growth. Unfavourable temperatures reduced annual photosynthesis and growth only by 10-15 per cent as they occur primarily in months with limited solar radiation. Drought and vapour pressure deficits do not appear to offer major constraints on growth. Soil fertility, however, significantly limited growth on a number of sites. The modelling approach provides a measure of nutrient availability by predicting the maximum accumulation of nitrogen in the canopy. The modelling analysis suggests that foresters could improve forecasts of Sitka spruce plantation growth throughout the UK with better estimates of solar radiation and canopy nitrogen content than with more conventional sets of measurements.