Modelling the response of tree growth to temperature and CO sub(2) elevation as related to the fertility and current temperature sum of a site

A methodology for simulating climate change impacts on tree growth was introduced into a statistical growth and yield model in relation to variations in site fertility and location implemented with current temperature sum. This was based on a procedure in which the relative enhancement in stem volume growth was calculated from short-term runs of a physiological simulation model for Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.) and silver birch (Betula pendula Roth.) stands. These simulations were made for a set of stands with species-specific variations in stand characteristics, location and fertility type first in current climatic conditions and then in different combinations of CO sub(2) and temperature elevations. Based on these simulations, the relative enhancement of volume growth induced by the climate change (relative scenario effect, RSEv) was calculated and modelled in relation to: (i) CO sub(2) and temperature elevation, stand density and the competition status of the tree in its stand, and (ii) variations in site fertility type and current temperature sum of a stand. Finally, these transfer functions for RSEv were applied to adapt the stem volume growth in the statistical growth and yield model to reflect the response to climate change. The simulations for Scots pine, Norway spruce and silver birch showed that the RSEv transfer functions worked logically in combination with the statistical growth and yield model, and that the results were logical with respect to the differences in productivity between sites and locations. However, the impact of location, as implied by current temperature sum, clearly had larger impact on growth reaction to climate change than did the variation between the site types. Regarding regional variations in current thermal conditions, the simulations of productivity under climate change were well in line with previous predictions; i.e. stem volume growth was enhanced more in northern than in southern Finland, because current temperatures have a greater limiting effect on growth in the north.