Effects of temperature on the site productivity of Pinus sylvestris and lodgepole pine in Finland and Sweden

The influence of temperature regime on forest production in the northern boreal forest was studied by analysing the variation in site index for 20 provenance tests of lodgepole pine (Pinus contorta Dougl. var, latifolia Engelm.) in Finland and Sweden with respect to temperature. The site indices were regressed with several different temperature variables, which were obtained from interpolated monthly averages over 30 years from permanent meteorological stations nearby. There were strong correlations between all tested temperature variables and site index (r = 0.80-0.94). Site fertility influenced site index to some extent but the slope of the lest sites did not. An increase in temperature sum from 600 to 1200 degree days (corresponding to an increase of the mean temperature in July from 13 to 15 degrees C) was associated with an increase in site index of Scots pine (Pinus sylvestris L.) from 12.2 to 21.6 m (77%). Such an increase in site index can be assumed to correspond to an increase of 6,5 m(3) ha(-1) wood (183%) in mean annual increment. The effect of temperature was greater on Door sites than on fertile sites, and in harsh temperature regimes than in mild.