TREE LIMIT DYNAMICS OF BETULA-PUBESCENS SSP TORTUOSA IN RELATION TO CLIMATE VARIABILITY - EVIDENCE FROM CENTRAL SWEDEN

The tree limit of Betula pubescens ssp. tortuosa (mountain birch) in the southern Swedish Scandes was monitored during 1972-1992. The study included various aspects of growth, vigour and reproduction of Betula. The main focus was on the character and mechanics of the tree limit/climate equilibrium system. The tree limit, which changed in response to a temperature rise early this century, remained constant in position and tree physiognomy did not change, although the past 50 yr or so have been colder. Indeed. growth and reproductive effort and capacity decreased in this period. Obviously, most resources in Betula were used to resist stress in the mature phase, resulting in delayed recession of trees. Unless drastic warming occurs, stem dieback is predicted for the near future. Even a minorclimatic disturbance would have an effect. because resources are gradually being depleted. Radial growth correlated most closely with the mean temperature in July. Particularly at the present-day tree limit, the variance in annual growth could be largely explained by climatic factors. The hypothesis is that long-term tree limit dynamics during the late Holocene is merely a matter of fluctuations in vegetative vigour and stature of old individuals. At the population level, response to climatic variability appears to be greatly delayed.