CLIMATE-CHANGE, TREE SPECIES DISTRIBUTIONS AND FOREST DYNAMICS - A CASE-STUDY IN THE MIXED-CONIFER NORTHERN HARDWOODS ZONE OF NORTHERN EUROPE

Three bioclimate variables (growing degree days after budburst, temper ature of the coldest month and a moisture index) are used in a model ( STASH) to predict potential range limits of north European tree specie s. CO2-induced climate warming scenarios cause major changes in these limits. The dominant conifers of the mixed conifer/northern hardwoods zone, Picea abies and Pinus sylvestris, retreat from the south and wes t while Fagus sylvatica and other temperate hardwoods spread to the no rth. A gap model (FORSKA 2) is used to simulate the associated transie nt responses of a forest reserve within this zone. In both dry Pinus- and moist Picea-dominated forest communities, Picea continues to incre ase while Pinus sooner or later declines. These changes are continuing successional (non-climatic) responses to the cessation of disturbance 150 years ago. Climate warming speeds up the succession, and allows F agus to establish and increase. The eventual decline of Picea however is delayed due to the persistence of old-growth stands. Picea forests subject to continuing disturbance show a more rapid shift to dominance by Fagus and other temperate hardwoods. Delayed immigration of new sp ecies, including Fagus, would favour early- successional species such as Betula pendula and Quercus spp. in a forest with reduced biomass an d diversity. The results emphasise the complex and sometimes counter-i ntuitive nature of transient responses, and the importance of consider ing disturbance history and potential migration rates when predicting the impacts of rapid climate change on forests.