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, temperature of the coldest month and a moisture index) are used in a model (STASH) to predict potential range limits of north European tree species. 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 west while Fagus sylvatica and other temperate hardwoods spread to the north. A gap model (FORSKA 2) is used to simulate the associated transient responses of a forest reserve within this zone. In both dry Pinus- and moist Picea-dominated forest communities, Picea continues to increase 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 Fagus 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 species, including Fagus, would favour early- successional species such as Betula pendula and Quercus spp. in a forest with reduced biomass and diversity. The results emphasise the complex and sometimes counter-intuitive nature of transient responses, and the importance of considering disturbance history and potential migration rates when predicting the impacts of rapid climate change on forests.