A bioclimatic model for the potential distributions of north European tree species under present and future climates

A bioclimatic model based on physiological constraints to plant growth and regeneration is used here in an empirical way to describe the present natural distributions of northern Europe's major trees. Bioclimatic variables were computed from monthly means of temperature, precipitation and sunshine (%) interpolated to a 10′ grid taking into account elevation. Minimum values of mean coldest-month temperature (T_c) and 'effective' growing degree days (GDD*) were fitted to species' range limits. GDD* is total annual growing degree days (GDD) minus GDD to budburst (GDD°). Each species was assigned to one of the chilling-response categories identified by Murray, Cannell & Smith (1989) to calculate GDD°. Maximum T_c values were fitted to continental species' mild-winter limits and other deciduous species' warm-winter limits. Minimum values of relative growing-season moisture availability (α*) were estimated from silvics. Growth indices were calculated based on potential net assimilation (a quadratic in daily temperature) and α*. Growth can be rapid near a range limit, e.g. Picea abies (L.) Karsten in southern Sweden. Climate changes expected for CO₂ doubling were projected on to the grid. Simulated distribution changes reflected interspecific differences in response to changing seasonality. Chilling responses proved important, e.g. the predicted range limit of Fagus sylvatica L. contracts in the west while expanding northwards as winters warm more than summers. Transient responses to climate change can be modelled using the same information provided that fundamental and realized mche limitations are distinguished - a caveat that underlines the dearth of experimental information on the climatic requirements for growth, and especially regeneration, of many important trees.