Response of forest trees to rapid climate change, at the retracting (xeric) limits: Lessons from common gardens
This presentation deals with quantitative (growth) responses of provenances of four species (Norway spruce, Scots pine, European beech and sessile oak) adapted to various macroclimatic conditions. Analysis of phenotypic behavior as observed in common gardens (provenance tests) is indispensable, as it provides the only realistic possibility to estimate and validate quantitative responses to climatic changes. Climatic adaptation is understood as being determined by the inherited potential and constraints of the genetic system of the species, and of the extant diversity of the population.
Populations at the low elevation xeric limits of distribution ranges will experience the most rapid and extreme impacts of expected climatic changes. Ultimately, the strategy of forest reproductive material deployment and of nature conservation depends on the answer: how much human assistance is necessary to support adaptive adjustment within one generation of trees.
Asymmetry of response indicated by response functions (both reaction norms and transfer functions) implies most probably the simultaneous role of plasticity and selection in climatic adaptation, and maintains an adaptive non-equilibrium;
Limits of climatic tolerance are genetically determined; population fitness declines and may lead to mass mortality when the potential for adaptation is exhausted, i.e. there is no realistic opportunity for spontaneous evolution;
Differences in sensitivity to climatic factors exist between investigated species. Beech stands out with higher phenotypic plasticity than the other tree species;
It is probable that plasticity is enhanced by certain ecological/climatic conditions;
At the retracting xeric limit of distribution, selection pressure of extreme events may narrow genetic variation and lead to decline of plasticity, and ultimately to local extinction;
Expected rapid changes in the next decades will affect first of all present forest stands in their lifetime. Their adaptation potential will mainly depend on the level of their phenotypic plasticity. The importance of this trait should be recognized both in breeding and also in use of forest reproductive material;
Survival trends which differ from growth trends demonstrate the need for caution against all-too-courageous selection of distant provenances based on early growth data. Growth data have to be considered together with criteria of tolerance and adaptability to anticipated random climate extremes. Prudent provision for adaptability and plasticity increases the probability of success in the next generation.