Historical Biogeography of Picea abies (L.) Karst. at Its Subarctic Limit in Northern Sweden

Late Holocene behaviour of the spruce (Picea abies (L.) Karst.) at its forest-limit in northem Sweden was assessed from age structure analyses and comparison of present-day distribution with older geobotanical surveys. At a landscape scale, spruce has maintained a dynamic equilibrium with climate during recent centuries. On very short time scales (years to decades), non-equilibrium responses are perceivable. Thus, for example, a recent episode of canopy dieback due to severe ground cooling has occurred. Ground frost may be part of the mechanism which limits further expansion of spruce against the westward/elevational gradient. Pulses of population growth, mainly in pre-existing stands, were inferred for the 1830s40s and the 1940s-60s, which reasonably related to shortterm climatic variability. Regeneration lagged slightly behind thermal optima. Possibly, instant responses to warming were precluded since generative processes presupposed a short period of vegetative recovery after cold episodes. Additionally, the peaking of precipitation after the temperature may be of some importance for this delay.