Neoglacial Climate Control of Subarctic Picea abies Stand Dynamics and Range Limit in Northern Sweden

Abstract

The study focused on the performance of the spruce (Picea abies [L.] Karst.) range limit in northern (subarctic) Sweden during the Neoglacial period of the Holocene, mainly after the climax of the Little Ice Age. Subfossil wood remains suggest that the geographic spruce tree limit has remained fairly stable for slightly more than the past 2000 14C yr. Previous postulates about delayed immigration and continual spread are contradicted in favor of a dynamic climate/spruce equilibrium. The mechanism restricting the spruce distribution was inferred to be severe annual ground frost characterizing this climatically continental and snowpoor region, which is crossed by the limit of discontinuous permafrost. An extensive (landscape scale) age structure analysis showed near-exponential population build-up over the past 100 yr or so. This manifested as densification of outlying stands and sparse range limit advance by some tens of kilometers. These responses clearly related to increased snowfall during the early (December) and late winter (March). In consequence, annual ground frost and permafrost declined in general, which made the least ground frost prone sites equable for establishment and persistent growth (less risk of winter desiccation) of spruce. These inferences are strengthened by tendencies for decreased spruce regeneration and vitality during the past decades, coincident with some exceptionally cold and snow-poor early winters. It is speculated that the structure and performance of the studied system, i.e. outliers checked by severe ground frost, is a small-scale analogy to the situation preceding the general late Holocene expansion of spruce in Fennoscandia.