Late Holocene reproductional patterns of Pinus sylvestris and Picea abies at the forest limit in central Sweden

Abstract

The age structure was studied of two stands of Pinus sylvestris L. and Picea abies (L.) Karst. growing on "fire-safe," opposing (north- and south-facing, respectively) slopes at the forest limit in the Scandes Mountains, in central Sweden. It is suggested that the spatial species segregation is due to different microclimatic demands during seedling establishment and early survival stages. Establishment of pine correlated positively with the mean "tetraterm" (June – September) air temperature over 20-year periods (1861 – 1980). A sequence of thermally favourable years appears to be important for population net growth at the forest limit. Establishment of spruce may relate to an absence of early summer drought, stable snow cover during the winter, and certain high-temperature demands for seed maturation. The extant pine population died off almost entirely during the climatic deterioration of the Little Ice Age (prior to A.D. 1700). From the mid-18th century onwards a new pine population was established, the seed parents being the few remaining survivors from the foregoing decimation. This process conforms broadly with the known pattern of climatic change. After the climax of the Little Ice Age spruce colonized new territory, a consequence of the summer warming combined with the still fairly humid conditions in early summer. No spruce established during the climax phase of warming during the 20th century (A.D. 1925 – 1940).