REGENERATION PULSES AND CLIMATE VEGETATION INTERACTIONS IN NONPYROGENIC BOREAL SCOTS PINE STANDS

1 Age and diameter distributions of nonpyrogenic, virgin Scots pine (Pinus sylvestris) populations were studied at six different sites of the Empetrum-Vaccinium type along a latitudinal gradient (65 degrees 54'-68 degrees 15'N) in northern Sweden. 1910 trees, including saplings and seedlings, were aged at their root collars in order to reconstruct regeneration patterns. 2 All populations had multimodal age structures. Obvious regeneration pulses occurred in the 1770s-80s and 1950s-70s. The three southernmost stands exhibited an additional regeneration pulse in the 1870s-80s. A regeneration cycle of about 100 years in the south and 200 years in the north were thus clearly expressed. Correlation with temperature changes over the past 300 years showed that recruitment peaks of Scots pine lagged 20-30 years behind the warm climate periods and fell to zero in the intervening cold periods. 3 A much smaller number of experimentally planted pine and spruce seeds established and survived in vegetation dominated by Empetrum hermaphroditum than in that dominated by Cladina spp. After two growing seasons, fresh weights of pine seedlings were also much lower in E. hermaphroditum vegetation than in Cladina spp. Spruce established less frequently than pine in both vegetation types. Naturally established pine seedlings were found almost exclusively in vegetation dominated by Cladina spp. 4 The wave-like regeneration pattern of pine may therefore result from successful establishment in periods with good seed production on ground dominated by Vaccinium spp. and Cladina spp. This vegetation type is favoured by warm dry conditions but its establishment possibly lagged behind climate change. The extreme amplitude between regeneration failures and successes found over the period studied may be caused by the specific complex of vegetation responses to climate variability found in this forest type. 5 The results have implications for predicting the effect of climatic warming.