STAND STRUCTURE AND SUCCESSIONAL TRENDS IN VIRGIN BOREAL FOREST RESERVES IN SWEDEN

Fire history and stand structure was examined in twelve virgin forest stands situated within forest reserves in northern Sweden. The selecte d stands represented fire refuges as well as different successional st ages after fire. Six of the stands were dominated by Norway spruce (Pi cea abies L. Karat.), three were dominated by Scots pine (Pinus sylves tris L.), and three were dominated by hairy birch (Betula pubescens Eh rh.) or aspen (Populus tremula L.). In 3 of the southernmost stands, t he average fire interval was 34 to 65 years during the late 1600s to l ate 1800s, but since 1888 no fires had occurred in any of the stands. The absence of fire disturbance since 1888 is probably caused by the f ire suppression in the overall landscape. The standing volume of livin g trees ranged between 87 and 511 m(3) ha(-1) while the volume of dead trees, including both snags and logs, ranged between 27 and 201 m(3) ha(-1). The volume of dead trees constituted ca. 30% of the total stem volume. In the conifer dominated stands, there was a statistically si gnificant relationship between total stem volume, including both livin g and dead trees, and site productivity. A comparison between the amou nt of dead and living trees indicated substantial changes in tree spec ies composition in several stands. It is suggested that data on the am ount of dead trees, especially logs, and its distribution over decay c lasses could be used to examine the continuity of certain tree species . All stands had a multi-sized tree diameter distribution, which in mo st cases was similar to a reversed J-shaped distribution. In general s pruce was numerous in the seedling cohort and in small diameter classe s, indicating that its proportion in the stands was stable, or was inc reasing at the expense of pioneer tree species such as pine, aspen and silver birch (Betula pendula Roth.). The most numerous species in the seedling cohort, rowan (Sorbus aucuparia L.), was almost totally miss ing in the tree layer, indicating a high browsing pressure preventing rowan seedlings from growing into trees. The general increase of spruc e and the sparse regeneration of pioneer species, in the stands previo usly affected by fire, are discussed in relation to natural disturbanc e regimes, biological diversity and nature conservation policies. It i s proposed that reintroduction of fire disturbance is a necessity for future management plans of forest reserves. Other management practices to increase species diversity within forest reserves are also discuss ed. (C) 1997 Elsevier Science B.V.