Stand structure and successional trends in virgin boreal forest reserves in Sweden

Abstract

Fire history and stand structure was examined in twelve virgin forest stands situated within forest reserves in northern Sweden. The selected stands represented fire refuges as well as different successional stages after fire. Six of the stands were dominated by Norway spruce ( Picea abies L. Karst.), three were dominated by Scots pine ( Pinus sylvestris L.), and three were dominated by hairy birch ( Betula pubescens Ehrh.) or aspen ( Populus tremula L.). In 3 of the southernmost stands, the average fire interval was 34 to 65 years during the late 1600s to late 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 fire suppression in the overall landscape. The standing volume of living 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 significant relationship between total stem volume, including both living and dead trees, and site productivity. A comparison between the amount of dead and living trees indicated substantial changes in tree species composition in several stands. It is suggested that data on the amount of dead trees, especially logs, and its distribution over decay classes could be used to examine the continuity of certain tree species. All stands had a multi-sized tree diameter distribution, which in most cases was similar to a reversed J-shaped distribution. In general spruce was numerous in the seedling cohort and in small diameter classes, indicating that its proportion in the stands was stable, or was increasing 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 missing in the tree layer, indicating a high browsing pressure preventing rowan seedlings from growing into trees. The general increase of spruce and the sparse regeneration of pioneer species, in the stands previously affected by fire, are discussed in relation to natural disturbance regimes, biological diversity and nature conservation policies. It is 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 discussed.