Variation in environmental conditions, understorey species number, abundance and composition among natural and managed Picea abies forest stands

We studied four south-facing and three north-facing boreal spruce forest stands (ca. 0.1 ha each) in SE Norway with the aim of testing the hypothesis that former logging has long-term effects on boreal forest-floor vegetation. The stand series comprised unlogged natural forests and forests that were selectively or clear cut 60–70 years prior to our study. Each stand was described with respect to history of forestry impact and tree-stand structure. Environmental, species number, species abundance and species composition (vegetation gradients obtained as ordination axes) variables obtained for 25m x 1m plots in each stand were tested for among-stand differences. Significant among-stand differences were found, partly related to former forest management and partly due to among-stand differences in topography. Differences among stands related to management were found for tree stand density, highest in managed stands, and for Dryopteris expansa agg. and Luzula pilosa abundances, peaking in formerly clear-cut stands. Species number (at plot or stand scales) was weakly related to former management. On southerly as well as northerly aspects, gradients in species composition were found that separated plots according to former management. Differences among stand conditioned on topography resulted in opposite patterns in the two series of stands because among southerly stands the clear cut was the least while among northerly the clear cut was the most strongly sloping. Low-inclination sites tended more strongly to be paludified and to have high Sphagnum cover, and to have low abundance of specific microsites with small mosses and hepatics. Vegetation gradients related to soil moisture and microtopography were found for both aspects. A strong gradient in species composition related to tree influence at within-stand scales was found, with variation in species number. Existence of such a gradient should provide for significant biotic effects (of short or long duration) of the environmental changes that take place during forest re-growth: (1) the immediate creation of small or large tree-layer gaps by tree felling; and (2) the closing of the tree layer during the regeneration phase. Most notably, the phases at which the tree layer reaches minimum and maximum cover, respectively, may act as ‘bottlenecks’ for survival of forest-floor species. We conclude that forestry impacts understorey vegetation by way of changes in tree-layer structure and, to a lesser extent, substrate availability and the local environment, during forest regrowth. The extent and duration of this impact will depend on a complex set of factors. Our results are consistent with the view that if maintenance of species diversity is aimed at, environmental considerations should be built into forest management practices, preferably by mimicking the natural structural dynamics of the tree layer.