DYNAMICS OF UNDERSTORY VEGETATION IN AN OLD-GROWTH BOREAL CONIFEROUS FOREST, 1988-1993

Understorey vegetation changes in a South Norwegian old-growth conifer ous forest were studied between 1988 and 1993 in 200 1-m(2) vegetation plots. Our aims were to quantify the amount of between-year compositi onal change, and to elaborate the environmental basis for long-term ve getation change, including the previously identified gradient structur e with a major gradient related to topography (and soil nutrient statu s and soil depth) and a minor gradient reflecting paludification and c anopy coverage. Species richness (yearly mean and cumulative species n umber) and change in species richness differed between vascular plants and cryptogams, and between forest types. The number of vascular plan t species decreased in pine forest in dry years; bryophyte species num ber increased in spruce forest. Statistically significant vegetation c hange, as tested by constrained ordination (CCA) with time as the cons training variable, is demonstrated for most one-year periods and for t he five-year period in most forest types. Vegetation change along iden tified gradients, measured as plot displacement along DCA ordination a xes, also occurred. The magnitude of year-to-year vegetation change wa s related neither to forest type nor to one-year period; different res ponses to climatic and environmental change were observed in each fore st type. The largest average displacement observed, from medium-rich s pruce forest towards poor spruce forest, was interpreted as a long-ter m trend. Humus-layer pH decreased by ca. 0.25 units from 1988 to 1993, most strongly in medium-rich spruce forest where exchangeable Ca decr eased and Al and Mn increased strongly. Our study supports the hypothe sis that vascular plants show a long-term and broad-scale response to soil acidification. Change in bryophyte composition is linked to some very long growing-seasons. Detailed analysis of short-term vegetation dynamics enhances the interpretation of long-term changes and stresses the complementarity of univariate and multivariate methods in the ana lysis of vegetation change.