Dynamics of understory vegetation in an old‐growth boreal coniferous forest, 1988–1993

Abstract. Understorey vegetation changes in a South Norwegian old‐growth coniferous forest were studied between 1988 and 1993 in 200 1‐m 2 vegetation plots. Our aims were to quantify the amount of between‐year compositional change, and to elaborate the environmental basis for long‐term vegetation change, including the previously identified gradient structure with a major gradient related to topography (and soil nutrient status and soil depth) and a minor gradient reflecting paludification and canopy coverage. Species richness (yearly mean and cumulative species number) and change in species richness differed between vascular plants and cryptogams, and between forest types. The number of vascular plant species decreased in pine forest in dry years; bryophyte species number increased in spruce forest. Statistically significant vegetation change, as tested by constrained ordination (CCA) with time as the constraining variable, is demonstrated for most one‐year periods and for the five‐year period in most forest types. Vegetation change along identified gradients, measured as plot displacement along DCA ordination axes, also occurred. The magnitude of year‐to‐year vegetation change was related neither to forest type nor to one‐year period; different responses to climatic and environmental change were observed in each forest type. The largest average displacement observed, from medium‐rich spruce forest towards poor spruce forest, was interpreted as a long‐term trend. Humus‐layer pH decreased by ca. 0.25 units from 1988 to 1993, most strongly in medium‐rich spruce forest where exchangeable Ca decreased and Al and Mn increased strongly. Our study supports the hypothesis 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 analysis of vegetation change.