Vegetation composition, gradients and environment relationships of birch forest in six reference areas in Norway

Terrestrial Monitoring of boreal birch forest ecosystems (TOV) was initiated in 1989 by the Directorate for Nature Management. The programme has a multidisciplinary approach and integrates studies of precipitation, soil water, soil, understorey vegetation composition, lichens on birch trunks, population studies of birds and mammals and environmental pollutants in plants and animals. Here we present studies of forest floor vegetation at establishment, which supplements and complements two studies established in boreal coniferous forests in 1988: ‘The effect of acid precipitation on forest and forest understorey vegetation in Gjerstad, South Norway’ and ‘Vegetational and environmental monitoring of boreal spruce forest in ten reference areas’, the latter initiated by the Norwegian Institute of Land Inventory (NIJOS) as part of a forest health monitoring programme.

The reference areas were selected to span regional gradients in climatic conditions and deposition of airborne pollutants, in old-growth bilberry-dominated birch forest in Norway. Ten macro plots in each reference area were located to span differences in nutrient and soil moisture conditions, terrain features, etc. by a sampling design similar to the one used in coniferous forests. Fifty 1×1 m meso sample plots, randomly chosen within the ten macro plots, were subjected to vegetation analysis, using frequency in subplots as well as percent cover as species abundance measures.

The main vegetational gradients were found by parallel use of DCA and GNMDS ordination methods; the results of which were subjected to environmental interpretation by means of non-parametric correlation and split-plot GLM analyses. Both ordination methods gave to large degree similar, interpretable, vegetation gradients. The most important ecoclines were related to variation in nutrient conditions, best expressed by pH, Ca, K and S. Tree influence, topographic (un)favourability, soil moisture and soil depth were other factors which were correlated with one of the two main vegetation gradients (ecoclines).

The main vegetational gradients and environmental/climatic/geographical complex gradients in the total data set were found by DCA and subsequent interpretation of the axes. The main complex gradient corresponded to the variation in the vegetation from sites with low pH and low content of nutrients (low concentrations of macro nutrients like C, Ca, Mn, S and Total N) and high loss on ignition to vice versa. The second gradient corresponded to variation in the vegetation from sites with high effective temperature sums at low latitudes and high soil concentrations of Mn and S, to sites with opposite characteristics. Most of the variation (> 80%) in the vegetation compositions could be ascribed to the between macro plots scale level, leaving a small residual variation on the between area and in the plots within macro plot scale level.