Vegetation-environment relationships of boreal spruce forests in ten monitoring reference areas in Norway

Vegetational and environmental monitoring of boreal spruce forest was initiated in 1988, as a part of the programme “Contrywide Monitoring of Forest Health” at the Norwegian Institute of Land Inventory (NIJOS). As a basis for monitoring, relationships between trees, understory vegetation and environmental conditions (vertical relationships) were analysed for each of ten reference areas. The reference areas were selected to span regional gradients, in climatic conditions and deposition of airborne pollutants, in old-growth, so-called “bilberry-dominated”, “small-fem” and “low-herb”, also paludified, spruce forests south of the Polar Circle. Fifty 1-m² meso sample plots, randomly chosen within ten 50-m² macro sample plots in each reference area, were subjected to vegetation analysis, using frequency in subplots as species abundance measure. Environmental (including soil chemical) and tree parameters were recorded for meso as well as macro sample plots.

The main vegetational gradients were found by parallel use of DCA and LNMDS ordination methods and subjected to environmental interpretation, mainly by means of non-parametric correlation analyses. DCA and LNMDS in most cases revealed the same main gradients in vegetation, but outliers were more frequent in LNMDS ordinations, due to higher vulnerability of this method to plots with deviating number of species. A complex-gradient in nutrient conditions, with pH and the concentration of nitrogen as the most constantly contributing variables, but with considerable between-area variation with respect to important cations, was evident in nine reference areas. Soil moisture varied along the second vegetational gradient in most areas. In the three most humid reference areas, the Ca concentration was related to variation in soil moisture and gradients from below to between trees, while unrelated or inversely related to the same vegetational gradient as pH. Species abundances were plotted on plot positions in DCA ordinations in order to summarize the species· responses to environmental variation in each area.

Variation in vegetation in the total data set (500 meso sample plots) was partitioned onto two sets of explantory variables (environmental and climatic/geographical) by use of CCA, in order to find the relative importance of environmental and climatic/geographical variation. The fraction of variation exclusively explained by environmental variables was about 1 7%, while only 5% of the variation was explained exclusively by climatic variables. The variation shared by both sets of variables was about 8%.

The main vegetational gradients and environmental/climatic/geographical complex-gradients in the total data set were found by DCA and subsequent interpretation of axes. The main complex-gradients found by separate analyses of data from each reference area, were reflected along the DCA axes in total ordinations, but differences between areas with respect to positions along both environmental and climatic/geographical gradients were also evident.

Meso plot occurrences of selected species were plotted in a DCA ordination of the total data set, with variation exclusively due to climatic/geographical variables removed, in order to express regional similarities and differences in the species· responses to the environment. The different patterns of species· distributions in the DCA ordination were discussed in the light of their use as indicators of specified environmental conditions.