Dynamics of understory vegetation in boreal forests: experiences from Swedish integrated monitoring sites

This study reports results from a first analysis of vegetation dynamics in permanent plots in Swedish boreal forests. The plots are a part of the international cooperative programme on integrated monitoring of air pollution effects on ecosystems (lCP IM), which is part of the long-range transboundary air pollution convention (LRTAP) of the united nation's economic commission for Europe. In Sweden, there are four ICP IM monitoring sites, established 1982-1999. Vegetation measurements are made about every second year to be able to detect and follow possible changes in species composition. Ellenberg indices for light, moisture, pH and nitrogen were used as indirect metrics of environmental conditions. Vegetational gradients were explored by multivariate ordination methods. All gradients were short, less than 1.5 units in detrended correspondence analyses. Despite the short gradients, all sites showed spatial variation that could be related to different Ellenberg indices. A temporal trend in species composition was detected only at one site. At this site time accounted for 8% of the variation in the data set and a spatial variable accounted for 13%. Correlations between ordination plot scores and Ellenberg indices indicated that the temporal trend was partly caused by increasing light regime and decreasing soil moisture. (C) 2004 Elsevier B.V. All rights reserved.