20th century vegetation changes in an island archipelago, SW Finland

We studied the long-term changes in the flora in the archipelago of SW Finland by comparing historic plant species lists from 331 islands surveyed in the 1930s to 1940s, to lists compiled in 1996–2004. We used plants as indicators to indirectly detect changes in the environment based on changes in species frequencies. Detrended correspondence analysis (DCA) was used to examine if there was a change in species composition during this 60–70 yr period. More species increased in frequency (180, 31.1%) than decreased (77, 13.6%). The number of species occurrences increased by 10.6%. Species of shores, mires, wetlands, and woods increased, while species of shore meadows, pastures, and heaths decreased. For the non-shore species the interpretation of the DCA and the comparisons of plant indicator values between the old and the new data indicate lower cultural impact, decreased light conditions, a higher level of moisture and nitrogen, and a higher proportion of perennials on the new data islands. The shore plants indicate that productivity has increased over the study period, probably reflecting the eutrophication of the Baltic Sea.