Compositional changes in thermophilous oak forests in Poland over time: do they correspond to European trends?

Questions Have compositional changes occurred in thermophilous oak forests in Poland during the past two decades? What kinds of drivers might be responsible for these changes? Are the thermophilous oak woods still rich in species or have they rather lost their diversity? Location Sudetes Mts., SW Poland. Methods The vegetation of thermophilous oak forests within four distinct areas was surveyed on 92 semi-permanent plots in the time periods from 1992 to 1994 and from 2010 to 2014. The overall pattern in vegetation changes, trends in species composition and site parameters, as well as the diversity of studied communities were analysed using ordination and permutation techniques. Results The species composition of thermophilous oak forests has changed significantly. We recorded a decrease in the share of oligotrophic light-demanding species, whereas mesotrophic light-demanding and nitrophilous species showed an upward trend. Regarding the fourth group of shade-tolerant species, their share varied depending on the site. The analysis of habitat parameters revealed that the studied oak forest communities generally shifted from communities of moderately oligotrophic and mesic character towards forests growing on nutrient-richer but drier habitats. Interestingly, the observed change in species composition was not accompanied by the loss of biodiversity that has generally been reported in other types of thermophilous forests in Europe. Conclusions Changes observed within thermophilous and previously coppiced oak forests of southwest Poland are going in a direction which has not been recognized so far in analogous communities. The transition of studied communities is probably a result of the cumulative influence of several drivers, the most crucial of which seem to be soil regeneration, after the period of intensive acidity caused by sulphur emission between 1960 and 2000, and decreasing moisture. The observed changes are independent of the type of forest management (or the lack thereof) within the study sites.