Using historical data to determine the interactive effects of historical land-cover changes and recent environmental changes on herb-layer plant diversity in temperate forests

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Land-cover changes are considered one of the most important threats to biodiversity. In many parts of temperate Europe, extensive land-cover changes occurred during the 18th and 19th century, when massive forest clearance for agriculture, followed by agricultural abandonment and reforestation took place. There is reason to believe that these historical land-cover changes are causing delayed responses in the forest herb layer vegetation today, as extinction debt and colonisation credit in ancient and post-agricultural forest fragments, respectively, are paid. However, knowledge on the magnitudes of extinction debt and colonisation credit in temperate forests is very limited. Even less is known about the degree to which they are being paid, probably due to a lack of adequate historical baseline data. Simultaneously to their response to historical land-cover changes, forest plant species need to respond to more recent environmental changes. Numerous resurvey studies have shown that herb layer communities are changing due to canopy cover changes, atmospheric deposition, enhanced deer browsing and climate warming. However, most studies so far handled these drivers separately, while neglecting their interactions both among each other and with the legacies of historical land-cover changes. With this research project, we will address these important knowledge gaps. Our main objectives are (a) to quantify the magnitude of extinction debt and colonisation credit in ancient and post-agricultural forest fragments, respectively; (b) to determine to which degree extinction debt and colonisation credit have been paid over the last five decades; and (c) to identify important interactions between the legacy effects of historical land-cover changes and recent environmental changes. The study will be conducted in the c. 3600 km² Prignitz region in eastern Germany, for which exceptional historical data are available. Historical topographical maps from the late 18th and late 19th century will allow to reconstruct the land-use history of the region. The historical and present-day landscape configuration can then be related to the present-day herb layer species diversity in forest fragments to quantify extinction debt and colonisation credit. Moreover, at least 200 semi-permanent vegetation plots from c. 1960, distributed across the deciduous forests of the region will be resurveyed to quantify changes in species diversity, the degree of floristic differentiation among forest communities (beta diversity) and functional composition. Using both measurements of site conditions and species performance traits we will be able to attribute the observed changes in species diversity and composition to the interactive effects of historical land-cover changes and recent environmental changes.