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
Zusammenfassung der Projektergebnisse
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 are indications that these historical land-cover changes are causing delayed responses in the forest herb layer vegetation today, as ‘extinction debt’ and ‘colonization credit’ in ‘ancient’ and post-agricultural forest fragments, respectively, are paid. However, knowledge on the magnitudes of extinction debt and colonization credit in temperate forests was very limited so far. Forest plants do not only respond to historical land-cover changes but also have to cope with more recent environmental changes, such as nitrogen deposition, climate warming or enhanced deer browsing. All these factors do not influence the forest plant diversity independently, but interact with each other and with the legacies of historical landcover changes. Our two main objectives of this project were, therefore, (a) to quantify the magnitude of the extinction debt and the colonization credit in ancient and post-agricultural forest fragments, respectively; and (b) to identify interactions between the more recent environmental changes and the historical landcover changes in driving compositional shifts in the forest herb layer. We conducted the study in the Prignitz region (NE Germany) which is a typical agricultural landscape of the European lowlands. For the first objective, we reconstructed the land-use history based on historical topographical maps from the late 18th and late 19th century. Then we inventoried the vascular plant species of 104 ancient (with no record of agriculture) and 110 post-agricultural forest patches and modelled species richness as a function of historical and present-day patch configuration (area and connectivity) to quantify the extinction debt and the colonization credit. For the second objective we resurveyed 119 vegetation relevés from c. 1960, 83 of which were located in ancient and 36 in post-agricultural stands. We also collected data on environmental conditions in both the surveyed forest patches and the resurveyed vegetation relevés. At the time when this final report was due, only the first objective had been achieved, while data analysis and production of results for the second objective were still owing. Present-day patch configuration explained species richness in ancient forest patches generally better than historical patch configuration, indicating that any extinction debt has already been paid. This was true for the richness of all plant species, forest specialists (those closely tied a closed forest habitat) and functional groups for which delayed responses to habitat destruction were considered most likely. This result is in contrast to previous studies that detected an extinction debt in deciduous forests 120-225 years after forest fragmentation. Our study shows that when the degree of forest area loss is very high (here c. 94%), the extinction debt can be paid off faster (here after less than c. 160 years). When present-day habitat area was not included as a predictor, historical patch connectivity had a significant positive effect on the richness of forest specialists (particularly on those with a low dispersal potential), whereas present-day connectivity was never a significant predictor. We interpret this as a last remainder of the fragmentation effect. It indicates that an extinction debt due to connectivity loss might persist longer than an extinction debt due to area loss because connectivity has decreased less dramatically in the Prignitz region than forest area. We found on average 17 and 14 forest specialist species in ancient and post-agricultural forest patches, respectively. The colonization credit in an average post-agricultural forest patch amounted to 4.7 forest specialist species and ranged up to 9 species in highly isolated patches. Species with a low dispersal potential and a low seed output contributed most to the credit. In better connected patches, the colonization credit was negative, indicating that the patches had been colonized by more species than predicted. For some groups of forest specialists the colonization credit was highest on relatively productive sites. The colonization credit was generally not smaller in older patches than it was in younger ones. The missing effect of present-day connectivity on the species richness in ancient patches and the missing effect of patch age on the colonization credit in post-agricultural patches indicate that patch (re-)colonization and thus metapopulation functioning has largely ceased, at least in strongly fragmented parts of the region. We, therefore, advocate active species introductions to preserve the forest specialist flora.
Projektbezogene Publikationen (Auswahl)
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2015. Colonization credit of post-agricultural forest patches in NE Germany remains 130– 230 years after reforestation. Biol Conserv 182, 155-163
Naaf, T., Kolk, J.
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2015. Herb layer extinction debt in highly fragmented temperate forests – Completely paid after 160 years? Biol Conserv 182, 164-172
Kolk, J., Naaf, T.
