Background: Acidification by airborne pollution strongly degraded terrestrial and aquatic ecosystems until the late 20^th century. Although geochemical recovery has been observed since then, recovery of ecological communities has failed to appear with unclear causes. Positive feedbacks between anthropogenic acidification and ecosystem engineers are likely to contribute to this lag in recovery but are under-investigated so far. Here we hypothesize, that positive feedbacks between past anthropogenic acidification and the ongoing, biogenic habitat modification by peat moss species (genus Sphagnum) will have long-term effects on the plant community composition of acidified spring fens.

Location: Spring fens located in the forest landscapes of Central Germany’s siliceous mountain ranges. These spring fens were impacted by anthropogenic acidification until the late 1980s and showed strong increases in abundance of Sphagnum species during the last 25 years of investigation.

Methods: Vegetation surveys (vascular plants, mosses and liverworts) and hydro-chemical water measurements of 54 spring fens over a study period of 25 years (1989-2013). Measurements of water pH and discharge were used in combination with Ellenberg Indicator values of the occurring plant species to investigate the joint effect of anthropogenic acidification and biogenic habitat modification.

Results: Changing plant communities (excluding Sphagnum spp.) indicated significant increases in acidification for spring fens with high Sphagnum cover although pH measured for the outpouring groundwater showed no temporal trend. A path analysis revealed significant enhancing effects of Sphagnum abundance on the community-indicated acidity which was independent from the abiotic acidity regime (water pH). With increasing species richness through time, community assemblies became increasingly similar and shifted towards acid tolerant species. Characteristic plant species of non-acidified spring fens like Chrysosplenium oppositifolium were replaced by generalist grass, sedge and tree species (e.g. Picea abies, Fagus sylvatica, Calamagrostis villosa and Carex remota).

Conclusions: Based on long-term monitoring [as1] data we show that biogenic habitat modification by ecosystem engineers can significantly change plant community composition on a decadal scale in a landscape where historic anthropogenic acidification heavily impacted ecosystem functioning. These complex functional legacies stress the importance to consider the history and memory of ecosystems in global change impact research.