LAUSR

Aims Acidification by airborne pollution strongly degraded terrestrial and aquatic ecosystems until the late 20th century. Although geochemical recovery has been observed since then, there has been little recovery of ecological communities, and the reasons are unclear. 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 on-going biogenic habitat modification by peat moss species (genus Sphagnum) will have long-term effects on plant community composition of acidified spring fens. Location Spring fens in forest landscapes of central Germany's siliceous mountain ranges impacted by anthropogenic acidification until the late 1980s, and with strong increases in abundance of Sphagnum during the last 25 yrs. Methods Vegetation surveys (vascular plants, mosses and liverworts) and hydro-chemical water measurements of 54 spring fens over a study period of 25 yrs (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 effects of anthropogenic acidification and biogenic habitat modification. Results Changing plant communities (excluding Sphagnum spp.) indicated significant increases in acidification of spring fens with high Sphagnum cover, although pH of the outpouring groundwater showed no temporal trends. Path analysis revealed significant enhancing effects of Sphagnum abundance on the community-indicated acidity, which was independent of the abiotic acidity regime (water pH). With increasing species richness through time, community assembly 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 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 of considering the history and memory of ecosystems in global change impact research.