‘Poor rich fen mosses’: atmospheric N-deposition and P-eutrophication in base-rich fens

Published online: 
22 October 2012

Article Id

Annemieke M. Kooijman

A. M. Kooijman (a.m.kooijman@uva.nl), Inst. for Biodiversity and Ecosystem Management, Univ. of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, the Netherlands.

Base-rich fens in the Netherlands are threatened by acidification and replacement of rich-fen bryophytes by Sphagnum spp. Acidification is a natural process when input of base-rich water is reduced, and is probably accelerated by high atmospheric deposition, leading to lower pH at similar calcium levels, and increased acidification capacity of Sphagnum. However, acidification may also be due to eutrophication, especially with P, which leads to a shift in stable states from base-rich to Sphagnum-dominated fen. Under nutrient-poor conditions, Scorpidium scorpioides fen is stabilized as long as sufficient base-rich water is supplied. The species is tolerant to rainwater and may even counteract acidification. Its successor Sphagnum subnitens, however, is intolerant to groundwater and has low acidification capacity, and can only become dominant after changes in hydrology and (local) accumulation of rain water. Under nutrient-rich conditions, however, Scorpidium scorpioides is replaced by Calliergonella cuspidata. In contrast to Scorpidium scorpioides, Calliergonella cuspidata is intolerant to rainwater. Moreover, its successor Sphagnum squarrosum grows well under base-rich conditions. High growth rates and high acidification capacity of S. squarrosum further lead to rapid expansion of this species, acidification of the fen, and loss of characteristic rich-fen species. For conservation of rich fens it is thus very important to keep the habitat base-rich, but nutrient-poor.