Basic and Applied Ecology 10 (2009) 697–706 Sphagnum re-introduction in degraded peatlands: The effects of aggregation, species identity and water table Bjorn J.M. Robroek a,b,Ã , Jasper van Ruijven a , Matthijs G.C. Schouten a,c , Angela Breeuwer a , Patrick H. Crushell d , Frank Berendse a , Juul Limpens a a Nature Conservation and Plant Ecology, Wageningen University, The Netherlands b School of Geography, Faculty of Environment, University of Leeds, UK c National Forest Service of the Netherlands, the Netherlands d Department of Zoology, Ecology and Plant Science, University College Cork, Ireland Received 31 October 2008; accepted 20 April 2009 Abstract In European peatlands which have been drained and cut-over in the past, re-vegetation often stagnates after the return of a species-poor Sphagnum community. Re-introduction of currently absent species may be a useful tool to restore a typical, and more diverse, Sphagnum vegetation and may ultimately improve the functioning of peatland ecosystems, regarding atmospheric carbon sequestration. Yet, the factors controlling the success of re-introduction are unclear. In Ireland and Estonia, we transplanted small and large aggregates of three Sphagnum species into existing vegetation. We recorded changes in cover over a 3-year period, at two water levels (5 and 20 cm). Performance of transplanted aggregates of Sphagnum was highly species specific. Hummock species profited at low water tables, whereas hollow species profited at high water tables. But our results indicate that performance and establishment of species was also promoted by increased aggregate size. This mechanism (positive self-association) has earlier been seen in other ecosystems, but our results are the first to show this mechanism in peatlands. Our results do not agree with present management, which is aimed at retaining water on the surface of peat remnants in order to restore a functional and diverse Sphagnum community. More than the water table, aggregate size of the reintroduced species is crucial for species performance, and ultimately for successful peatland restoration. r 2009 Gesellschaft fu ¨ r O ¨ kologie. Published by Elsevier GmbH. All rights reserved. Zusammenfassung In den europa ¨ ischen Mooren, die in der Vergangenheit trockengelegt und gema ¨ ht wurden, stagniert die Wiederbesiedlung ha ¨ ufig nach der Ru ¨ ckkehr zu einer artenarmen Sphagnum-Gesellschaft. Die Wiederansiedlung von bisher abwesenden Arten kann ein sinnvolles Werkzeug sein, um eine typische und diversere Sphagnum-Vegetation wiederherzustellen, und ko ¨ nnte letztendlich die Funktionsfa ¨ higkeit von Mooro ¨ kosystemen in Hinsicht auf die Festlegung von atmospha ¨ rischem Kohlenstoffdioxid verbessern. Dennoch sind die Faktoren, die den Erfolg einer Wiederansiedlung bestimmen, bisher unklar. In Irland und Estland verpflanzten wir kleine und große Aggregate von drei Sphagnum-Arten in existierende Vegetation. Wir nahmen die Vera ¨ nderungen im Deckungsgrad u ¨ ber eine Zeit von ARTICLE IN PRESS www.elsevier.de/baae 1439-1791/$ - see front matter r 2009 Gesellschaft fu ¨ r O ¨ kologie. Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.baae.2009.04.005 Ã Corresponding author. Current address: Utrecht University, Department of Biology, Institute of Environmental Biology, Section of Landscape Ecology, P.O. Box 800.84, 3508 TB Utrecht, The Netherlands. Tel.: +31 302536852; fax: +31 302518366. E-mail address: b.j.m.robroek@uu.nl (B.J.M. Robroek).