Variable sensitivity of plant communities in Iceland to experimental warming INGIBJO ¨ RG S. JO ´ NSDO ´ TTIR *, BORGTHO ´ R MAGNU ´ SSON w , JO ´ N GUDMUNDSSON z, A ´ SRU ´ N ELMARSDO ´ TTIR w andHREINNHJARTARSON§ *The University Centre in Svalbard, UNIS, PO Box 156, N-9171 Longyearbyen, Norway, wIcelandic Institute of Natural History, Hlemmur 3, PO Box 5320, 125 Reykjavı ´k, Iceland, zIcelandic Agricultural Research Institute, Keldnaholt, 112 Reykjavı ´k, Iceland, §The Icelandic Meteorological Office, Bu´ stadavegur 9, 150 Reykjavı ´k, Iceland Abstract Facing an increased threat of rapid climate change in cold-climate regions, it is important to understand the sensitivity of plant communities both in terms of degree and direction of community change. We studied responses to 3–5 years of moderate experimental warming by open-top chambers in two widespread but contrasting tundra communities in Iceland. In a species-poor and nutrient-deficient moss heath, dominated by Racomitrium lanuginosum, mean daily air temperatures at surface were 1–2 1C higher in the warmed plots than the controls whereas soil temperatures tended to be lower in the warmed plots throughout the season. In a species-rich dwarf shrub heath on relatively rich soils at a cooler site, dominated by Betula nana and R. lanuginosum, temperature changes were in the same direction although more moderate. In the moss heath, there were no detectable community changes while significant changes were detected in the dwarf shrub heath: the abundance of deciduous and evergreen dwarf shrubs significantly increased (450%), bryophytes decreased (18%) and canopy height increased (100%). Contrary to some other studies of tundra communities, we detected no changes in species richness or other diversity measures in either community and the abundance of lichens did not change. It is concluded that the sensitivity of Icelandic tundra communities to climate warming varies greatly depending on initial conditions in terms of species diversity, dominant species, soil and climatic conditions as well as land-use history. Keywords: climate change, community change, diversity, microclimate, phenology, subarctic tundra Received 21 November 2003; received in revised form and accepted 7 April 2004 Introduction The increasing threat of rapid climate change in cold- climate regions as predicted by the International Panel of Climate Change (IPCC) (Houghton et al., 2001), calls for understanding of the sensitivity of cold-climate vegetation to changes. There are certain key areas that need special attention in this context, i.e. areas that are either exceptionally susceptible to rapid changes or areas where the biota is expected to be highly sensitive to changes. Being an island situated at the border between arctic and Atlantic waters and air masses, i.e. the polar front, Iceland is one of these key areas. This paper aims at elucidating the sensitivity of two wide- spread plant communities in Iceland to warmer climate and the type of responses that can be expected. Boundaries between climatic zones usually also reflect boundaries between vegetation zones (Elvebakk et al., 1999; Walker, 2000) (i.e. ecotones where southern and northern distribution limits for many plant species converse). Consequently, ecotones are of particular interest for monitoring biotic responses to climate change (e.g. Callaghan et al., 2002). According to Ko¨ ppen’s climate definitions, large parts of Iceland belong to the Low Arctic today, i.e. the Central Highland and the northern coastal areas, whereas other parts of the island belong to the ‘cold tempered zone’ (Ko¨ppen, 1931). As a result, the ecotone where the subarctic birch forest meets the low arctic tundra is evident in Iceland, although not always clearly pro- nounced because of extensive degradation of the birch forest during last millennia. Correspondence: I. S. Jo´ nsdo´ttir, fax 1 47 7902 3301, e-mail: isj@unis.no Global Change Biology (2005) 11, 553–563, doi: 10.1111/j.1365-2486.2005.00928.x r 2005 Blackwell Publishing Ltd 553