Disturbance in Boreal Peatlands

Abstract

16.1 Introduction Over the past several hundred years, approximately 50 % of wetlands glob-ally have been degraded (IUCN 2000), largely owing to agricultural inten-sification, pollution, energy and forestry practices, and urban develop-ment. Wetland loss has occurred over the past several centuries in Europe (Immirzi et al. 1992; Jones et al. 1993), more recently since settlement in North America (Tiner 1984; National Wetlands Working Group 1988; Dahl 1990), and may be intensifying today in many less industrialized nations (Brinson et al. 2002). Disturbances are particularly important to the boreal forest biome, where the structure and function of the boreal forest both determines and is influenced by its disturbance regime. In pristine boreal forest, distur-bances such as fire and insect outbreaks are common and alter landscape structure and species composition. In many boreal regions, however, human activities also influence ecosystem structure and function, and can alter natural disturbance regimes. Fires, for example, are a function of both natural conditions and human activities. The increasing area burned in Canada is most likely due to a combination of changing climate and increased human activities in formerly remote areas (Podur et al. 2002; Stocks et al. 2003). Most of our knowledge on disturbances in boreal regions is based on upland ecosystems, while less attention has been devoted to the role of disturbance in poorly drained landscapes, especially peatlands that cover significant areas of the boreal forest region. As noted in other chapters in this volume, peatlands play a significant role in many ecosystem services, including C sequestration, water storage and supply, and species and landscape diversity. Changes in disturbance regimes could compromise these ecosystem functions. Here, we briefly review the most common disturbances that influence boreal peatlands, focusing on disturbance impacts on plant structure, hydrology, and bio-Ecological Studies, Vol. 188 R.K.Wieder and D.H.Vitt (Eds.) Boreal Peatland Ecosystems © Springer-Verlag Berlin Heidelberg 2006 geochemical cycling in peatlands. Similar to well-drained ecosystems, dis-turbances in peatlands are a function of both natural conditions and human activities. We classify disturbances into perturbations that involve (1) herbivory and grazing, (2) biomass burning, (3) flooding or inunda-tion, (4) water table drawdown, and (5) pollution. Disturbance can be defined as discrete events in space and time that disrupt ecosystem structure (Pickett and White 1985) or induce wide-spread mortality of dominant species in an ecosystem (Aber and Melillo 1991). These definitions distinguish disturbances from more chronic stresses or variability present in many ecosystems. However, as noted throughout this chapter, determining whether a perturbation is discrete or is caused by an external versus an internal agent (Pickett et al. 1989) can be difficult and is often influenced by the temporal or spatial scale being considered. 16.2 Herbivory/Grazing