ABSTRACT
Grasslands cover about a fifth of the terrestrial surface of the world (Hadley, 1993), and the majority of this area is grazed by animals. The impact of an increasing concentration of CO2 in the atmosphere on these grasslands has assumed importance, first because of the direct effects on food production (Gregory et al., 1999), and second because of the influence terrestrial ecosystems can have on the composition of the atmosphere and therefore on our climate (Pielke et al., 1998). In the case of grasslands this includes not only C sequestration, N2O release, and CH4 uptake by soils, but also CH4 emissions from ruminants. Consequently, many research programs have been developed to explore these impacts, and our knowledge of the likely outcomes is progressing rapidly. However, our understanding is based almost exclusively on cut (as opposed to grazed) grassland (e.g., Wolfenden
and Diggle, 1995; Casella et al., 1996a,b; Newton et al., 1996; Clark et al., 1997; Hebeisen et al., 1997; Potvin and Vasseur, 1997; Taylor and Potvin, 1997; Clark et al., 1998; Leadley et al., 1999; Navas et al., 1999, but see Edwards et al., 2000), and grazed swards are very different in their botanical and soil characteristics (Watkin and Clements, 1976; Haynes and Williams, 1993). In addition, some of these experiments involved the transfer of previously grazed areas to a cutting management (Newton et al., 1996; Clark et al., 1997; Potvin and Vasseur, 1997; Taylor and Potvin, 1997; Clark et al., 1998; Leadley et al., 1999) and therefore do not necessarily display the responses typical of a cut system but of a system in transition. In these examples it is probable that the change in management resulted in a process of succession, one consequence of which would likely be a loss of early successional species. Clearly, any interpretation of a response to elevated CO2 in these transitional systems must be made with this background change in mind.
