The grassland–forest continuum: trends in ecosystem properties for woody plant mosaics?

www.frontiersinecology.org © The Ecological Society of America F an airplane, we often look out the window and survey the landscape as we begin our final descent. As we get closer to the ground, our focus changes from an initial broad survey of the topography to an increasingly detailed picture of vegetation patterns. Perhaps the first thing we notice is the extent to which the land is covered by trees or shrubs (Figure 1). If the location is devoid of woody plants, we might be landing in a grassland, and if it is completely covered by trees, we are landing amidst forest with a closed canopy. These two states represent the extremes of a continuum of coverage by woody plants (Belsky and Canham 1994; Breshears and Barnes 1999). Much of the terrestrial biosphere has an intermediate level of coverage by woody plants and therefore lies between the two extremes – within a “grassland–forest continuum” (Figure 2). Examples of such systems include savannas, shrublands, and woodlands, as well as grasslands with a few woody plants, and forests with nearly complete canopy closure. Each of these types is fundamentally differentiated on the basis of coverage by woody plants, even though the percentages of canopy coverage used to distinguish them are highly variable (Anderson et al. 1999). The amount of coverage, as well as the associated stature and spatial patterns seen in the woody plants, forms a mosaic within an ecosystem. This woody plant mosaic not only represents a first-order descriptor of major vegetation types, but is also a fundamental determinant of many key ecosystem processes and associated abiotic patterns (Scholes and Archer 1997; Aguiar and Sala 1999; Martens et al. 2000; Sankaran et al. 2005). It is well known that woody plants modify the environment beneath them – the canopy patches – quite dramatically. They intercept incoming precipitation and solar radiation, thereby creating patterns that are heterogeneous between the canopy patches and the adjacent intercanopy patches. Because woody plants have major effects on the ecosystem properties of the patches beneath their canopies, and because these effects can provide feedbacks to woody and herbaceous plants (Scholes and Archer 1997; Breshears and Barnes 1999), it is important to understand and quantify these effects for individual sites as well as for sites along the grassland–forest continuum. Understanding trends in ecosystem properties along the grassland–forest continuum is not simply of esoteric interest, but rather is highly applicable to a diverse and apparently disparate set of environmental issues (Figure 2). For REVIEWS REVIEWS REVIEWS