Abstract
Walter (Jahrb Wiss Bot 87:750-860, 1939) proposed a two-layer hypothesis, an equilibrium explanation for coexistence of savanna trees and grasses. This hypothesis relies on vertical niche partitioning and assumed that grasses are more water-use efficient than trees and use subsurface water while trees also have access to deeper water sources. Thus, in open savannas, grasses were predicted to predominate because of their water use efficiency and access to subsurface water. This hypothesis has been a prominent part of the savanna literature since first proposed. We review the literature on Walter's hypothesis and reconsider his original intentions. Walter intended this hypothesis to be restricted to dry savannas. In his opinion, mesic and humid savannas were controlled by biotic factors and disturbances. We surveyed the global savanna literature for records of vertical niche partitioning by grasses and trees. We find that, within the scope of Walter's original intentions, this hypothesis works remarkably well, and in some cases is appropriate for deserts as well as for dry temperate systems and even some mesic savannas.
Highlights
Savannas cover about 20 % of the earth’s land surface (Sankaran et al 2005)
Our review has shown that the two-layer hypothesis is an adequate model to explain soil–water partitioning and tree– grass codominance in dry savannas, as long as it is carefully appreciated in its original context and with all its subtleties (Walter 1939, 1954, 1971, 1989)
The two-layer hypothesis was never proposed for moist savannas because, in these biotic feedback-dominated systems, tree–grass codominance is primarily mediated by disturbance
Summary
Savannas cover about 20 % of the earth’s land surface (Sankaran et al 2005). They are generally described as biomes with continuous grass strata and discontinuous tree or shrub strata (Walker 1985; Belsky 1990). Among a variety of concepts seeking to explain tree–grass codominance, spatial resource partitioning is one of the most favoured and most disputed (Walter 1939; Walker and Noy-Meir 1982; Jeltsch et al 2000; Ward 2005; Lehmann et al 2009). According to this concept, tree and grass roots occupy different vertical niches, providing each vegetation stratum with almost exclusive access to essential resources such as water and nutrients (Breshears and Barnes 1999; Kambatuku et al 2012). In its purest form, when woody and herbaceous niches do not overlap, no competition for resources between the plants in these strata would exist and, tree–grass codominance would be in a stable equilibrium
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