Revisiting and evaluating the validity of Walter’s two-layer hypothesis requires the exclusion of both edaphic and anthropogenic savannas (e.g. Ward et al. ; Wiegand et al. ; Wu and Archer ). We explicitly excluded edaphic savannas because, for example, shallow soils on bedrock prevent vertical soil–water partitioning in the arid savanna of Namibia (Wiegand et al. ). While fine-textured soils usually favour grasses with their shallow roots (Nano and Clarke ) and coarse-textured soils favour trees with their extensive and deeper-reaching root systems (Ward and Esler ), these tree–grass ratios may be reversed if topographic redistribution of water overrides soil texture effects (Wu and Archer ). In Van Wijk’s () ecohydrological model, roots were predicted to be deeper with higher rainfall (consistent with Jackson et al.’s and Schenk and Jackson’s empirical data) because of a shift in the importance of evaporation relative to drainage. As rainfall increased, drainage was more important than evaporation, resulting in an optimal distribution of roots at greater depths. Van Wijk () also found that this result was consistent with the pattern resulting from increasing coarseness (sandiness) of the soil because more water is lost from drainage in coarse soils (see also empirical data from Kambatuku et al. ). We note that there may be some overlap between climatic and edaphic savannas, as recognized by Walter (, ).
The second criterion for evaluating the two-layer hypothesis is the concept of the ‘superior competitor’ (Knoop and Walker ) in the soil layer, which is more important than niche partitioning based on a strict threshold. Even if tree and grass roots overlap to a certain extent, the two-layer model can still operate, provided grasses and trees are the superior competitors in their layers, for example due to different relative distributions at variable depths (Knoop and Walker ; Dodd et al. ; Van Langevelde et al. ; Sankaran et al. ; Kambatuku et al. ).
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To answer questions about resource uptake and plant competition, information about fine root length density and spatial distribution is important (Millikin and Bledsoe ). Consequently, most empirical studies that have investigated Walter’s two-layer hypothesis must be interpreted with caution because the excavation techniques that were applied, such as separating roots from soil with pressurized water, almost certainly underestimate fine roots (Jackson et al. ). In our view, the study of Mordelet et al. () was path-breaking because they assessed 13C abundance for fine roots that allowed the separation of C4-grasses and C3-trees through the difference in their ‘signatures’ (isotopic values). The studies of Le Roux et al. () and Weltzin and McPherson () were also important approaches to investigating the two-layer hypothesis in savannas; they were the first to use stable isotopes of water [18O and 2H (deuterium)] in savannas. Meanwhile, techniques for molecular identification of fine tree and grass roots at species level and relative species abundances have been developed (e.g. Bohrer et al. ; Mommer et al. ), but have not yet been used to investigate the two-layer hypothesis.