The two-layer hypothesis is dead: Long live the two-layer hypothesis

Background/Question/Methods

Understanding when and where plants access soil resources remains a fundamental, but poorly understood aspect of plant biology.  Walter’s two-layer hypothesis has long been used to describe niche partitioning by trees and grasses, but the measurements of root activity needed to test this hypothesis have rarely been made.  We develop a depth-controlled tracer injection technique that allows the description of the location, timing and extent of water use by individual plants or species in the field.  We use this technique and present quantitative estimates of tree and grass water use by depth for mesic and xeric savanna sites on both sand and clay soils in Kruger National Park, South Africa.  

Results/Conclusions

Several patterns emerge from this data.  First, we found that, in contrast to the two-layer hypothesis, all plants rely on soil water in the top 20 cm of the soil.  Second, trees and grasses partition soil resources over short vertical distances (i.e., 0-10 vs. 10-20 cm for grasses and trees, respectively).  Third, grasses consistently demonstrate a geometric decline in tracer uptake with depth while trees shift root activity up and down the soil profile each year.  Fourth, both trees and grasses use only small amounts of soil water from the 60-120 cm depths.  Finally, in contrast to expectations, rooting activity was deeper in xeric, clay sites than in sandy, mesic sites.  Results demonstrate that across a wide range of climate and soil conditions, trees and grasses do partition resources but this partitioning occurs in shallow depths and trees do not have exclusive access to deep (e.g., 60-120 cm) soil water.  Results from this research are providing the data needed to parameterize niche partitioning and ecohydrological models.