From what depths do plants get soil water? This is a deceptively difficult question, but answering this question has the potential to improve understanding of plant competition, plant response to climate change, biosphere/atmosphere exchange and aquifer recharge. For over 100 years root biomass has been used to infer patterns of resource uptake. In savannas, observations of root biomass led to the two-layer hypothesis which suggests that grasses outcompete trees due in part to dense shallow root mats that have been presumed to provide grasses with more soil water than trees.
Results/Conclusions
Here we show that root biomass can be a poor indicator of resource uptake. More specifically, we combine a hydrologic tracer experiment with a soil water model to measure the amount of water plant species remove from different soil depths. We use this approach in mesic, xeric, sandy and clay sites and reveal surprisingly similar rooting patterns among sites. Our approach revealed that the depth from which grasses and trees absorbed half of their annual water was twice as deep (23 and 43 cm, respectively) and niche partitioning was greater (0.82 vs. 0.95) than would be inferred from root biomass or tracer uptake data alone. Contrary to long-standing assumptions based on root biomass distributions, our approach suggested that shallow rooting patterns provided grasses with 32% less water than slightly deeper tree rooting patterns. Our results provided good predictions (R2 = 0.82) of soil water content throughout the soil profile and throughout the growing season and helped predict tree and grass growth responses to treatments in separate precipitation manipulation experiments. Results provide an example of how quantifying water uptake can reveal patterns of root activity that are not apparent from root biomass distributions and help resolve long-standing questions about species coexistence and water use.