Dominant and subdominant rooting niches

Abstract Hydrologic niche separation may explain why some plants are dominant while most are rare. For example, deep roots may provide more water than shallow roots and result in dominance in some sites, but less water and subdominance in other sites. Yet, because it remains difficult to measure where different species absorb water, relatively little is known conceptually or experimentally about the niches of dominant and subdominant species. Does water availability ‘choose’ dominants, or do dominants ‘choose’ the most productive niches? Here, I test these ideas using tracer uptake and water modelling data to describe the water use niches of dominant and subdominant trees and grasses across a range of savanna conditions. Species with the deepest roots were dominant in sites with the deepest percolation and species with the shallowest roots were dominant in sites with the shallowest percolation. Species with intermediate rooting depths typically absorbed less water and were subdominant. Synthesis. Results support the idea that species are often constrained to a narrow range of traits and that species with root distributions that happen to best match water availability at a site will become dominant in water‐limited systems. Remaining species are subdominant because their root traits absorb less water at a site. Results provide an example of how resource niches can explain why some species are dominant while others are subdominant. Results support the use of mechanistic hydrologic niche models for predicting species abundance and coexistence on the landscape and over time. In short, results support the idea that water availability ‘chooses’ dominant species.