Hydrologic Niches Explain Species Coexistence and Abundance in a Shrub-Steppe System

Differences in vertical root distributions are often assumed to create resource uptake tradeoffs that determine plant growth and coexistence. Yet, most plant roots are in shallow soils, and data linking root distributions with resource uptake and plant abundances remain elusive. Here we used a tracer experiment to describe the vertical distribution of absorptive roots of dominant species in a shrub‐steppe ecosystem. To describe how these different rooting distributions affected water uptake in wet and dry soils across a growing season, we used a soil water movement model. Root traits were then correlated with plant landscape abundances. Deeper root distributions extracted more soil water, had larger unique hydrological niches and were more abundant on the landscape. Though most (\u3e 50%) root biomass and tracer uptake occurred in shallow soils (0–32 cm), the depth of 50% of tracer uptake varied from 11 to 32 cm across species and species with deeper rooting distributions were more abundant on the landscape (R2 = 0.95). The water flow model revealed that deeper rooting distributions should extract more soil water (i.e., a range of 60 to 113 mm of soil water) because shallow roots were often in dry soils. These potential water uptake values were tightly correlated with species’ abundances on the landscape (R2 = 0.90). Finally, each species’ rooting distribution demonstrated a depth and time at which it could extract more soil water than any other rooting distribution, and the size of these unique hydrological niches indices was also well correlated with species’ abundances (R2 = 0.89). Synthesis. Our results demonstrate not only a correlation between root distributions and species abundance, but also the mechanism through which differences in rooting distributions can determine resource uptake and niche partitioning, even when most roots are found in shallow soils