Shrub and grass soil-resource partitioning as modulated by precipitation amount and size of individual

Plant-root uptake underpins the flow of water and nutrients through terrestrial ecosystems. Water and nutrients are heterogeneously distributed in soil and vary among wet and dry years. Shrubs and grasses have different root distributions extending to different soil depths. We asked three questions: (1) What are the depth profiles of root absorption of the dominant shrub and grass species of the northern Chihuahuan drylands? (2) How do their absorption profiles change from wet to dry years? (3) How does the absorption profile of the dominant shrub Prosopis glandulosa change with shrub size? Using a tracer, we measured soil-resource uptake by neighboring shrubs and grasses in the top 120 cm of the soil. Grass absorption profiles were shallower and more flexible than those of shrubs, which showed little change between dry and wet years. Shrub-rooting depth increased swiftly with size; all shrubs above the 20th percentile of aboveground volume displayed deeper absorption than grasses. We showed that shrub–grass competition for soil resources is minimal and limited to early shrub establishment stages. Our results assist in understanding woody-plant encroachment and future plant community structure and ecosystem functioning under a hotter, drier, and more variable climate.