Water matching: an explanation for plant growth and coexistence in water-limited systems

The ability to absorb water from the soil is likely to be a primary determinant of plant growth in water-limited ecosystems. Vertical root biomass distributions have long been used to understand how plants absorb water. However, a growing body of evidence suggests that root water uptake is more complicated. Some roots absorb water quickly, others slowly (i.e., root activity). Some species move their active roots through the soil more quickly than others (i.e., root plasticity). Here, a framework for understanding how vertical root distributions, root activity, and root plasticity affect plant growth and coexistence is developed. It is suggested that plants with a large transpiration demand use a deep and dynamic rooting strategy to provide a large, stable soil water supply. Plants with a smaller transpiration demand use a shallow and pre-emptive strategy to intercept enough water to produce herbaceous aboveground growth. The root strategy that best matches water availability at a site will have a competitive advantage. Tracer experiments can describe these rooting strategies and ecohydrologic models can predict their effects on water uptake over time. This framework provides a new way of understanding and measuring plant water uptake that can be expected to improve predictions of plant growth on the landscape and in response to changing climate conditions.