Conservatives and Gamblers: Interpreting plant functional response to water stress in terms of a single indicator

Water availability has a decisive impact on plant growth, survival and distribution. Climate change is expected to alter both the amount and variability of precipitation. To predict and understand plant responses to water stress, efficient and robust mechanisms for describing their functional responses to water availability are needed. However, most ecohydrological processes and models which take into account these responses are inherently complex, difficult to understand and require large amounts of data. We develop a novel straightforward approach and hypothesize that: (1) Plants exhibit two archetypical response patterns under water stress, one typical to slow-growing plants and one typical to fast-growing ones, with most plants being situated between these two. (2) Differences within and between these functional types can be adequately described by a single parameter - the threshold of relative soil water content - at which plants reduce their transpiration in response to water stress. This indicator is straightforward and relies on data which is relatively easy to measure. Its effect has been previously described and it is already used in several models to simulate the effect of water stress on plants. In our approach, we combine this indicator with a description of reactions patterns. This combination provides a general and efficient way of classifying plant responses and allows the assessment of the impact of water stress on a wide variety of plants. Due to its simplicity, our approach offers the opportunity to include water relations of plants in a larger set of models and descriptions than it is possible with more complex ecohydrological descriptions. It also can be used to explain biodiversity in fluctuating environments