Urgent need for a common metric to make precipitation manipulation experiments comparable.

In recent decades, researchers have increasingly focused attention on how climatic change will affect ecosystem functioning. Much of this research has centred on the effects of rising atmospheric CO2 concentrations and increasing temperatures (e.g. Ainsworth & Long, 2005; Wu et al., 2011). The consequences of altered precipitation patterns, by contrast, have received far less attention. The results of precipitation manipulation experiments were only recently synthesized for the first time (Wu et al., 2011), and the conclusions that can be drawn regarding global patterns remain preliminary. In general, reduced water inputs slow ecosystem processes, while increased rainfall enhances plant productivity (Wu et al., 2011), but it remains unclear how this response differs among ecosystems. When synthesizing water manipulation experiments, firm conclusions are not only precluded by the insufficient number of data points (Wu et al., 2011), but also by the difficulty of defining the magnitude of the perturbation experienced by the biota (further coined the ‘actual’ treatment). Unlike in the case of, for example, elevated CO2, the magnitude of the imposed treatment (e.g. )20% precipitation vs +300 ppm CO2) does not clearly indicate the actual treatment. Many factors influence the way an ecosystem experiences a change in rainfall; of greatest importance to the ecosystem is not the amount of incoming precipitation, but rather the amount of water that plants have access to. This ‘plant available water’ strongly depends on factors such as soil texture and rooting depth (Tolk, 2003), and the latter can show substantial seasonal and interannual variation (Knapp et al., 2008) and may differ between treatments. Additionally, runoff water and stem flow can complicate estimations of the magnitude of an imposed manipulation (Cotrufo et al., 2011), and plants that access groundwater supplies can complicate interpretations of treatment effects. If we want to understand why plant responses to altered precipitation differ among ecosystems, comparisons of effects of precipitation manipulations must use a more ecologically meaningful metric to describe the actual treatment than merely the change in precipitation. Without such a relevant ‘common denominator’, observed differences in ecosystem responses to altered precipitation may reflect differences not only in ecosystem properties, but also in the actual treatment. Potential metrics for quantification of the actual treatment