Meta-analysis of ANPP and rain-use efficiency confirms indicative value for degradation and supports non-linear response along precipitation gradients in drylands

Questions In drylands above-ground net primary production (ANPP) and rain-use efficiency (RUE) are common ecological indicators for assessing ecosystem state, including degradation and supply of key ecosystem services. However, both indicators have been criticized as lumped parameters, since they aggregate complex information. Their value as ecological parameters in decision-making and their use in ecological modelling therefore have been challenged and their explanatory power remains unclear. Furthermore, there is no consensus about the response of ANPP and RUE along precipitation gradients. Methods Taking advantage of several long-term studies in (semi-)arid environments where ANPP and RUE were recorded, we compiled a data set of 923yr. We used meta-analysis to disentangle the effects of different ecological layers (climate, soil and land use) on ANPP and RUE. Linear piece-wise quantile regression (LPQR) was used to analyse the response of maximum and median ANPP and RUE as functions of precipitation. We assumed that looking at maximum response (instead of average response) stratified for land-use intensity was an ecologically more plausible way to understand ANPP constrained by precipitation and land use. Results We separated the impact of different environmental factors into distinct, quantitative effect sizes with the aid of meta-analyses. ANPP was affected by recent and previous precipitation, land use, soil and biome. LPQR revealed that both parameters displayed several sequential linear intersects, which together formed a unimodal trend, peaking around precipitation of 200mmyr-1. Unimodal response was more pronounced for maximum values (ANPPmax, RUEmax) than for median values. Peak ANPPmax and RUEmax, as well as post-peak decline (>200mmyr-1) were affected by land use: higher land-use intensity decreased intercepts and increases post-peak decline. Conclusions Our results have important consequences for the use of RUE as an ecosystem indicator and a tool in ecosystem monitoring and decision-making. Most importantly, grasslands, shrublands and savannas significantly differ in their primary production, with a biome-specific importance of precipitation, land use and previous year's precipitation. We thus propose to establish biome-specific reference values of maximum and average RUE. Our study also contributes to reconcile contradictory findings for ANPP and RUE response along precipitation gradients of varying length