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

Abstract

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 923 yr. 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 200 mm yr−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 (>200 mm yr−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.

Rain-Use-Efficiency: What it Tells us about the Conflicting Sahel Greening and Sahelian Paradox

“…These trends in the residuals may be positive or negative, and reflect changes in vegetation composition, land cover (to name a few), and finally indicate land degradation [4]. Moreover, the ANPP residuals can be examined to look for a non-random distribution and possible dependency of residuals on rainfall amount [2,7,27,30]. Indeed, if the hypothesis of a linear relationship between ANPP and rainfall does not hold, it should be evidenced by the shape of the distribution of RUE plotted against rainfall.…”

“…It has been increasingly used to analyze the variability of vegetation production in arid and semi-arid biomes, where rainfall is a major limiting factor for plant growth [1][2][3][4][5]. Indeed, RUE was designed to separate rainfall contribution to vegetation production from other factors such as plant life-form, nutrient status or anthropogenic effects, like management and cropping practices [6].…”

Assessing Land Degradation/Recovery in the African Sahel from Long-Term Earth Observation Based Primary Productivity and Precipitation Relationships

“…They further suggest that a quadratic or cubic relationship might replace the assumption of proportionality when going beyond 200-300 mm precipitation. Other publications based on in situ measurements suggest that biome-specific RUE values should be applied depending on the rainfall regime [26][27][28]. However [28] concludes that inter-annual variation in RUE is not correlated with precipitation at the site level.…”

Assessing Land Degradation and Desertification Using Vegetation Index Data: Current Frameworks and Future Directions

“…Rainfall increases may potentially drive an increase in both RUE and biomass, rendering RUE irrelevant as a detrending technique [84]. However, this interpretation has been criticised as being limited to sites with anomalous precipitation regimes [24,113,114]. RUE values have been demonstrated to correlate with inter-annual precipitation fluxes [65,88].…”