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

Cécile Dardel,Manuela Grippa,Pierre Hiernaux,Laurent Kergoat,Eric Mougin,Philippe Ciais,Cam-Chi Nguyen

doi:10.3390/rs6043446

Abstract

Rain Use Efficiency (RUE), defined as Aboveground Net Primary Production (ANPP) divided by rainfall, is increasingly used to diagnose land degradation. Yet, the outcome of RUE monitoring has been much debated since opposite results were found about land degradation in the Sahel region. The debate is fueled by methodological issues, especially when using satellite remote sensing data to estimate ANPP, and by differences in the ecological interpretation. An alternative method which solves part of these issues relies on the residuals of ANPP regressed against rainfall (“ANPP residuals”). In this paper, we use long-term field observations of herbaceous vegetation mass collected in the Gourma region in Mali together with remote sensing data (GIMMS-3g Normalized Difference Vegetation Index) to estimate ANPP, RUE, and the ANPP residuals, over the period 1984–2010. The residuals as well as RUE do not reveal any trend over time over the Gourma region, implying that vegetation is resilient over that period, when data are aggregated at the Gourma scale. We find no conflict between field-derived and satellite-derived results in terms of trends. The nature (linearity) of the ANPP/rainfall relationship is investigated and is found to have no impact on the RUE and residuals interpretation. However, at odds with a stable RUE, an increased run-off coefficient has been observed in the area over the same period, pointing towards land degradation. The divergence of these two indicators of ecosystem resilience (stable RUE) and land degradation (increasing run-off coefficient) is referred to as the “second Sahelian paradox”. When shallow soils and deep soils are examined separately, high resilience is diagnosed on the deep soil sites. However, some of the shallow soils show signs of degradation, being characterized by decreasing vegetation cover and increasing run-off coefficient. Such results show that contrasted changes may co-exist within a region where a strong overall re-greening pattern is observed, highlighting that both the scale of observations and the scale of the processes have to be considered when performing assessments of vegetation changes and land degradation.

Highlights

A number of issues may impair the derivation of Aboveground Net Primary Production (ANPP) from long time series of AVHRR data, which did not aim originally at monitoring vegetation, and require significant challenges to be overcome before vegetation cover trends can be successfully derived
Since increasing run-off coefficient is observed in many places in the Sahel and since the present study gives more confidence in Rain Use Efficiency (RUE) observed from space, which is often stable [27], we propose that divergent indicators of degradation may co-exist in the Sahel as soon as shallow soils are present, especially if their detection is complicated by a typical patch size smaller than the resolution of the remote sensing dataset
Interannual variability of ANPP in pastoral Gourma and temporal trends following the extreme drought of the early 1980s are both consistently portrayed by ground and remote sensing time series, when aggregated over the 3° × 1.2° Gourma area

Summary

Introduction

Rain Use Efficiency is defined as the ratio between net primary production (NPP), or aboveground. 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]. 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]. For a given ecosystem preserved from any kind of functional change through time, the RUE values should be stable over time [6].

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