1982–2010 Trends of Light Use Efficiency and Inherent Water Use Efficiency in African vegetation: Sensitivity to Climate and Atmospheric CO2 Concentrations

Abdoul Traore,Cecile Dardel,Benjamin Poulter,Nicolas Viovy,Natasha Macbean,Martin Jung,Shilong Piao,Philippe Ciais,Nicolas Vuichard,Joshua Fisher,Ranga Myneni

doi:10.3390/rs6098923

Abstract

Light and water use by vegetation at the ecosystem level, are key components for understanding the carbon and water cycles particularly in regions with high climate variability and dry climates such as Africa. The objective of this study is to examine recent trends over the last 30 years in Light Use Efficiency (LUE) and inherent Water Use Efficiency (iWUE*) for the major biomes of Africa, including their sensitivities to climate and CO2. LUE and iWUE* trends are analyzed using a combination of NOAA-AVHRR NDVI3g and fAPAR3g, and a data-driven model of monthly evapotranspiration and Gross Primary Productivity (based on flux tower measurements and remote sensing fAPAR, yet with no flux tower data in Africa) and the ORCHIDEE (ORganizing Carbon and Hydrology In Dynamic EcosystEms) process-based land surface model driven by variable CO2 and two different gridded climate fields. The iWUE* data product increases by 10%–20% per decade during the 1982–2010 period over the northern savannas (due to positive trend of vegetation productivity) and the central African forest (due to positive trend of vapor pressure deficit). In contrast to the iWUE*, the LUE trends are not statistically significant. The process-based model simulations only show a positive linear trend in iWUE* and LUE over the central African forest. Additionally, factorial model simulations were conducted to attribute trends in iWUE and LUE to climate change and rising CO2 concentrations. We found that the increase of atmospheric CO2 by 52.8 ppm during the period of study explains 30%–50% of the increase in iWUE* and >90% of the LUE trend over the central African forest. The modeled iWUE* trend exhibits a high sensitivity to the climate forcing and environmental conditions, whereas the LUE trend has a smaller sensitivity to the selected climate forcing.

Highlights

Terrestrial gross primary productivity (GPP) and evapotranspiration (ET), two critical components of the terrestrial carbon and water cycles, are driven by solar radiation and limited by soil moisture availability [1,2,3,4,5,6,7]
Lower model tree ensemble (MTE)-iWUE* values are found in the central African forest with values of less than 12 gC · hPa · mm · m
Previous studies have reported the sensitivity of light use efficiency (LUE) and inherent water use efficiency to atmospheric CO2 increase, climate trends and land use change [27,28,29,30,31,32,33,34] with very few studies over Africa

Summary

Introduction

Terrestrial gross primary productivity (GPP) and evapotranspiration (ET), two critical components of the terrestrial carbon and water cycles, are driven by solar radiation and limited by soil moisture (and nutrient) availability [1,2,3,4,5,6,7]. LUE is defined as the ability of the vegetation to use GPP per unit of Absorbed Photosynthetically Active Radiation (APAR) that is limited by temperature and water shortage [14,15]. This definition integrates limiting environmental factors through the fraction of absorbed photosynthetically active radiation (fAPAR) and provides LUE value below its theoretical potential value used in several studies [1,16,17]. The dependence of iWUE* on environmental conditions indicates possible adaptive adjustment of ecosystem physiology in response to a changing environment

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Conclusion