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

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 8924 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.

Using satellite data to improve the leaf phenology of a global terrestrial biosphere model

“…Knorr et al (2010) pointed out that evaporative demand, and not just moisture availability, should be considered in phenology models. (Traore et al, 2014b) evaluated the inter-annual variability of the soil moisture of ORCHIDEE across Africa using satellite-derived estimates, and found that the new 11-layer hydrology version performed better than the 2-layer version that was used in this study. The latest version of ORCHIDEE (Naudts et al, 2015) has a more mechanistic representation of plant water stress using water potential in the soil-plant continuum, which may lead to better predictions of leaf dynamics in drought-prone regions.…”

“…Aside from making predictions of the carbon, water and energy budgets, TBMs are routinely used in trend attribution studies. A good example in this context would be the exploration of the causes of "greening" or "browning" trends in vegetation productivity (Hickler et al, 2005;Piao et al, 2006), or the impact of such trends on resource use efficiency (Traore et al, 2014b) or the C cycle (Piao et al, 2007). The fact that the optimisation resulted in changes in the strength and location of these trends (Fig.…”

Characterizing ecosystem water-use efficiency of croplands with eddy covariance measurements and MODIS products

“…C 4 crops are generally found to have high phosphoenolpyruvate carboxylase activity (Still et al, 2003). In addition, many studies have found that C 4 plants exhibited increasing WUE in response to rising CO 2 with reductions in stomatal conductance and transpirational water loss compared to C 3 species (Manzoni et al, 2011;Traore et al, 2014). Nevertheless, WUE RS only captured the broad trend of WUE.…”

Assessing hydro-ecological vulnerability using microwave radiometric measurements from WindSat

“…Several studies (Boko et al, 2007;Christensen et al, 2007;Müller, Cramer, Hare, & Lotze-Campen, 2011) have shown that P in many regions in sub-Saharan Africa will change, while others (Faramarzi et al, 2013;Müller, Waha, Bondeau, & Heinke, 2014) show that dry regions will be impacted from climate change more severely than wetter areas, thus posing additional challenges to the crop security of those regions (Fisher et al, 2013;Traore et al, 2014). Overall, there is a consensus that changes in P regimes and frequency of extreme events (Haddeland et al, 2014;Llopart, Coppola, Giorgi, da Rocha, & Cuadra, 2014;Schewe et al, 2014), coupled with an increasing water usage to meet human needs, will fundamentally change water supply in many regions across the globe, with severe implications to the environment, food security, and economic prosperity in many countries (Schewe et al, 2014;Haddeland et al, 2014;Smith et al, 2014;Konar, Jason Todd, Muneepeerakul, Rinaldo, & Rodriguez-Iturbe, 2013;Morrongiello et al, 2011;Arthington, Naiman, Mcclain, & Nilsson, 2010;Vinya, Malhi, Brown, & Fisher, 2012;Vinya et al, 2013).…”