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

Changing Sensitivity of Diverse Tropical Biomes to Precipitation Consistent with the Expected Carbon Dioxide Fertilization Effect

Global environmental changes have implications for the terrestrial ecosystem functioning, but disentangling individual effects remains elusive. The impact of vegetation responses to increasing atmospheric CO2 concentrations is particularly poorly understood. As the atmospheric CO2 concentration increases, the CO2 acts as a fertilizer for plant growth. An increase in atmospheric CO2 reduces the amount of water needed to produce an equivalent amount of biomass due to closing or a narrowing of the stomata that reduces the amount of water that is transpired by plants. To study the impacts of climate change and CO2 fertilization on plant growth, we analyzed the growing season sensitivity of plant growth to climatic forcing from alpine to semi-desert eco-climatic zones of Ethiopia for various plant functional types over the period of 1982–2011. Growing season 3rd generation Normalized Difference Vegetation Index of Global Inventory Modeling and Mapping Studies (NDVI) was used as a proxy of plant growth, while mean growing season precipitation (prec), temperature (temp), and solar radiation (sr) as the climate forcing. The sensitivities of plant growth are calculated as a partial correlation, and a derivative of NDVI with respect to prec, temp and sr for earliest and recent 15-year periods of the satellite records, and using a moving window of 15-year. Our results show increasing trends of plant growth that are not explained by any climate variables. We also find that an equivalent increase in prec leads to a larger increase in NDVI since the 1980s. This result implies a given amount of prec has sustained greater amounts of plant foliage materials over time due to decreasing transpiration with increasing CO2 concentration as expected from the CO2 fertilization effect on water use efficiency and plant growth. Increasing trends of growth in shallow-rooted vegetation tend to be associated with woody vegetation encroachment.

The variation of vegetation productivity and its relationship to temperature and precipitation based on the GLASS-LAI of different African ecosystems from 1982 to 2013

In recent years, many studies have focused on the changes of partial or single African ecosystems and the drivers of those changes. However, focusing only on partial or single ecosystems has limited the understanding of the relationships between the vegetation and climate changes in all of the African ecosystems. In this study, the temporal trends of the satellite-derived annual mean leaf area index (GLASS-LAI) were analyzed, and the inter-annual relationships were developed between the annual mean LAI and the climate variables (precipitation and temperature) for the time period ranging from 1982 to 2013. Additionally, this study applied seasonal curves and step-wise multiple regression methods to investigate the relationships between intra-annual LAI and climate changes. It was found that the GLASS-LAI over half of Africa had shown general significant greening or browning trends during the period from 1982 to 2013. From the results of inter-annual analysis, with mean annual precipitation lower than 600 mm, the greening of the savannas and grasslands in the Sahel was found to highly correspond with the increased precipitation. In contrast, the evergreen broadleaf forests in the Gulf of Guinea and Congo Basin showed strongly positive responses to the annual temperature when the mean annual temperature was below 25 °C. In regard to the intra-annual responses, the precipitation with 1-month lags was found to be helpful for the vegetation growth, with the exception of the evergreen broadleaf. The results of this research study indicated that the different land-covers in Africa had displayed clear differences in their annual trends during the examined 32-year period and had responded differently to the inter- and intra-annual climate drivers. This difference was evident by the characteristics of the vegetation covers and the geographic distributions. Therefore, further examinations of these differences can potentially improve the understanding of the land surface-atmosphere interactions among the different African ecosystems.

Spatial-temporal patterns of water use efficiency and climate controls in China's Loess Plateau during 2000-2010

Accurate assessments of spatial-temporal variations in water use efficiency (WUE) are important for evaluation of carbon and water balances. In this study, the spatial and temporal patterns of WUE and associated climate controls in China's Loess Plateau are investigated over 2000-2010 by utilizing remote sensing data and multiple statistical methods; which provides a greater understanding about how WUE changed after the Grain to Green Program (GTGP) launched. Carbon sequestration (i.e., net primary productivity, NPP) is estimated with the CASA model and water consumption (i.e., evapotranspiration, ET) is obtained from the MODIS product (i.e., MOD16). Our results identify an increasing trend in the regional mean NPP that amounted to 7.593gC/m(2)·yr with an average value of 310.035gC/m(2)·yr. Changes in ET are segmented into three stages, the growth (2000-2003), decline (2004-2006) and stable (2007-2010) stages. Regional WUE is measured at 0.915gC/mm·m(2) and shows an upward trend at a rate of 0.027gC/mm·m(2)·yr. Spatially, significant regional heterogeneity is found in both NPP and WUE with gradients decreasing from the southeast to the northwest, but sharp rises detected in northern Shaanxi. At the biome level, the annual average WUE of the four groups decrease in the order of grasslands>woodlands>shrublands>croplands. Moreover, all biomes in the grassland ecosystems exhibit a growth in WUE as does the arid desert zone in the northwestern region, suggesting that vegetation in moderately water-deficient areas may have a higher tolerance to drought. Among different meteorological factors, precipitation and drought severity index (DSI) in the Loess Plateau show a latitudinal zonality and influences the WUE, which indicated that the moisture rather than temperature would be the major control factor of the regional WUE. Finally, significant variation in vegetation WUE sensitivity in response to meteorological factors is noted. Temperature is found to be the dominant driving factor of shrublands WUE, whereas precipitation primarily influenced the WUE of grasslands, croplands, and woodlands.