Relative effects of climate variation and human activities on grassland dynamics in Africa from 2000 to 2015

Highlights

• The relative effects of climate variation and human activities on grassland dynamics in Africa were studied.

• Precipitation was the decisive climatic factor driving the grassland dynamics in most areas of Africa

• Overgrazing was the main form of human activities influencing grassland variations.

Abstract

Grassland plays a key role in socioeconomic development and environmental protection in Africa. Climate variation and human activities are two main drivers of grassland dynamics. Quantitatively assessing the contributions of these two factors and understanding the driving mechanisms are important in ecosystem adaptation and management. In this research, the spatiotemporal patterns of grassland dynamics in Africa during 2000–2015 were analyzed based on the net primary productivity (NPP), M-K test, linear regression analysis, and correlation analysis. In addition, the potential NPP (PNPP), human-induced NPP (HNPP) and actual NPP (ANPP) were employed to establish scenarios to distinguish the relative impacts of climatic and human factors on grassland dynamics. An overall grassland ANPP increase than decrease (62.91% vs 37.09%) was found during 2000–2015. 21.80% of the total grassland area showed increases in ANPP, which was influenced by climate variation, whereas 23.61% were affected by human activities. The ANPP decreases induced by climate variation, human activities and the combination of these two factors occupied 19.31%, 8.39% and 9.39% of the total grassland area, respectively. Therefore, the contributions of climatic and human factors on ANPP increase were almost consistent, while climate variation was the dominated factor on ANPP decrease. In addition, the respective roles of these two factors were quite different in five grassland types. The dynamics of ANPP in closed shrublands, non-woody grasslands, and open shrublands were mainly attributed to the climate variation. Meanwhile, the human-dominated increases in ANPP were observed in woody savannas. Further analysis demonstrated that the increases in African grassland ANPP are likely due to the mitigation of drought and reduction in land use intensity, while the decreases in ANPP were related to unbalance of local hydrothermal condition and overgrazing. This study expects to improve the understanding of the respective contributions of climatic and human factors on grassland dynamics in Africa.

Introduction

Grassland, as one of the most widespread terrestrial ecosystem, occupies approximately 25% of land covers and is of critical importance in carbon cycling, food security and climate change (Conant et al., 2001; Field et al., 1995; Houghton, 1994). The grassland dynamics is the significant natural phenomenon resulting from both climatic and human factors (Chen et al., 2019). With the dramatic changes in land use management, climatic and atmospheric composition in recent decades (Xu et al., 2016), the grassland ecosystems have deviated from its previous steady state and formed typical variation processes and spatiotemporal patterns (Maclean and Wilson, 2011). Heightened human activities and dramatic climate variation can cause grassland degradation, even lead to the serious economic and ecological losses (Zhou et al., 2015), especially in the semiarid and arid regions (Chen et al., 2014; Zhang et al., 2018). Therefore, accurately assess the relative effects of climate variation and human activities on grassland dynamics has a great significance to ecosystem adaptation and management at both national and global scales (Aldous et al., 2011; Liu et al., 2019).

Africa has widely spread grassland, and local residents has been exploiting the grassland resource for forage, food, herbal medicine, fuel and building materials (Scurlock and Hall, 1998; Ugbaje et al., 2017). However, considering the rural poverty and the demand of the increasing population in most regions of Africa, agriculture land expanded at the consumption of forest and grassland, which had adversely altered the carbon, water and nitrogen cycles of ecosystems (Gang et al., 2014; Jordan, 1986; Symeonakis and Drake, 2004). Meanwhile, Africa is a example of continent suffering from grassland degradation caused by overgrazing over past decades (Doran et al., 1979). In addition, the climate have undergone a significant changes in Africa, which exected an enormous effect on distribution, composition, and structure of the grassland ecosystem (Batjes, 2004; Ugbaje et al., 2017). Therefore, the grassland has attracted growing concern because of its significant effect on the ecological security and the development of local economy in Africa (Gang et al., 2014).

In recent years, the dynamics of grassland net primary productivity (NPP) have been reported at different regions due to global warming, urbanization, resources demand and population pressure (Liu et al., 2019; Wang et al., 2016). Meanwhile, the influence of climate variation across different grassland types and seasons were also reported (Shen et al., 2015). Moreover, the mechanism of grassland dynamics in the future will be more complex with increasing human interference. Although previous studies have revealed the underlying drivers of vegetation dynamics in Africa (Bosch, 1989; Hoscilo et al., 2015; Wessels et al., 2007), the quantitative assessment of the individual contributions of underlying drivers forcing on vegetation variations is still lacking, particular in the African grassland. Hence, there is a gap in the understanding of where and the degree to which climatic and human factors affect the African grassland.

Recently, numerous methods have been used to distinguish the respective effects of climatic and human factors on grassland dynamics (Han et al., 2018; Xu et al., 2016; Zhang et al., 2018). Traditional method is based on the field investigation, which is costly and inaccurate because of the vastness of grassland (Zhou et al., 2017). Comparatively, remote sensing and model simulation are widely used to investigate the relative role of climate variation and human activities in the changes of grassland because of their convenience, low cost and wide survey areas. Some studies selected rainfall use efficiency (RUE) as a proxy to differentiate the human effect on grassland degradation (Symeonakis and Drake, 2004; Zhou et al., 2017). Meanwhile, the normalized difference vegetation index (NDVI)-based residual trend method was utilized to explore the climatic and human impacts on grassland (Li et al., 2012). However, the pre-mentioned indicators (RUE and NDVI) are oversimplified and may cause uncertainty to the results (Chen et al., 2019; Zhou et al., 2015). NPP is defined as the net amount of organic matter captured by vegetation through photosynthesis, which is sensitive to the environment change (Potter et al., 1999; Wang et al., 2017). Therefore, NPP have been selected as an indicator to investgate the effects of climate variation and human activities on grassland dynamics in current studies (Chen et al., 2014; Xu et al., 2016). As a quantitative and objective method, NPP coupled scenario simulation are widely employed at the regional and global level (Gang et al., 2014; Mu et al., 2013; Yang et al., 2016).

Therefore, the purposes of this study are (1) to explore the spatiotemporal variation of grassland NPP in Africa from 2000 to 2015. (2) To quantitatively differentiate and assess the individual contributions of climate variation and human activities on grassland dynamics. (3) To reveal the main drivers in the variations of different grassland types. The results of this study are expected to provide a reference for pastoral management and precaution of land desertification and degradation in African grassland.