非洲人类活动导致的土地利用及地表覆盖变化造成了严重的森林损失和森林退化，并引起了碳排放增加和植被生态系统服务功能减弱等负面效应。然而，非洲森林损失和森林退化对植被地上碳储量动态的影响及其作用机制尚不明确。本文研究了非洲森林损失和森林退化对植被地上碳储量时空动态变化的贡献，并揭示了火灾叠加森林边缘效应加剧森林退化的作用机制，主要结果如下：基于多套遥感数据产品，本文首先分析了非洲森林损失和森林退化面积的时空分布格局。结果表明，非洲每年新增森林损失面积在2001-2019年间呈现出显著增加的趋势（2.2×10^3 km^2 yr^-1），主要是由森林损失分布增多和平均损失面积扩大共同变化导致，并且成片森林地区的森林损失也在增加。在此期间，非洲的森林退化面积表现出逐年减少的趋势（-1.1×10^3 km^2 yr^-1）。森林破碎化导致的边缘森林退化是非洲重要的森林退化来源之一。2010年非洲边缘森林面积总共为2.1×10^6 km^2，其退化产生的植被地上碳储量亏缺累计为4.06 Pg C。边缘森林的退化程度会随着时间不断加剧。本文进一步利用微波遥感植被光学深度数据计算了2010-2019年非洲不同植被类型地上碳储量的时空动态变化，并分析了森林损失、森林退化和其他驱动因素的贡献。2010-2019年间，非洲植被地上碳储量表现为净碳源（-8.7 Tg C yr^-1），其中，由于森林损失导致的碳排放为-187.3 Tg C yr^-1。在扣除森林损失的影响后，森林退化和气候相关变量是影响非洲植被地上碳储量动态的重要因素，起到的贡献分别为12.7 ~ 14.0%和49.3 ~ 52.1%。本文最后解析了森林边缘效应和火灾对边缘森林退化的交互作用机制。研究显示，火灾加剧了边缘森林的退化，造成了0.9 Pg C的额外地上碳储量亏缺。火灾可以通过直接作用（火灾直接侵入边缘森林）和间接作用（火灾影响局地大气环流）两种方式加剧边缘森林退化。其中，火灾主要通过直接作用影响干森林，而通过直接和间接作用共同影响湿森林。本文还预测了不同的未来情景下，2010-2100年非洲所有新产生的边缘森林将导致0.54 ~ 3.14 Pg C的植被地上碳储量损失。本文阐明了非洲森林损失和森林退化面积的时空分布格局及其对植被地上碳储量动态的影响，并揭示了森林边缘效应与火灾对森林退化的交互作用机制。研究结果可以为未来森林管理政策的制定和森林保护项目的实施提供科学依据。
Land use and land cover change (LULCC) in Africa have caused severe forest loss and forest degradation, resulting in substantial carbon emissions and loss of ecosystem service functions. However, the impacts of forest loss and forest degradation on the temporal changes in vegetation aboveground carbon stocks in Africa remain unclear. This study quantifies the contributions of forest loss and forest degradation on the spatial-temporal changes of vegetation aboveground carbon stocks in Africa, and analyzes the impacts of fire and forest edge effect on forest degradation and the associated carbon emissions.Using multiple remote sensing products, the spatial-temporal changes of forest loss and forest degradation areas in Africa were analyzed. During 2001-2019, the area of annual forest loss in Africa showed an increasing trend of 2.2×10^3 km^2 yr^-1, due to both the increasing number of forest loss events and the expanding area per event. Large forest patches also suffered from area losses in recent years. By contrast, forest degradation area in Africa showed a decreasing trend of -1.2×10^3 km^2 yr^-1 during 2001-2019. In 2010, the area of edge forests resulted from forest fragmentation was 2.1×10^6 km^2, and it caused a cumulative carbon deficit of 4.06 Pg C. After the creation of edges, the forest edge effect will become severer over time.The temporal changes in aboveground carbon stocks during 2010-2019 were further estimated for different vegetation types in Africa using the L-band microwave Vegetation Optical Depth (L-VOD) data. The contributions of forest loss, forest degradation and other factors to the aboveground carbon stock changes were also analyzed. Aboveground carbon stock in Africa acted as a net carbon source (-8.7 Tg C yr^-1) during 2010-2019, and the aboveground carbon loss due to forest loss was -187.3 Tg C yr^-1. After excluding the impacts of forest loss, the main drivers of the remaining aboveground carbon changes were forest degradation and climate-related factors, which explained 12.7 ~ 14.0% and 49.3 ~ 52.1% of the variations in the aboveground carbon changes resulted from non-deforestation drivers, respectively.Combining the results of forest edge effects and the fire data, the interactions between forest edge effect and fire and their impacts on forest degradation were analyzed. Fire enhanced forest degradation in the edge forests, and led to an extra carbon emission of 0.9 Pg C. Two possible mechanisms for the enhanced forest degradation by fires were identified: the direct effect, i.e., fire intrudes into forest patches directly, and the indirect effect, i.e., fire changes the local circulations near the edges. The enhanced degradation in edge forests by fire is mainly driven by the direct effect in the dry forests, while it is mutually driven by both direct and indirect effects in the moist forests. In different future scenarios, the carbon emissions due to forest degradation in future created edge effects during 2010-2100 was also predicted. This study characterized the spatial-temporal patterns of forest loss and forest degradation area in Africa and their impacts on aboveground carbon dynamics, and also revealed the mechanisms of interactions between forest edge effect and fire on forest degradation. The findings can support the policy-making of forest managements and the implementation of future forest conservation projects.
