Revealing the main factors affecting global forest change at distinct altitude gradients

Anthropogenic activities and climate change play crucial roles in the productivity and sustainability of global forests. Although the impacts of anthropogenic activities and climate change on forests in specific regions have been reported in previous studies, their impact on the global forests along the elevation gradient remains poorly understood. Thus, here, we aimed to investigate anthropogenic activities and climate change effects on global forests by focusing on the drivers on the altitude gradient. To this end, we used spaceborne remote sensing and statistical methods to elucidate the forest areas experiencing significant change and the main factors governing the changes. Our analysis demonstrated that: (1) With respect to the altitude distribution of forest greening/browning, the higher the altitude is, the smaller is the indicated area of forest, and the smaller is the area of forest, the more significant is the greening/browning. In addition, although the area with significant forest greening was generally larger than that of browning, this trend was not consistent through the months or altitude gradients. (2) Except for the high-altitude areas in August and September (>2400 m), the impact of human global forest governance activities on the greening of forests in most altitude areas and months was greater than that of climate change. However, at most altitudinal gradients, the area of forest with significant browning caused by anthropogenic activities was smaller than that caused by climate change. This was because the scope of the impact of climate change was usually larger than that of anthropogenic activities. (3) Although temperature was the main factor responsible for significant monthly forest greening, it was not always the most crucial factor at specific altitudes. Temperature and sunshine duration were the most influential factors causing significant forest greening for the most months of the year in five terrestrial ecoregions. Sunshine duration, as the largest influencing factor causing significant forest browning, had the longest monthly impact in five terrestrial ecoregions. The findings of our study can provide a reference for better global forest management and sustainability.