Xiaowei Tong

Forest cover dynamics are studied on a routine basis, but how changes in forest cover impact forest fragmentation has rarely been studied over a long time period resolution. This is, however, important because forest fragmentation critically impacts ecosystem services, such as biodiversity and cooling effects. Here, we apply a long time series of Landsat images from 1986–2018 and study how forest fragmentation has changed along with forest cover dynamics in southern China. Furthermore, we attribute drivers and study the impact on local air temperature changes. The region is particularly relevant as it was largely deforested three decades ago, and most of the current forests are the result of protection and forestation measures. We found a reduction in the forest fragmentation index FFI (−34.4%) from 1986 to 2018. In 81.2% of the area, forest cover increased and fragmentation decreased, while 18.5% of the area showed increases in both forest cover and fragmentation. The contribution of human activities to forest fragmentation increased by 9%, with a distinct spatial correlation between areas of increasing forest fragmentation and high levels of human disturbance. Furthermore, we found that the average level of cooling effects in areas with increased forest cover of less than 40% is heavily dominated by forest fragmentation, whereas the cooling effects are primarily controlled by changes in forest cover. These findings underscore the role of human disturbance in driving forest fragmentation, which in turn affects the functioning of forest ecosystems. The results emphasize the need for integrated land management strategies that balance forest restoration with the mitigation of human-induced fragmentation to sustain ecosystem services in the face of ongoing environmental change.

Forest structure complexity is an essential variable in forest management and conservation, as it has a direct impact on ecosystem processes and functions. Previous studies have primarily focused on tree cover as a proxy, which often falls short in providing comprehensive information on the structural complexity of forests. Sub-meter resolution remote sensing data and tree crown segmentation techniques hold promise in offering detailed information that can support the characterization of forest structure and complexity. In this study, we generated a dataset with over 5 billion tree crowns, and developed an Overstory Complexity Index (OCI) to characterize forest structure complexity from a horizontal perspective, by analyzing spatial relationships among neighboring trees from remote sensing optical images. We first extracted the location and crown size of overstory trees from optical satellite and aerial imagery at sub-meter resolution. We subsequently calculated the distance between tree crown centers, their angles, the crown size and crown spacing and linked this information with individual trees. We then used Principal Component Analysis (PCA) to condense the structural information into the OCI and tested it in China’s Guangxi province, Rwanda, and Denmark. In addition, we conducted a comparative analysis of OCI between protected and unprotected areas and among different forest types across these regions. Finally, we explored the relationships of terrain slope, distance to settlement and aboveground biomass with the OCI. Our result showed that the distribution of OCI values varies across the different bioclimatic regions, closely related to their respective forest characteristics. Higher OCI values were observed in protected areas as compared to unprotected areas, and OCI showed a positive correlation with terrain slope, distance to settlement and aboveground biomass. The proposed OCI is derived directly from standard tree-level attributes and supports a deeper understanding on forest structure and complexity in diverse ecosystems as compared to existing proxies.

Forest expansion has been observed in China over the past decades, but the typically applied coarse resolution satellite data does not reveal spatial details about China’s forest transition. By using three decades of satellite observations at a 30-m spatial resolution, we reveal here the complex spatiotemporal patterns of individual forest stands forming the forest return history of southern China. We calculate forest age, forest densification rates, and annual forest fragmentation and show that the observed forest area surge around 2010 is a result of trees planted after 2000 that formed dense forests about a decade later. We document that old forests in the 1980s were mostly fragmented into scattered patches located on mountain tops, but forests rapidly expanded downhill by 729,540 km² and alleviated the clear-cut and logging pressure from old forests. Our study provides a detailed documentation of forest densification and expansion for a country that had been largely deforested three decades ago.

A large-scale greening and forest expansion has been observed in China over the past decades, which has accelerated since around 2010. This has been inferred by the use of satellite data, however with too coarse spatial resolution to reveal spatial details about China’s forest transition. By using three decades of dense satellite observations at a 30-m spatial resolution, we reveal here the complex spatio-temporal patterns of individual forest stands forming the forest return history of southern China. We calculate forest age, forest densification rates, and annual landscape fragmentation, and show that the observed forest area surge around 2010 is a result of trees planted after 2000 that formed dense forests about a decade later. We document that old forests in the 1980s were mostly fragmented into scattered patches located on mountain tops, but forests rapidly expanded downhill by 729,540 km² from 1986–2018, connecting forest patches and creating buffer zones that alleviated the clear-cut and logging pressure from old forests. This process decreased forest fragmentation and sextupled core forest areas. Our study contests several widely accepted assumptions that negatively relate humans actions with forest dynamics, and provides a detailed documentation of forest densification and expansion for a country that had been largely deforested three decades ago.