Highlights:
- An online real-time monitoring technology for carbon effects from forest cover change on the Google Earth Engine.
- The CCDC-SBK ensemble model was improved by incorporating the heterogeneous carbon response functions.
- The carbon effect of forest cover loss per hectare was about 4.5 times that of forest cover gain.
- The forest cover change might behave as a carbon source even under a net-zero or net increase in forest cover.
- Urban and agricultural expansion contributed 37% and 10% of carbon emissions, respectively.
Forests cover over 30% of the global land area and are essential carbon stocks that influence atmospheric CO2 concentrations and climate. As critical components of the terrestrial carbon sink, forests can offset carbon emissions from human activities. However, the regional distribution of forest dynamics and associated changes in carbon stocks are highly uncertain. This makes it difficult to accurately assess the contribution of forest ecosystems to the global carbon budget.
In the past decades, the Yangtze River Delta (YRD), one of China's most economically active and urbanizing regions, has experienced significant changes in forest cover due to urban expansion and ecological restoration projects. To address the gap in continuous and quantitative monitoring of forest change, the study integrated the continuous change detection and classification (CCDC) algorithm with an improved spatial carbon bookkeeping (SBK) model. They used Landsat long-term observations and ground measurements to track carbon emissions, uptakes, and net changes from forest cover changes in the YRD.
The study revealed that despite a net gain in forest cover of 10.95 × 104 hectares during 2000–2020, carbon emissions from forest losses were four times greater than carbon uptakes from forest gains. Urban and agricultural expansions accounted for 37% and 10% of carbon emissions, respectively, while the Grain for Green project contributed to 45% of carbon uptakes.
Cities with significant net gains in forest area, such as Suzhou and Shanghai, have shown continuous improvement in forest coverage rates through high-quality land greening initiatives. However, rapidly urbanizing areas still face significant challenges in forest resource protection. "It is imperative to conserve existing forests and stop forest loss while enhancing terrestrial carbon sinks through afforestation and reforestation efforts," noted Prof. Xiangrong Wang, the corresponding author.
The study's detailed and continuous monitoring of forest change provides valuable data for regional forest resource conservation and management. It underscores the necessity of understanding the asymmetric carbon effects of forest cover loss and gains to accurately assess the capacity of forest carbon sinks.
This work is supported by the National Key Research and Development Program of China (No. 2016YFC0502700) and the General Program of Education Department of Zhejiang (No. 23056209-F).
