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Between degradation and conservation: The carbon balance of the Chinese karst ecosystem

Periodic Reporting for period 1 - CabaKarst (Between degradation and conservation: The carbon balance of the Chinese karst ecosystem)

Reporting period: 2018-11-01 to 2020-10-31

Over the past decades, the karst region of southern China has been identified as a hot spot of global tree cover increase. Ecological engineering projects have converted this area that was characterized by a high population pressure, intensive agricultural use and accelerated land degradation into a carbon sink. While the overall increase in vegetation cover is not a new finding of this project, it had not been clear how this massively increasing vegetation cover has impacted the regional and national carbon balance. It was also unclear to which extend the increase in tree cover can be attributed to ecological engineering projects, forest management types, and how population migration impacted on these land transitions. This is however important to understand, as tree plantations and the restoration of degraded landscapes are key to mitigate climate change. Here we have the unique chance to study how large scale human forest management can help to increase the carbon sink and reduce atmospheric CO2. The reason why this topic has not been studied in detail is the fact that traditional satellite data do not go beyond variables describing the greenness of the landscape. This project aimed at developing new methods and applying new data sources to study changes in carbon stocks in southern China. These carbon dynamics are then related to management, like ecological engineering, climate, human migration, and other drivers. The ability of managed forests to act as a carbon sink and mitigate climate change by offsetting CO2 emissions from fossil fuel burning was further studied. We conclude that tree planting from ecological engineering and released pressure on the vegetation via rural depopulation created a large carbon sink in rural southern China. This carbon sink can offset about 1/3 of the CO2 emissions over the past decade, but we also observed saturation in tree growth, making this carbon sink a rather short term and transient solution to mitigate climate change.
First the annual aboveground biomass carbon maps were developed using passive microwaves (SMOS satellite) and optical data (MODIS and Landsat), covering the period 2000-2017. High resolution PlanetScope and Gaofen-1 images were also acquired for the study area and served here for visually inspecting the results and also for training the models. A detailed forest cover dataset based on MODIS was also developed. The forest cover data were used to classify the area into existing forests and new forests, as well as different forest management types, based on the growing speed, magnitude and direction. Then the datasets were combined to study the carbon gains and losses for different land cover and forest types. We studied the impact of drivers (geology, climate management, etc) on forest and carbon dynamics. Carbon gains were attributed to afforestation programs and human migration. CO2 emission data were used to calculate the relationship between the vegetation carbon sink and CO2 emissions from fossil fuel burning. We further studied how much of the carbon carrying capacity has been reached at the end of the study period. Here we predicted the amount of carbon that can potentially be stored, and compared it with the observed carbon stocks. The datasets have been made publicly available. Results were presented at workshops and conferences (e.g. AGU 2021) and published in four publications, including two of the most prestigious open access journals Nature Communication and Earth’s Future (under review), and also in Nature Sustainability. The media has taken up our work and published the stories in Chinese and Danish newspapers, and also on the faculty websites.
Both annual forest cover and aboveground biomass carbon maps are novel and beyond state of the art. Using these maps, we can show unprecedented patterns, as shown in our publications in Nature Communications and Nature Sustainability. We found that it is mainly human management in the form of afforestation and released pressure on existing forests that was causing major carbon stock increases over the past decades. Interestingly, not only conservation was a main driver, but also rural depopulation. We found urbanization causes people to move from rural areas to urban areas, which contributes to the carbon stock increase, by releasing human pressure on tress in rural areas and tree planting in urban areas in the context of China’s “ecological civilization”. We have shown that human management can indeed reverse degradation and that urbanization is not necessarily coupled with environmental destruction, but a sustainably managed urban expansion can even be an integral part of the pathway towards carbon neutrality.
This photo with my Danish colleagues was made during the field trip to China kast area in 2019.