Trees Combat Climate Change in China by Reducing CO2 Levels

Forest management in southern China generates short term extensive carbon sequestration

By: Xiaowei Tong, Martin Brandt, Yuemin Yue, Philippe Ciais, Martin Rudbeck Jepsen, Josep Penuelas, Jean-Pierre Wigneron, Xiangming Xiao, Xiao-Peng Song, Stephanie Horion, Kjeld Rasmussen, Sassan Saatchi, Lei Fan, Kelin Wang, Bing Zhang, Zhengchao Chen, Yuhang Wang, Xiaojun Li and Rasmus Fensholt

Summarized by Anna Geldert

What data were used? Researchers collected data on carbon storage (long-term carbon stocks) and sequestration levels (new uptake of carbon gasses) by forest type. Data was recorded between 2002 and 2017, and the area of study focused on eight provinces in southern China. This data was compared with existing published data on soil moisture levels and national CO2 emissions.

Methods: Researchers used satellite imagery data from MODIS (Moderate Resolution Imaging Spectroradiometer) for the basis of this study. Using approximately 10,000 MODIS images, they divided the area into a grid with a scale of 0.25km2. Using “training points” of known land cover, they trained a computer to estimate the probability of forest cover in each grid cell, as well as the level of change in forest cover over time. Based on this information, grid cells were classified into eight categories of forest types, including dense forest (probability of forest cover ≥ 0.8, with low change), persistent forest (probability ≥ 0.5, with low change), persistent non-forest (probability ≤ 0.5, with low change), recovery (regrowth of deforested areas, causing a gradual shift from non-forest to forest), afforestation (tree plantation in previously unforested areas, causing a rapid shift from non-forest to forests), deforestation (shift from forest to non-forest), rotation (harvested area, causing fluctuation between forest and non-forest) and rotationL (harvested area, causing fluctuations and low forest recovery). Researchers then estimated the carbon density of each forest type using data from a previous study 2015 from GLAS (Geoscience Laser Altimeter System, i.e., a satellite machine designed to measure the vertical structure of forests). MODIS data from this study was cross-referenced with existing passive microwave data from SMOS (soil moisture and ocean salinity), which also measured carbon density from this region, though on a broader scale. SMOS data were also used to determine the average soil moisture in the studied region.

Results: Both tree cover and fossil fuel emissions increased considerably between 2002 and 2017. Using the MODIS data, researchers estimates a carbon sink of 0.11 Pg C year-1 (i.e., petagrams of carbon per year, the equivalent of 0.11 billion metric tons per year) in the region studied. This accounted for roughly 33% of yearly carbon emissions since the year 2012. Unmanaged dense forest had the highest carbon density, accounting for 20.5% of carbon storage despite only occupying 8.8% of the land. However, dense forests had low levels of carbon sequestration, accounting for only 4% of the total uptake. Comparatively, persistent non-forests and managed forests (recovering, afforestation, and rotation areas) all had low levels of carbon storage but accounted for 65% the total carbon sequestration. Persistent forest areas lay somewhere in the middle, with moderate storage and sequestration levels. Heavily harvested forests (deforested and rotationL areas) had much lower sequestration rates, and served as carbon sources rather than sinks. Finally, SMOS data revealed that soil moisture levels tended to be lower in regions with lots of managed forests.

The figure shows a bar graph comparing the type of land to the level of sequestration of CO2 emissions. The x-axis is labeled “Type of land use,” and is numbered 1 through 8. A legend on the right of the graph indicates that each number corresponds to a type of forest or non-forest area: 1 represents dense forest, 2 represents forest, 3 is non-forest, 4 is recovery, 5 is afforestation, 6 is deforestation, 7 is rotation, and 8 in rotationL. A different y-axis is present on either side of the graph, so that both the relative percent of CO2 emissions sequestered and the numerical quantity of carbon sequestered per year are represented. The left axis represents the percentages, and spans from 0 to 7.5, increasing at increments of 1.25. The right axis represents the quantity of carbon sequestered in petagrams per year, spanning from 0.00 to 0.03 and increasing by a factor of 0.05. A separate legend on the bottom of the graph indicates that the dark orange portions of the bars represent the percentage/fraction of carbon sequestered compared to CO2 emissions from China as a whole, while light orange portions correspond to emissions from the eight provinces alone. The land use type with the highest percentage of carbon sequestered was non-forest, which accounted for approximately 6.5% of annual emissions for the eight provinces, or 0.026 total Pg of carbon per year. Non-forest was followed by forest and afforestation (both accounting for 5.2% of emissions and 0.21 total Pg), recovery (4% and 0.016 Pg), rotation (3.5% and 0.013 Pg) and dense forest (1% and 0.009 Pg). Deforestation and rotationL were the only types of land use to represent a negative percentage and quantity of carbon sequestered, indicating that they served as a carbon source rather than a carbon sink. Deforestation accounted for approximately -0.2% (or 0.001 Pg) of carbon sequestration, while rotationL was nearly negligible. The percentages of carbon sequestered when compared to national emissions (dark orange) were all about one fifth of the percentages when compared to the eight provinces alone.
Fig. 1. Average percent of CO2 emissions sequestered annually by each forest type. CO2 emissions from the eight provinces in the study region, as well as emissions from China as a whole, are both shown.

Why is this study important? This study compares the effectiveness of different types of forests in mitigating the impacts of climate change. While natural, dense forests were the best at storing carbon long-term, managed forests were most effective at rapidly removing CO2 from the atmosphere on a shorter timescale. Harvested forests, especially those classified as “rotation,” were especially successful as they were able to sequester relatively high levels of carbon while still providing significant economic revenue from timber for the region. Overall, changes in forest management policies in China in 2002 led to an impressive reduction in carbon emission levels (33%). However, it is important to note that an additional 3 million km2 of forested land would be needed to reach net zero carbon emissions, a number which is unreachable in this region. Likewise, reduced levels of soil moisture indicate that heavily managed forests may not be sustainable in the long run, and will likely be less effective during periods of drought. More research is needed to determine if these forest management policies have already reached maximum effectiveness, or if other adjustments can be made to further increase sequestration.

The big picture: As the main drivers of climate change, fossil fuel emissions continue to threaten our planet. Forestation and forest management policies, such as those established in China at the turn of the century, are a way to mitigate the impact of greenhouse gasses. Modeling future policies after these could help increase carbon sequestration worldwide, especially until renewable energy becomes available. However, as was revealed in this study, it is nearly impossible at current emission levels to reach net zero carbon emissions through forest management alone; in the long term, forest management will likely need to be combined with other policies to ensure a sustainable future.

Citation: Tong, X., Brandt, M., Yue, Y., Ciais, P., Rudbeck Jepsen, M., Penuelas, J., … Fensholt, R. (2020). Forest management in southern China generates short term extensive carbon sequestration. Nature Communications, 11(1).

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