Abstract
BackgroundThe assessment of change in forest ecosystems, especially the change of canopy heights, is essential for improving global carbon estimates and understanding effects of climate change. Spaceborne lidar systems provide a unique opportunity to monitor changes in the vertical structure of forests. NASA’s Ice, Cloud and Land Elevation Satellites, ICESat-1 for the period 2003 to 2009, and ICESat-2 (available since 2018), have collected elevation data over the Earth’s surface with a time interval of 10 years. In this study, we tried to discover forest canopy changes by utilizing the global forest canopy height map of 2005 (complete global coverage with 1 km resolution) derived from ICESat-1 data and the ATL08 land and vegetation products of 2019 (sampling footprints with 17 m diameter) from ICESat-2.ResultsOur study revealed a significant increase in forest canopy heights of China’s Beijing-Tianjin-Hebei region. Evaluations of unchanging areas for data consistency of two products show that the bias values decreased significantly from line-transect-level (− 8.0 to 6.2 m) to site-level (− 1.5 to 1.1 m), while RMSE values are still relatively high (6.1 to 15.2 m, 10.2 to 12.0 m). Additionally, 58% of ATL08 data are located in ‘0 m’ pixels with an average height of 7.9 m, which are likely to reflect the ambitious tree planting programs in China.ConclusionsOur study shows that it is possible, with proper calibrations, to use ICESat-1 and -2 products to detect forest canopy height changes in a regional context. We expect that the approach presented in this study is potentially suitable to derive a fine-scale map of global forest change.
Highlights
The assessment of change in forest ecosystems, especially the change of canopy heights, is essential for improving global carbon estimates and understanding effects of climate change
Comparison results Before comparing the data sets, forest canopy heights derived from every filtered ICESat-2 step are shown in Fig. 7a
It is acknowledged that several sampling points were not completely representative, but we consider it was Another factor that impacts the accuracy is the difference of land cover products employed by the canopy height map and the ICESat-2 ATL08 product
Summary
The assessment of change in forest ecosystems, especially the change of canopy heights, is essential for improving global carbon estimates and understanding effects of climate change. Spaceborne lidar systems provide a unique opportunity to monitor changes in the vertical structure of forests. NASA’s Ice, Cloud and Land Elevation Satellites, ICESat-1 for the period 2003 to 2009, and ICESat-2 (available since 2018), have collected elevation data over the Earth’s surface with a time interval of 10 years. A number of studies related to forest height changes utilized repeated airborne lidar data in local area, achieving bi-temporal or multitemporal observations (Dubayah et al 2010; Hudak et al 2012; Zhao et al 2018). Before the launch of Ice, Cloud and land Elevation Satellite (ICESat) -2, due to limited availability of data, it was difficult for spaceborne lidar to monitor forest change for a long period of time. Combining the NASA’s ICESAT − 1 and − 2 may alleviate this problem
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