Large uncertainty about forest area change in the early 21st century among widely used global land cover datasets

Abstract:

Currently available land cover datasets and forest datasets are not consistent in global forest area change, the question whether the global forest area has increased or decreased in the 21st century remains open. In this study, five global land cover datasets were compared to reveal uncertainties in the global forest area changes in the 21st century, including Vegetation Continuous Fields (VCF), Global Forest Change (Hansen), Terra and Aqua combined Moderate Resolution Imaging Spectroradiometer Land Cover Climate Modeling Grid (MCD12C1), Land Cover project of the Climate Change Initiative (CCI-LC), and the new generation of Land-Use Harmonization (LUH2). All datasets displayed large variability in terms of total area, spatial distribution, latitudinal profile, and annual area change during 2001-2012. For the change in total forest area, VCF displayed an increase of 1.7×106 km2, whereas Hansen, MCD12C1, CCI_LC and LUH2 stated decreases of 1.6×106 km2, 0.4×106 km2, 0.2×106 km2 and 0.1×106 km2, respectively. In the tropics, a negative trend in forest area is seen in all the datasets. The inconsistency mainly occurred between VCF and other datasets, the former has the inter-annual forest change with an order of magnitude greater than other datasets. A validation using the high resolution Google Earth images implies that the VCF data could not appropriately distinguish tree cover and short vegetation; the seasonal growth of herbs and shrubs may lead to an unreasonable large interannual changes in forest area. The results of this study reveal the large uncertainty in global forest change in the 21st century ands calls for more studies to reduce the uncertainty to support forest policies and to contribute to the global action against climate change.