## 4. Changes in Tropical Forest Cover
During the last decade (2010-2020), a significant reduction is reported, and the average rate of deforestation drops to 10.3 million ha during 2010-2015 and 9.3 million ha during 2015-2020. This reduction of net forest loss results from a decrease in deforestation as well as an increase in recovering secondary forests, expansion of existing forest and plantations [6]. In another study, a global forest cover change assessment from 2000 to 2012 was performed, using Google Earth Engine's cloud computing platform and Landsat satellite images [67]. They reported ~7.1 million ha deforestation every year in the tropical forests. These forest cover change, losses and gain maps are now updated yearly by the GLAD and hosted on the online portal of GFW. Most of these studies used satellite images for forest cover assessment, and the primary differences in estimates lie in the assessment approach, viz. inherent classification errors [147], sampling-based approach [145], sample size and distribution of samples, spatially contiguous or wall-to-wall assessment [15,63,67], systematic or stratified sampling methods, MMU as well as definitions of 'forest','deforestation' and 'degradation'. Differences in remote sensing methodologies, assessment period and forest definition impede comparative assessment of change statistics among different studies [148]. For example, [PERSON] et al. [145] and [PERSON] et al. [15] defined the forest as a patch of land with greater than one ha in area and comprising pixels with greater than 30% tree cover, while [PERSON] et al. [67] and FAO's FRA [149] considered 25% and 10% of tree cover in a greater than one ha patch to classify it as forest. It is important to note that the automatic mapping and forest definition can classify young (re-growing) vegetation as well as oil palm and rubber plantations as forests. For example, in the study by [PERSON] et al. [15], overestimation of the forest cover extent for the year 2000 leads to an underestimation of deforestation during the 1990s and an overestimation of forest cover loss during 2000s. Moreover, the unavailability of high temporal frequency Landsat images during the 1990s coerces to use longer time steps as compared to shorter time steps in the 2000s. The use of different time steps can lead to serious errors when estimating the rate of deforestation in different decades. For instance, [PERSON] et al. [15] used two different time steps for the 1990s (\(1\times 10\) year) and 2000s (\(2\times 5\) years) which can lead to picking up more changes during 2000s in dynamic tropical landscapes such as shifting cultivation areas and tree plantations. However, FAO's FRA 2020 also used different time steps to estimate forest  
Figure 4: Distribution of intact, fragmented/managed, deforested and degraded tropical woodlands and forests in 2011 [142]. loss in the 1990s (\(1\times 10\) year), 2000s (\(1\times 10\) year), and 2010s (\(2\times 5\) year) but reported a continuous reduction in deforestation [6]. Subsequent regrowth of forests is difficult to identify, especially using medium resolution (Landsat) images with a shorter assessment period (less than a decade), and it requires significant input from ground field surveys to evaluate whether the changes are temporary or permanent [150]. Similarly, as discussed in Section 3.1, a lack of cloud-free images during the 1980s and 1990s contributes to uncertainties in forest cover extents, which might contribute to the underestimation of deforestation rates during the period, especially when using Landsat images in humid tropics. Furthermore, it is difficult to avoid the analyst bias even during the interpretation of land cover with the best available reference data. This bias can be minimized by involving several interpreters and automatic methods [151]. In another study, [PERSON] et al. [152] found significant user and producer inaccuracy in tropical forest region of Global Forest Change (GFC) datasets.