Our study area comprises all Tropical & Subtropical coniferous forest, Tropical & Subtropical Dry Broadleaf Forests, Tropical & Subtropical Moist Broadleaf Forests, and Tropical & Subtropical Grasslands, Savannas & Shrublands (which contains some forested ecosystems) within +25 to -25 degree latitudes, excluding Australia. Using the Copernicus 2019 land cover raster (Buchhorn et al., 2020) we defined areas available for restoration as classes 121 (Open forest, evergreen needle leaf), 123 (Open forest, deciduous needle leaf), 122 (Open forest, evergreen broad leaf), 124 (Open forest, deciduous broad leaf), 125 (Open forest, mixed), 126 (Open forest, unknown), 20 (Shrubs), 30 (Herbaceous vegetation), and 40 (Cultivated and managed vegetation/agriculture (cropland)) (Buchhorn et al., 2020). This is summarised as the proportion of land available for restoration within each 1km2 resolution planning unit. All other classes were considered as unavailable for restoration.
The definition of the areas that are deemed available for restoration can have a profound impact on the spatial distribution of areas selected for forest restoration, and this issue is a key consideration for planning and policy related to large-scale forest restoration. The definition is uncontentious for the tropical and subtropical forest biomes where areas that were once forested but are no longer forced can be defined reliably, and where areas associated with land uses that are not available for forest restoration (e.g. urban areas, water, wetlands) can be excluded. However, it is more difficult to define areas available for forest restoration in other biomes. For example, the Tropical & Subtropical Grasslands, Savannas and Shrublands biome contains 58 ecoregions including woodlands, savannah, forest-savannah, pine forests, bushlands, shrublands, grasslands, and other types, and spanning montane, tropical, subtropical and xeric conditions.
There is a legitimate concern that including non-forest ecoregions such as savannahs in forest restoration planning could be perceived as promoting, or lead to, afforestation of these systems. However, there are three reasons we argue some of these ecoregions should not be unilaterally excluded from forest restoration planning. First, tree cover is an important component of several of these ecoregions and they may, therefore, represent areas that could support some level of forest restoration without compromising their ecological integrity.
Second, ecoregions and biomes are coarsely mapped and may contain a range of other ecosystem types at smaller scales. As some of the ecoregions are large (e.g. the Cerrado), excluding them may result in a substantial impact on the area considered available for forest restoration. Furthermore, transitions between ecoregions can sometimes be gradual, hence the boundaries are subjectively defined. For large ecoregions, error in the mapping of boundaries can substantially alter the area of the ecoregion and hence the area deemed available for forest restoration. For example, a ±1 km distance error in the definition of the boundary of the Cerrado translates to a potential ±38,000 km2 (3.8 M ha) variation in the ecoregion area (estimated using an inner and outer buffer of the Cerrado ecoregion polygon).
Third, there are many ecosystems that appear to be at risk of transitioning to alternative states as anthropogenic pressures have altered some of the biophysical processes (e.g. fire regimes, climate change, elephant abundance) that maintain some ecoregions in a non-forested, or partially forested, state. Climate change in particular has the potential to drive substantial changes in the distribution of some ecosystems over the coming decades. In that context, there may be some ecoregions that are currently non-forested, but fostering a transition to a forest ecosystem may be deemed appropriate if there is a high likelihood the current ecosystem would be lost anyway.
There is considerable subjectivity and uncertainty in the definition of areas deemed available for restoration. Many of the decisions are subjective, are sensitive to error in datasets, and may not be robust to climate change impacts. Ultimately, it is likely to fall to individual nations to make these decisions, though science could play an important role in informing those decisions. There is a research need to produce a high resolution (e.g. 30m-100m) estimate of the areas that could be deemed available for restoration under a variety of assumptions, based on a range of ecological and biophysical data, and that provides an assessment of risk in the context of climate change. Also needed is a spatially explicit assessment of the potential for perverse outcomes to arise from forest restoration.