The global climate limits on tree cover
Trees exist in nearly every terrestrial ecosystem found on Earth. In many ecosystems, tree cover promotes increased diversity and an increased ability to sequester and store carbon. Trees, however, do not exist in all climates found on Earth and, where they do exist, may not persist at high covers (e.g. grasslands). Clearly, climate is linked with the distribution of trees, but larger-scale analyses have failed to develop a comprehensive description of what climate conditions lead to diminished or absent tree cover. Complicating this analysis is the notion that extant tree cover may not be a consequence of climate alone, rather a consequence of site history, soil characteristics, or species-specific adaptations for dealing with current climate. Here, we examine global tree cover through the lens of environmental limiting factors (ELF) analysis: that extant tree cover is a consequence of multiple limiting factors, both measured and unmeasured, reducing a site’s ability to attain full canopy cover.
By applying the principle of limiting factors to a comprehensive, global database of remote sensing-derived tree cover and climate, we determined that the climate envelope of trees allows for a global terrestrial potential of 60% tree cover, whereas the actual tree cover of the planet is 22% tree cover, a 38% difference. By area, two ecosystems account for approximately 50% of the world’s extant tree cover: evergreen broadleaf forests and mixed forests. Four ecosystems account for approximately half of the world’s climate-limited potential tree cover: evergreen broadleaf forests, grasslands, open shrublands, and woody savannas. Three ecosystems accounted for nearly half of the unrealized potential tree cover: grasslands, open shrublands, and croplands. Understanding the spatial differences between climate limited potential tree cover and actual tree cover can be used to derive new hypotheses concerning whether climate, as opposed to other factors, predicts tree cover at a given location.