Bioclimatic limitations on global tree distributions

An important question of climate change impacts on ecosystems concerns the future global distribution of trees and forests. While models of ecosystem responses to climate change abound in the literature, there is increasing recognition that these models behave poorly when calibrated using historical data and validated using modern data. Theory predicts that the life history of an organism (ability to establish, grow, reproduce, and avoid dying) is limited by one, and only one, factor at a given location and time - the principle of limiting factors. Given that climate is a potential limitation on the distribution of forests, as are other non-climate factors such as natural and anthropogenic disturbance, soil properties, and taxonomic/evolutionary effects (when dealing with “functional types”) may be the factors limiting forests. Therefore, we take a step back from asking the question of “how does climate affect the global distribution of trees” (which is based on the assumption that climate does limit the distribution of trees) to a more basic question of “does climate limit trees globally?” To answer this, we applied boundary analysis to a fused, gridded global dataset of tree cover (derived from satellite imagery) and climate to predict the potential tree cover at a site under the assumption that the only limiting factor was climate. This potential maximum tree cover was compared to the actual tree cover at a given site to estimate the deviation between the two. We hypothesize that large deviations between the potential tree cover and actual tree cover indicate that a missing factor is present. Furthermore, we will test if one of the missing non-climate factors (fire) explains some of the deviation between potential and actual tree cover, particularly in sites hypothesized to be fire limited (e.g. savannas). We found that, contrary to assumptions many ecosystem modelers make, climate factors overpredict the tree cover by 40%: climate alone predicts a world of 60.7% tree cover, but the actual tree cover is 20.9%. Tree cover showed the smallest deviation from the climate-based potential in the world’s temperate and tropical forests, and high deviations in the world’s anthropogenic landscapes, shrublands and grasslands, and sparsely vegetated regions. By including fire frequency as a potential limiting factor in addition to the climate factors, we found that fire to be a major limiting factor in the world’s savannas. Our results suggest that climate variables are not the only factors determining the global distribution of forests, which challenges our interpretation of climate change impact models that based on these factors alone.