Climate change and human activities strongly influence forests, but uncertainties persist about the pervasiveness of these stressors and how they will shape future forest structure. Disentangling the relative influences of climate and human activities on global forest structure is essential for understanding and predicting the role of forests in biosphere carbon cycling and biodiversity conservation as well as for climate mitigation strategies. Using a synthetic forest canopy structure index, we map forest structural density at a near-global scale using a satellite dataset. We find distinct latitudinal patterns of multidimensional forest structure and that forests in protected areas (PAs) and so-called intact forest landscapes (IFLs) have an overall higher structural density than other forests. Human factors are the second-most important driver of forest structure after climate (temperature and rainfall), both globally and regionally, with negative associations to structural density. Human factors are the dominant driver of regional-scale variation in structural density in 35.1% of forests globally and even of forest structure in 31.4% and 22.4% of forests in PAs and IFLs, respectively. As anthropogenic forest degradation clearly affects many areas that are formally protected or perceived to be intact, it is vital to counteract human impacts more effectively in the planning and sustainable management of PAs and IFLs.
