Habitat fragmentation, in which contiguous forests are broken into smaller, isolated patches, threatens biodiversity by disrupting species movement, shrinking populations, and altering ecosystem dynamics. Past assessments suggested declining global fragmentation, but they relied on structure-based metrics that overlook ecological connectivity. We analyzed global forest fragmentation from 2000 to 2020 using complementary metrics that captured patch connectivity, aggregation, and structure. Connectivity-based metrics revealed that 51 to 67% of forests globally—and 58 to 80% of tropical forests—became more fragmented, which is nearly twice the rate suggested by traditional structure-focused methods (30 to 35%). Aggregation-focused metrics confirmed increases in 57 to 83% of forests. Human activities such as agriculture and logging drive this change. Yet protected tropical areas saw up to an 82% reduction in fragmentation, underscoring the potential of targeted conservation.

The emergence of alternative stable states in forest systems has significant implications for the functioning and structure of the terrestrial biosphere, yet empirical evidence remains scarce. Here, we combine global forest biodiversity observations and simulations to test for alternative stable states in the presence of evergreen and deciduous forest types. We reveal a bimodal distribution of forest leaf types across temperate regions of the Northern Hemisphere that cannot be explained by the environment alone, suggesting signatures of alternative forest states. Moreover, we empirically demonstrate the existence of positive feedbacks in tree growth, recruitment and mortality, with trees having 4–43% higher growth rates, 14–17% higher survival rates and 4–7 times higher recruitment rates when they are surrounded by trees of their own leaf type. Simulations show that the observed positive feedbacks are necessary and sufficient to generate alternative forest states, which also lead to dependency on history (hysteresis) during ecosystem transition from evergreen to deciduous forests and vice versa. We identify hotspots of bistable forest types in evergreen-deciduous ecotones, which are likely driven by soil-related positive feedbacks. These findings are integral to predicting the distribution of forest biomes, and aid to our understanding of biodiversity, carbon turnover, and terrestrial climate feedbacks.

Forest fragmentation could reduce carbon sequestration beyond losses caused by declining forest area alone, if smaller patches are intrinsically less productive per unit area than larger ones. Here we analyse 17 million forest patches across the conterminous USA and show that per-area net primary productivity increases systematically with patch size. A hectare embedded within a ~100,000 km² forest is, on average, 38% more productive than an isolated hectare under comparable environmental conditions. Counterfactual analyses indicate that existing fragmentation has already reduced total annual forest productivity across the conterminous USA by 0.16 GtC yr⁻¹ or 14% relative to an upper-bound configuration of large, contiguous forests. Random forest models identify patch size as a stronger predictor of net primary productivity than topographic and soil variables. Extending the analysis globally with coarser-resolution data, we find consistent positive relationships between patch size and per-area productivity for both net and gross primary productivity. These results demonstrate that forest fragmentation can reduce carbon sequestration without net forest-area loss, highlighting the need to account for fragmentation—not just forest cover—in climate mitigation strategies.

Deforestation leads to habitat fragmentation, which adversely affects global biodiversity. Although some studies, using a separation-focused definition, have reported a decrease in fragmentation across 75% of the world's forests over recent decades, a comprehensive and ecologically relevant understanding of global fragmentation patterns remains lacking. In this study, we analyzed global fragmentation trends from 2000 to 2020, employing metrics that emphasize connectivity, aggregation, or separation. Connectivity-focused metrics reveal that 51% of global forests, particularly in tropical regions (58%), have undergone increased fragmentation—a rate nearly double that suggested by previous separation-focused metrics. This increase is corroborated by aggregation-focused metrics, which indicate heightened fragmentation in approximately 58% of forests worldwide and across all biomes. Further analysis attributes this escalation primarily to human activities, such as shifting agriculture and logging. Importantly, tropical protected areas have exhibited reductions in fragmentation by up to 82% compared to non-protected areas, underscoring the success of conservation efforts in these regions.

Deforestation has exacerbated the fragmentation of habitats into smaller, more isolated patches, driving global declines in biodiversity. Yet, a comprehensive global perspective of the trends in forest fragmentation, and its key drivers in relation to forest cover change remains elusive. To provide a comprehensive global overview of recent changes in forest fragmentation, we compare multiple fragmentation metrics, including those that are sensitive to forest cover and those that are not. Our analysis reveals that, according to cover-sensitive metrics that reflect the ecological implications of forest fragmentation, 52% of the world's forests have become more fragmented over the last two decades, a trend that is primarily attributed to increased deforestation in tropical zones. This value is twice as high than estimates from previous research, which estimated that forest fragmentation is declining across 75% of the global forest area. This discrepancy arises from a mathematical artifact, as previous cover-insensitive metrics equate declines in forest cover with decreased fragmentation. By adjusting for this and focusing on metrics that capture the ecologically relevant aspects of forest fragmentation, our study highlights a worrying trend: the ecological integrity of the global forest system has been significantly deteriorating in recent decades. This underscores the importance of using appropriate metrics to accurately assess the ecological impacts of forest fragmentation, especially in the context of global environmental change.