A structural constraint drives carbon-biodiversity trade-offs under land-use retreat

Forest expansion through vegetation succession or tree planting is central to global nature-based climate solutions, yet its consequences for biodiversity remain uncertain. Although vegetation densification increases above-ground carbon storage, the associated canopy closure reduces sub-canopy light availability and constrains light-demanding species. Here, we used an integrated modelling framework—combining species distribution models, dynamic vegetation models, and species-specific light-niche filters—to examine how vegetation structure governs carbon storage, light regimes, and the set of plant species that can be supported. We explore this framework under a pan-European scenario of complete land-use retreat to exclude socioeconomic and management variation and isolate these structural dynamics.

Our simulations reveal a fundamental structural constraint in the biodiversity–climate nexus. Vegetation darkening following land-use retreat occurs far faster than carbon accumulation, producing a pronounced early reduction in the proportion of Europe’s flora whose light niches can be satisfied (“supportable plant richness”). This temporal asymmetry yields a strong, initially steep carbon–biodiversity trade-off of modest early carbon gains in exchange for disproportionally large declines in supportable plant richness, a trend that was broadly consistent across Europe’s environmental zones. To test mitigation potential, we ran disturbance-oriented sensitivity scenarios by adjusting mortality processes targeting different demographic stages. Reducing establishment or increasing early-stage mortality partly, though not entirely, reduced the severity of carbon-biodiversity trade-offs by slowing canopy closure and maintaining more light-rich, heterogeneous vegetation while still allowing continued carbon accumulation.

Because many disturbance-mediated biomes worldwide—such as savannas, temperate woodlands and alpine mosaics—harbour species pools dominated by light-demanding taxa, comparable structural dynamics are expected in such regions whenver land-use retreat or reforestation leads to rapid canopy closure. These results highlight that climate-focused forest expansion or land-use retreat can inadvertently reduce the space available for light-demanding biodiversity unless structural heterogeneity, through disturbance or other means, is explicitly considered. Recognizing this structural constraint is essential to reconcile nature-based climate solutions with biodiversity conservation. By linking mechanistic modelling to systemic trade-off analysis, this work advances nexus thinking from concept to quantification, offering a framework for integrated land-use strategies that align carbon storage, disturbance, and biodiversity outcomes.