Biodiversity side effects of carbon-focused reforestation under Paris-aligned transformation pathways

Large-scale afforestation/reforestation (AR) represents one of the most cost-effective approaches for carbon dioxide removal (CDR) and is therefore a central component of Paris-aligned land and energy transformation pathways. Using a dynamic land- and energy-system model, we assess the emergent biodiversity side effects and energy-system adjustments under varying scales of AR in Paris-aligned pathways. We show that increasing scales of AR markedly affect both the extent and pattern of habitat loss. While stringent climate action that avoids further conversion of forest and non-forest ecosystems offers substantial biodiversity co-benefits by reducing habitat loss, these benefits are largely offset at high levels of carbon-focused AR (>150 Mha) due to disproportionate losses of open habitats. Notably, we also find almost no effect of AR on energy-system transformations until 2050 and only limited effects of CDR from AR on long-term emissions. Our findings underline that near-term emission cuts remain critical for achieving the Paris Agreement and emphasise the need to shift from a dominant focus on large-scale tree planting to broader ecosystem restoration.