Decline in regeneration capacity in mature forests across Great Britain

Mature forests are key for climate change mitigation, preserving biodiversity and providing ecosystem services. The future of these services depends on the forest structure and composition which in turn largely depend on the continuity of the regeneration of new trees. However, long-term data on regeneration are rare in European temperate mature forests, hampering our ability to assess trends in regeneration and inform forest-management policies. Here we assess changes in the dynamics of saplings and small trees in 16 permanent plots located in 9 sites of semi-natural mature forests across Great Britain. All plots were measured at least three times, with the most recent census conducted in 2022. We use long term records of individual trees, analysed with Generalised Mixed-Effects Models, to test for temporal trends and drivers of change in recruitment and mortality rates of saplings and small trees over widely spaced censuses across of 30–63 years of forest monitoring. We observed failure of forest regeneration in 8 of the 9 sites monitored with a 46 % decrease in small tree recruitment rates and a 90 % increase in sapling mortality rates during the past 3–4 decades. Generalized regeneration declines were observed for most dominant species across different forest types and climate conditions in Great Britain, suggesting that it is likely caused by multiple drivers. Our results point to recovery from past management, droughts and rising winter temperatures as potential contributors to the regeneration failure. Although not formally tested, other plausible drivers may include the climatic aggravation of tree pathogens and widespread presence of uncontrolled population of herbivores. While more data will be necessary to confirm the drivers of these trends, our study raises concerns about the resilience of these temperate forests and their adaptability to future climate conditions.