Land-use legacies drive contrasting dynamics between new and pre-existing forests

In many European countries, land-use changes driven by socio-economic contexts have induced widespread forest expansion during the 20th century, with still poorly explored implications for ecosystem dynamics. Previous research has shown that these “new forests” differ from “pre-existing forests” in terms of structure and productivity. However, beyond their current state, it remains unclear whether and how the long-term dynamics of new forests diverge from or converge towards those of pre-existing forests.Here, we address this question using the framework of Ecological Dynamics Regimes (EDRs), which characterize ecosystem trajectories through time based on temporal changes in multiple state variables within a multidimensional state space. In this study, EDRs were defined using forest structural and compositional attributes. We assessed forest EDRs across two biogeographical regions of the Iberian Peninsula using data from 756 plots of the Spanish National Forest Inventory (1986–2023). Historical land-cover maps from 1956 were used to distinguish between pre-existing forests and new forests established after mid-20th-century land abandonment. Analyses focused on plots dominated by major Pinus and Quercus species in the region, from which we derived metrics describing the dispersion, length, and relative position of EDR trajectories.Our results show that, for most species, new forests exhibit more dispersed (between 0 and 37% according to the species) and longer trajectories (11-75%), and are positioned further behind in state space (4-56%) compared to pre-existing forests. This indicates that new forests start from more heterogeneous initial conditions and experience faster or more pronounced structural and compositional changes, while nonetheless showing a tendency to converge towards the dynamics of pre-existing forests over time. These findings highlight the lasting influence of land-use legacies on forest dynamics and help to the understanding of forest responses under ongoing global change and increasing uncertainty.