Status and drivers of oak population decline in Western European forests

In Western European temperate forests, oak (Quercus petraea and Quercus robur) covers vast areas, provide habitats for a wide range of organisms, deliver essential ecosystem services, and are often considered more resilient than other native species. Yet, oak populations are in decline because of changes in forest management, ungulate herbivory, and interspecific competition. In this presentation, we will synthesize different research studies - that we published between 2010 and 2023 – about the status and drivers of oak decline in Belgium. According to the regional forest inventory, oak-dominated stands account for 23 % of the forest area (32% of standing stock), in which beech (Fagus sylvatica) and hornbeam (Carpinus betulus) are often mixed. While oak is abundant, the lack of young oak trees with a diameter lower than 30 cm is obvious and concerning. In oak-beech forests managed with continuous cover forestry (CCF), we studied the competition between naturally regenerated oak, beech and hornbeam saplings in 23 sites and 242 fenced plots, between 2009 and 2011. We modelled and evidenced that oak saplings grew significantly more slowly than beech and hornbeam in the whole range of studied light conditions. Additionally, we monitored regeneration in 734 pairs of fenced and unfenced plots between 2016 and 2021. Ungulates strongly reduced the growth of oak, hornbeam, birch and rowan seedlings whereas the growth of beech and spruce were little or unaffected. Nevertheless, whatever the herbivory pressure was, oak seedlings grew more slowly than the mixed species. By simulating different sylvicultural scenarios, to successfully regenerate oak with CCF, we recommend maintaining a closed canopy (PACL < 5%) until oak seedlings emerge, then protecting seedlings against browsing, reducing overstory density (PACL > 20%) and manually releasing oak seedling from competition. In the study area, oak may regenerate spontaneously regenerate outside the forests (e.g., in edges) but may hardly regenerate in forest understories without manual interventions.