Thresholds of facilitation and competition—Climate modulates the growth–density relationship in Quercus robur

Tree density can have both positive (facilitation) and negative (competition) impacts on tree growth. Evidence indicates that non‐linear growth–density patterns often better capture the complexity of these interactions than linear forms. How such patterns vary along density and climatic conditions, including drought, remains unclear. In this study we used a unique and long‐term density manipulation experimental network to assess the impact of site climatic conditions and temporal climatic variability on the tree growth–density relationship.

For two decades we monitored growth of pedunculate oak ( Quercus robur L.) trees planted with systematic spacing (Nelder design) along a climatic gradient in Europe. We quantified relative growth (normalized to solitary trees) and absolute growth responses to density across age classes. We further disentangled the impact of site aridity and drought periods on growth.

Our results revealed that trees growing within intraspecific neighbourhoods can facilitate each other's above‐ground biomass growth, achieving a maximum relative growth increase of 30% compared with solitary trees. This unimodal optimum relationship was most pronounced during early developmental stages (ages 5–10), with its effect diminishing as trees matured. The observed shift of interaction, from facilitation to competition, could be attributed to increased canopy cover in a trees' neighbourhood. We identified aridity‐specific density thresholds (3000–4000 N ha ⁻¹ ), beyond which growth declines sharply during water‐limited periods. At dry sites, growth was highly responsive to drought, while the density threshold was relatively consistent across climatic conditions. On mesic to humid sites, drought and density were more tightly coupled, with density effects amplified beyond the initial low‐density optimum.

Synthesis. We found that density‐dependent growth follows a unimodal, non‐linear pattern, with optimal density thresholds narrowing under increasing site aridity, stand age and climatic variability. These findings enhance our understanding of how climate and tree development jointly shape growth responses to density. They highlight the need for adaptive density management strategies that account for shifting facilitation–competition dynamics under climate change.