Drought Severity and Nitrogen Addition Interactively Modulate Seedling Growth and Resource-Use Strategies of Quercus wutaishanica

Global climate change has intensified drought and increased nitrogen deposition, threatening forest tree seedling regeneration. To clarify how drought severity and nitrogen enrichment jointly affect seedling performance in Quercus wutaishanica, a dominant montane tree in northern China, we conducted a full two-factor pot experiment. We established three drought treatments (ambient precipitation [CK], chronic drought [CD], and intense drought [ID]), fully crossed with two nitrogen addition levels (0 and 10 g N m−2 yr−1), and measured functional traits related to growth, photosynthesis, and stress resistance. Our main results were threefold: (1) Both drought treatments significantly inhibited growth and biomass accumulation: total biomass decreased by 28% under CD and 38% under ID relative to CK, with suppression intensifying as drought severity increased. (2) Nitrogen addition increased total biomass by 12% under chronic drought, but this ameliorative effect fell to just 2% under intense drought. (3) As drought stress increased, the seedlings underwent a shift from stomatal to non-stomatal photosynthetic limitation, and from active physiological acclimation to irreversible metabolic damage. (4) Random forest modeling confirmed that biomass variation was primarily driven by traits related to water-use efficiency and resource acquisition. Overall, intensifying drought consistently constrains Q. wutaishanica seedling growth, and the beneficial effect of nitrogen addition declines sharply with increasing drought severity. These findings provide new empirical insights for predicting seedling regeneration and guiding sustainable forest nutrient management under ongoing climate change.