Conceptual framework of tree nitrogen uptake–growth–reproduction–defense and its implications in forest health regulation

Nitrogen （N） is a key limiting nutrient for forest primary productivity and tree growth. Trees' N acquisition capacity largely determines resources competition and stress resilience, as well as their role in maintaining the stability of forest ecosystem. Recent research has shifted from focusing solely on root uptake to an integrative view that recognizes rhizosphere–microbial cooperation and subterranean nutrient networks. This review synthesizes pathways for inorganic and organic N uptake, elucidates the roles of root functional traits （e.g., fine-root morphology, specific root length, and lifespan）, mycorrhizal symbiosis, and root exudate regulation in N uptake, and examines the coupling between N uptake, carbon assimilation, growth and defense. Based on current evidence, we propose an integrated conceptual framework （N uptake–growth–reproduction–defense） and classify four representative strategy types of trees （high N uptake–fast growth, moderate N uptake–balanced strategy, low N uptake–high defense, microbial dependent N uptake–synergistic and mutualistic）. We further discuss implications for adaptive forest management via optimized N supply and microbial manipulation. Finally, we identify the key knowledge gaps in this field and outline future research directions, including cross–scale gene–metabolism–ecology coupling analyses, dynamic isotope tracing, and model–based predictions under multiple stress conditions. This review aims to provide theoretical references for understanding N–driven tree adaptive strategies and for forest health management.