Physiological and Molecular Mechanisms of Tree Responses to Combined Drought and Low-Nitrogen Stress: A Review

Drought and low nitrogen often co-occur in forests, yet their combined effects cannot be inferred reliably from single-stress studies. This review synthesizes physiological and molecular evidence for tree responses to combined drought and low nitrogen at four levels, namely stress perception, signal transduction, metabolic reprogramming, and adaptive phenotypes. It compares single-stress baselines and evaluates reported interactions in photosynthesis, water-use efficiency, root traits, and carbon–nitrogen allocation. The review distinguishes direct tree evidence from mechanistic context derived from crops and model plants. Abscisic acid–nitrogen signaling crosstalk is considered alongside reactive oxygen species, energy, hydraulics, and transcriptional regulation as candidate integration routes. Candidate genes are classified by species context and experimental evidence; responsiveness to the two single stresses is not interpreted as proof of a combined-stress function. The review also assesses the opportunities and current limitations of multi-omics and genome editing for trees. It identifies priorities for factorial experiments, multi-year validation, and molecular breeding for climate-resilient forestry.