Atmospheric nitrogen deposition alters forest water relations and nutrient balances in ways that can both temporarily boost growth and ultimately reduce drought resilience. Human-driven increases in reactive nitrogen change soil chemistry, plant allocation patterns, and microbial communities. Peter M. Vitousek of Stanford University has described how anthropogenic nitrogen inputs have reshaped terrestrial nutrient cycles, creating ecological responses that vary by species and region. These shifts influence how trees take up and use water during dry periods.
Physiological and soil mechanisms
Added nitrogen often increases leaf area and photosynthetic capacity, a response documented in syntheses by Kathleen K. Treseder University of California, Irvine that show many plants are nitrogen-limited. Greater leaf area raises canopy water demand, increasing transpiration and sensitivity to soil moisture deficits. At the same time, long-term nitrogen enrichment can reduce fine root production and lower root to shoot ratios, weakening the plant’s ability to access deep soil water. Soil acidification and losses of base cations under chronic nitrogen inputs, described in work by Mark E. Fenn U.S. Forest Service, alter root function and can impair water uptake and xylem function, compounding drought stress.
Microbial and mycorrhizal interactions
Nitrogen deposition changes soil microbial communities and mycorrhizal partnerships that support tree water and nutrient acquisition. Some studies report declines in ectomycorrhizal abundance where nitrogen is high, reducing phosphorus acquisition and altering hydraulic conductance. These microbial shifts can make trees less adaptable when droughts intensify, especially in ecosystems where symbiotic fungi mediate drought tolerance.
Regional, cultural, and environmental nuance matters. Temperate forests that are historically nitrogen-limited may show transient growth increases after deposition, which can be perceived as beneficial by timber-dependent communities. In contrast, Mediterranean and boreal systems often face combined pressures of nitrogen pollution and changing fire regimes, exacerbating risks to subsistence users and biodiversity. In landscapes with acidic soils or thin organic layers, nitrogen-driven cation losses can be especially damaging to water relations and long-term forest health.
Consequences include increased mortality during severe droughts, shifts in species composition toward more drought-tolerant or nitrophilous species, and altered carbon storage dynamics as trees die and decomposition rates change. Management responses that reduce atmospheric emissions, protect soil base cations, and maintain mycorrhizal diversity can help preserve forest resilience.