Responses of functional traits to seven-year nitrogen addition in two tree species: coordination of hydraulics, gas exchange and carbon reserves

Atmospheric nitrogen (N) deposition has been observed to impact plant structure and functional traits in terrestrial ecosystems. Although the effect of N deposition on plant water use has been well-evaluated in laboratories and in experimental forests, the linkages between water and carbon relations under N deposition are unclear. Here, we report on hydraulics, gas exchange and carbon reserves of two broad-leaved tree species (Quercus mongolica and Fraxinus mandshurica) in mature temperate forests after a seven-year experiment with different levels of N addition (control (CK), low (23 kg N ha(-1) yr(-1)), medium (46 kg N ha(-1) yr(-1)) and high (69 kg N ha(-1) yr(-1))). We investigated variation in hydraulic traits (xylem-specific hydraulic conductivity (K-s), native percentage loss of conductivity (PLC) and leaf water potential), xylem anatomy (vessel diameter and density), gas exchange (maximum net photosynthesis rate and stomatal conductance) and carbon reserves (soluble sugars, starch and total nonstructural carbohydrates (NSC)) with different N addition levels. We found that medium N addition significantly increased K-s and vessel diameter compared to control, but accompanied increasing PLC and decreasing leaf water potential, suggesting that N addition results in a greater hydraulic efficiency and higher risk of embolism. N addition promoted photosynthetic capacity via increasing foliar N concentration but did not change stomatal conductance. In addition, we found increase in foliar soluble sugar concentration and decrease in starch concentration with N addition, and positive correlations between hydraulic traits (vessel diameter and PLC) and soluble sugars. These coupled responses of tree hydraulics and carbon metabolism are consistent with a regulatory role of carbohydrates in maintaining hydraulic integrity. Our study provides an important insight into the relationship of plant water transport and carbon dynamics under increasing N deposition.

Automated summary

Our study found that medium nitrogen addition significantly increased the wood-specific hydraulic conductivity and vessel diameter of trees, but also led to higher risk of embolism and decreased leaf water potential. Nitrogen addition promoted photosynthetic capacity through increasing leaf nitrogen concentration, while not affecting stomatal conductance. Additionally, there were positive correlations between wood traits (vessel diameter and PLC) and foliar soluble sugars, indicating a regulatory role of carbohydrates in maintaining tree hydraulic integrity under increasing nitrogen deposition.