Non-structural carbohydrates (NSC) play a central role in plant functioning as energy carriers and building blocks for primary and secondary metabolism. Many studies have investigated how environmental and anthropogenic changes, like increasingly frequent and severe drought episodes, elevated CO₂ and atmospheric nitrogen (N) deposition, influence NSC concentrations in individual trees. However, this wealth of data has not been analyzed yet to identify general trends using a common statistical framework. A thorough understanding of tree responses to global change is required for making realistic predictions of vegetation dynamics. Here we compiled data from 57 experimental studies on 71 tree species and conducted a meta-analysis to evaluate general responses of stored soluble sugars, starch and total NSC (soluble sugars + starch) concentrations in different tree organs (foliage, above-ground wood and roots) to drought, elevated CO₂ and N deposition. We found that drought significantly decreased total NSC in roots (−17.3%), but not in foliage and above-ground woody tissues (bole, branch, stem and/or twig). Elevated CO₂ significantly increased total NSC in foliage (+26.2%) and roots (+12.8%), but not in above-ground wood. By contrast, total NSC significantly decreased in roots (−17.9%), increased in above-ground wood (+6.1%), but was unaffected in foliage from N fertilization. In addition, the response of NSC to three global change drivers was strongly affected by tree taxonomic type, leaf habit, tree age and treatment intensity. Our results pave the way for a better understanding of general tree function responses to drought, elevated CO₂ and N fertilization. The existing data also reveal that more long-term studies on mature trees that allow testing interactions between these factors are urgently needed to provide a basis for forecasting tree responses to environmental change at the global scale.

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 a 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 non-structural 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.