Magnitude and mechanisms of nitrogen‐mediated responses of tree biomass production to elevated CO2: A global synthesis

Abstract Elevated atmospheric CO 2 concentration (eCO 2 ) typically stimulates tree growth, which is mediated by nitrogen (N) availability; but how N regulates tree biomass responses to eCO 2 remains uncertain, which limits our prediction of forest carbon (C) cycling under future global change scenarios. A meta‐analysis of a global dataset including 3,399 observations from 283 papers published from 1980s to February 2021 was conducted with the aim of elucidating N‐mediated responses of tree biomass production to eCO 2 and the underlying mechanisms. We found that eCO 2 stimulated tree biomass production (+32.0%), while it induced accumulation of non‐structural carbohydrates in leaves rather than in woods and roots, suggesting that the production may be C‐limited but depend on the sink strength of organs. Biomass responses to eCO 2 of N‐fertilized trees (+39.6%) were 68.4% greater than those of non‐fertilized trees (+23.5%), confirming that tree growth is also N‐limited. Such N limitation was alleviated by the eCO 2 ‐induced increases in N uptake and N‐use efficiency (NUE), with the former being more important. Increases in tree N pool arose from the enhanced production of fine roots with a lower specific root length, whereas increases in NUE resulted from the flexibility in tissue C:N ratios instead of N resorption efficiency. The positive responses of tree biomass production to eCO 2 were greater for ectomycorrhizal trees and conifers than for arbuscular mycorrhizal trees and angiosperms, respectively. Synthesis . Our findings suggest that eCO 2 stimulates tree biomass production by increasing C availability, and alleviating N limitation in a feedback way via enhancing N uptake and NUE, and they improve our mechanistic understanding of responses of forest productivity and C sequestration to eCO 2 under global change.