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

1. Elevated atmospheric CO2 concentration (eCO2) typically stimulates tree growth, which is mediated by nitrogen (N) availability; but how N regulates tree biomass responses to eCO2 remains uncertain, which limits our prediction of forest carbon (C) cycling under future global change scenarios. 2. A meta-analysis of a global dataset including 3399 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 eCO2 and the underlying mechanisms. 3. We found that eCO2 stimulated tree biomass production (+32.0%), while it induced accumulation of nonstructural 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 eCO2 of N-fertilized trees (+39.6%) were 68.4% greater than those of non-fertilized trees (+25%), confirming that tree growth is also N-limited. Such N limitation was alleviated by the eCO2-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 eCO2 were greater for ectomycorrhizal trees and conifers than for arbuscular mycorrhizal trees and angiosperms, respectively. 4. Synthesis: Our findings suggest that eCO2 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 eCO2 under global change.