Rising CO₂ and warming reduce global canopy demand for nitrogen

Area of Science:

• Ecology
• Plant Physiology
• Global Change Biology

Background:

• Nitrogen (N) limitation is a key factor influencing terrestrial carbon uptake under rising CO2 and climate change.
• Declining leaf N content and carboxylation capacity (Vcmax) have been interpreted as evidence of increasing N limitation.

Purpose of the Study:

• To predict Vcmax and estimate changes in leaf-level photosynthetic N from 1982-2016.
• To investigate alternative explanations for declining leaf N trends beyond increasing N limitation.

Main Methods:

• Utilized the coordination hypothesis to predict Vcmax.
• Estimated leaf-level photosynthetic N based on Vcmax proportionality at 25°C.
• Derived whole-canopy photosynthetic N using satellite-derived leaf area index (LAI) and an empirical extinction coefficient, converting to N demand via leaf turnover times.

Main Results:

• Predicted Vcmax spatial patterns aligned with reconstructions from remotely sensed leaf chlorophyll content.
• Predicted leaf photosynthetic N declined by 0.27% yr−1, consistent with observed leaf N declines (0.2-0.25% yr−1).
• Global canopy N and N demand declined from 1996 onwards, despite increasing LAI, suggesting reduced canopy N requirements.

Conclusions:

• Leaf-level responses to elevated CO2 and temperature may decrease canopy N requirements more than LAI increases N demand.
• This provides an alternative explanation for declining leaf N that does not rely on increasing N limitation.
• Re-evaluating the role of N limitation in terrestrial ecosystem responses to global change is warranted.