Differences between tree stem CO₂ efflux and O₂ influx rates cannot be explained by internal CO₂ transport or storage...

Area of Science:

• Forest ecology
• Plant physiology
• Biogeochemistry

Background:

• Tree stem respiration (Rs) is crucial for forest carbon budgets.
• Discrepancies exist between mass balance and oxygen-based methods for measuring Rs.
• The fate of respired CO2 within tree stems remains unclear, impacting carbon dynamics.

Purpose of the Study:

• Investigate the reasons for inconsistent Rs measurements between different approaches.
• Clarify the internal CO2 fluxes and their role in reconciling measurement discrepancies.
• Assess the potential role of phosphoenolpyruvate carboxylase (PEPC) in local CO2 removal.

Main Methods:

• Collected data on CO2 efflux, O2 influx, xylem CO2, sap flow, sap pH, stem temperature, and nonstructural carbohydrates in beech trees.
• Analyzed the ratio of CO2 efflux to O2 influx along a vertical gradient.
• Measured potential PEPC capacity in tree stems.

Main Results:

• The ratio of CO2 efflux to O2 influx was consistently below 1 (0.7) across a 3-m stem gradient.
• Internal CO2 fluxes did not account for the difference between influx and efflux.
• No significant changes in respiratory substrate use were detected.
• PEPC capacity was similar to that found in young twigs.

Conclusions:

• The study could not reconcile the discrepancies between the mass balance and oxygen-based methods for measuring tree stem respiration.
• Results suggest that internal CO2 fluxes do not fully explain the observed differences.
• High PEPC capacity indicates a potential role in local CO2 removal within tree stems, warranting further investigation.