Interpretation of stem CO2 efflux measurements

Area of Science:

• Plant Physiology
• Forest Ecology
• Biogeochemistry

Background:

• Stem CO2 efflux and stem respiration are distinct processes, with their relationship not fully understood.
• Accurate measurement of tree respiration is crucial for understanding forest carbon cycling and climate change impacts.

Purpose of the Study:

• To develop and validate a physical model explaining the relationship between stem CO2 efflux and actual stem respiration.
• To investigate the influence of xylem sap flow, diffusion, and stem characteristics on CO2 transport within trees.

Main Methods:

• Development of a physical model incorporating CO2 diffusion and advection via xylem sap flow.
• Comparison of model predictions with field-measured CO2 efflux data from a mature Scots pine (Pinus sylvestris L.) tree.

Main Results:

• Model predicts a higher ratio of CO2 efflux to CO2 production in the upper tree sections due to upward sap flow.
• Predicted temperature dependency of stem respiration is higher than that of CO2 efflux, attributed to diffusion limitations.
• The relationship is strongly influenced by sap flow rate, radial diffusion resistance, and stem dimensions.

Conclusions:

• The developed model provides a mechanistic understanding of stem CO2 efflux and its divergence from stem respiration.
• The model can be utilized to scale point measurements of CO2 efflux for estimating whole-tree and forest respiration rates.
• This research enhances the accuracy of carbon flux estimations in forest ecosystems.