Understanding the variability of CO2 efflux from woody tissue within and among forest trees

Background/Question/Methods

Stem and branch CO₂ efflux is very uncertain in western coniferous forests. We measured this flux over two years in young- and old-growth stands within the Wind River Experimental Forest, Washington, USA, which consists primarily of Pseudotsuga menziesii, Thuja plicata, and Tsuga heterophylla. Our objective was to identify and explain the variability of forest tree woody-tissue CO₂ efflux to refine our modeling of stand-level carbon fluxes.

Results/Conclusions

Stem CO₂ efflux (R₁₅, normalized to 15°C) at breast height (mmol m^-3 s^-1) increased 1.5 – 2 times during summer months, which was possibly related to tissue growth, or reallocation of photosynthate to stem storage and roots. Stem R₁₅ for P. menziesii was 1.5 – 5 times higher in younger stands, supporting the hypothesis that respiration peaks early with tree age. Canopy branch R₁₅ (mmol m^-3 s^-1) was highly variable and there was no consistent pattern across seasons. Additionally, branch R₁₅ was 5 – 10 times higher than for stems, suggesting that branch surface area may be a better basis than volume for extrapolating to the stand level. We tested whether the extreme variability of branch R₁₅ resulted from green-bark photosynthesis by comparing R₁₅ of shaded vs. un-shaded branches and found no difference. However, nighttime branch R₁₅ was about twice as high as that during daytime – especially in upper branches – suggesting an influence from the internal, relocation of CO₂ via the transpiration stream to photosynthesizing leaves. To separate woody tissue CO₂ efflux from other influences, such as the transpiration stream or diffusion from inside to outside the stem, we compared two methods for measuring CO₂ efflux and monitored stem sapflow. CO₂ efflux from extracted woody tissues using a temperature-controlled laboratory method (RP₁₅, measured at 15°C) was 10 – 15 times higher than R₁₅ from intact stems. RP₁₅ and R₁₅ were linearly correlated for the old- growth, but not for the younger trees. In part, stem R₁₅ may be lower because of a loss of sapwood-respired CO₂ to the transpiration stream, which reduces the amount that diffuses out of the stem. This hypothesis was supported by an inverse relationship between sap flux density (m³ H₂O / m² h) and stem R₁₅, yet this pattern was seasonally dependent and may only occur during days of active photosynthesis. Determining the relevance of this observed variation when extrapolated to the stand level will be important to accurately model ecosystem level carbon fluxes.