A branch bag and CO₂ control system for long‐term CO₂ enrichment of mature Sitka spruce [Picea sitchensis (Bong.) Carr.]

This paper describes the construction and performance of branch bags and a CO 2 control system used to fumigate branches of mature Sitka spruce trees with air enriched in CO 2 (700 μmolmol ‐1 ). It contains some examples of results obtained using the system over the course of the first two growing seasons. The branch bags have run continuously for 2 years with very few problems. CO 2 concentrations were within 20 μmol mol ‐1 of the target concentration for more than 90% of the time. Temperatures within the bags were slightly higher than ambient (1–2 °C) and this had some effect on phenology. Attenuation of quantum flux density (photosynthetically active radiation) was 10–15%. The branch bag system has enabled investigation into the effects of elevated CO 2 on mature tissue without the problems and expense of fumigating whole trees. Growth in elevated CO 2 resulted in an increase in starch and a decrease in soluble protein content of needles. Stomatal conductance was higher in elevated CO 2 grown needles, and there was some evidence of an increase in photosynthetic capacity.