1. We investigated the functional controls on light-dependent refixation of respired CO2 in photosynthetic bark of Western White Pine (Pinus monticola Dougl. ex D. Don) branches.
2. Gross photosynthesis increased with increasing intensity of photosynthetically active radiation (PAR) to > 2000 µmol m−2 s−1 and with increasing bark surface temperature from 15 to > 45 °C. Maximum observed refixation rates were 76 ± 3% (mean ± SE) of dark respiration.
3. At a given temperature and irradiance, dark respiration rate was the physiological parameter that correlated most strongly with gross photosynthesis (R = 0·89, P < 0·0001, n = 43). Dark respiration rates varied 20-fold among branches. Over this range, refixation averaged 55 ± 2% of dark respiration at 25 °C and 1000 µmol PAR m−2 s−1. However, refixation was not correlated with internal CO2 concentrations in illuminated branches; these concentrations remained fairly constant.
4. Specific bark area, bark conductance to water vapour, bark chlorophyll a/b ratio, and bark nitrogen concentration varied markedly with canopy depth in mature trees.
5. Results are summarized in a simple model of refixation driven by light, temperature and dark respiration rate. The model may prove useful in assessing the quantitative significance of refixation in the carbon balance of forest trees.