Discrimination in Tree Stems O 2 Uptake and the Dole Effect

The difference in δ 18 O between the atmospheric O 2 and seawater is named the Dole effect (DE). Interpretation of past DE variations, a proxy for monsoonal intensity, is based on current understanding of global O 2 fluxes. One such O 2 flux that was poorly studied is the discrimination in O 2 uptake by tree stems ( D stem ). Stems differ from leaves in their lower gas permeability, which reduces the internal [O 2 ] and poses diffusion limitations to local respiration. Diffusion limitations are expected to lower the effective discrimination below enzymatic values (17–30‰). Given fixed permeability, the lowest internal [O 2 ] and thus the greatest diffusional effect are expected in high O 2 influx. D stem and O 2 influx values were obtained by measurements of stem‐chamber air and a numerical model that resolves the gas exchange in the chamber. According to 101 results of temperate, Mediterranean, and tropical trees, the global D stem is 19.2‰. Assuming basal O 2 diffusion limitation in the stems, the results indicate involvement of the alternative oxidase respiration, with enzymatic discrimination factor of above 20‰. Small seasonal variations in D stem were uncoupled from the O 2 influx rates that tracked the temperature changes. This indicates a possible adjustment between the aeration capability of the stems and O 2 uptake rate, which helps to maintain sufficient O 2 supply. The mean D stem value agrees with the assumed discrimination for leaf respiration, thus supporting current DE understanding. The small D stem seasonal variations give an indication that climate variations on millennial and interglacial scales did not have a strong effect on D stem values.