Methods for the determination of CO_2 flux from non-photosynthetic organs of trees and their influences on the results

The CO 2 exchange feature of non-photosynthetic organs of trees is important in monitoring forest CO 2 flux. However, up to date, no standard method has been suggested in these measurements. Six categories of 10 methods for separating root respiration from soil respiration, and 2 methods for measuring stem or branch respiration were reviewed. The advantage and disadvantage of each method and its recent advance were also discussed. Under the light of these discussion, data from previous references (65 data of root respiration and 59 stem respiration data of Pinus ponderosa)were collected to analyze the influence of different methods to the results. The results showed that root respiration contributed 5% to 90% of total soil respiration, and near 60% data ranged from 40%～70%. Study methods showed somewhat influences on the results of root respiration. The contribution of root respiration to total soil respiration was ca. 40% measured by the isotopic labeling methods, but this value was enhanced by 33% for those measured by the root exclusion methods and the stem girdling method, and 7% higher than that measured by the root separation method. This finding indicates that the root exclusion methods and the stem girdling method may overestimate the root respiration since they disturbed the root system and rhizosphere. Comparing with in vivo method, the stem respiration rate was overestimated by 72% using the in vitro method based on stem surface area; while, it was overestimated by 56 times based on sap wood volume for the in vitro method in same study. Therefore, it is necessary to fully considerate the method-induced difference when comparisons are carried out by different methods.