Effects of girdling on stem CO2 efflux and its temperature sensitivity in Chinese fir and sweetgum trees

The effect of carbohydrate supply on stem CO2 efflux and its temperature sensitivity is crucial in predicting forest carbon cycling under climate change and forest management but has been rarely reported. In this study, we altered carbohydrate supply via stem girdling and investigated the effect of nonstructural carbohydrate concentration on stem CO2 efflux and its temperature sensitivity in Chinese fir and sweetgum trees in spring and winter. The stem CO(2 )efflux of Chinese fir increased significantly above the girdle in spring and winter, but decreased below the girdle in winter. The stem CO(2 )efflux of sweetgum increased above the girdle and decreased below the girdle in spring but not in winter. Seasonal variations in the response of stem CO(2 )efflux to girdling were associated with the changes in stem soluble sugar and starch concentration, especially in sweetgum tree. Girdling altered Q(10) of the stem CO2 efflux, and this effect was dependent on location, season and species. Compared with the Q(10) of the stem CO2 efflux in non-girdled trees, that of CO2 efflux in Chinese fir increased by 45% above the girdle but decreased by 16% below the girdle in winter. These values also exhibited 6% and 11% responses in spring. By contrast, the Q(10) of the stem CO(2 )efflux in sweetgum increased by 95% above the girdle and decreased by 45% below the girdle in spring. However, these values were nearly constant in winter. We concluded that the effect of carbohydrate supply on stem CO2 efflux and its temperature sensitivity depends on season and species. Therefore, the vital role of carbohydrate supply that regulates stem CO(2 )efflux and its temperature sensitivity should be considered for the understanding and modeling of forest carbon cycling response to forest management, especially under global climate changes.