Stem CO2 efflux (ES), the diffusion of CO2 from plant stems to the atmosphere, is an important component in regional and global carbon cycles. Historically, ES was considered a direct measure of stem respiration as it was assumed that ES represented the respiratory activity of local phloem, cambium, and ray cells; however, it is now accepted that it also incorporates some of the respiratory activity of cells located below the point of measurement, including those of roots and omits some of the locally produced CO2 that remains within the stem. Intensive management and predicted declines in precipitation are likely to affect the carbon budgets of loblolly pine (Pinus taeda) plantations in the southeastern United States. However, little is known about how ES responds to decreasing soil moisture and changes in soil fertility. This was examined from 2014 to 2015 in an experiment in a loblolly pine plantation in Washington, GA. The experimental design was a 2 × 2 factorial combination of fertilization (2 levels) and precipitation (throughfall exclusion, 2 levels) replicated in four blocks. We measured ES, leaf area index, sap flow, and root CO2 efflux along with stem temperature soil. We also measured annual diameter increment. Our objectives were to (1) quantify impacts of throughfall exclusion and fertilization on ES, and (2) determine physical and biological controlling factor of ES.
Throughfall reduction decreased ES by 18% while fertilization had no effect on ES. Overall mean ES was 2.75, 2.85, 2.84 and 2.26 µmol CO2 m-2 s-1 for control, fertilization, fertilization & throughfall reduction and throughfall reduction treatments, respectively, over the total measurement period. Over the total measurement period, between 24% (fertilization) and 41% (throughfall reduction) of the variation in ES was explained by stem temperature. When the cofounding effect of temperature was controlled, there was a positive correlation between the residuals of ES and residuals of root CO2 efflux in control plots. There also was a weak correlation between ES normalized at 15oC and leaf area index in the control, fertilization and throughfall reduction treatments. We concluded that (1) nutrient supply and water availability had different effects on ES and (2) considering the linkage between ES and leaf area index and root CO2 efflux will improve our mechanistic understanding of of ES.