Effects of four years of throughfall reduction and fertilization on stem CO2 efflux in a loblolly pine (Pinus taeda) plantation

Background/Question/Methods

Stem CO₂ efflux (E_S), the diffusion of CO₂ from plant stems to the atmosphere, is an important component in regional and global carbon cycles. Historically, E_S was considered a direct measure of stem respiration as it was assumed that E_S 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 E_S 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 E_S, 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 E_S, and (2) determine physical and biological controlling factor of E_S.

Results/Conclusions

Throughfall reduction decreased E_S by 18% while fertilization had no effect on E_S. Overall mean E_S 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 E_S was explained by stem temperature. When the cofounding effect of temperature was controlled, there was a positive correlation between the residuals of E_S and residuals of root CO₂ efflux in control plots. There also was a weak correlation between E_S normalized at 15^oC 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 E_S and (2) considering the linkage between E_S and leaf area index and root CO₂ efflux will improve our mechanistic understanding of of E_S.