Contribution of root and rhizosphere respiration to the annual variation of carbon balance of a boreal Scots pine forest

Abstract. A large part of gross primary production (GPP) is consumed in root and rhizosphere respiration (R_r). To measure R_r, a group of evergreen coniferous Scots pine (Pinus sylvestris) trees were girdled in a 45-year-old even aged forest in Hyytiälä, Southern Finland. In the girdling, phloem and bark were removed from breast height around the trees. We measured soil CO₂ effluxes with a dynamic chamber at the girdled plot and at a non-girdled control plot in close vicinity. Before the girdling, effluxes were 22% higher at the plot to be girdled compared to the control plot. We scaled the measurements so that before girdling the effluxes representing total soil respiration (R_s) were at the same level. We compared the R_r and R_d to GPP measured with eddy covariance system. Our results show that R_r has higher seasonal variation than R_d, and also spatial variability was higher for R_r. The annual R_r:R_s and R_r:GPP-ratios were 0.36 and 0.21, respectively. R_r:R_d varied seasonally and in late summer and in autumn R_r exceeded R_d. R_r followed GPP with a delay of several weeks. During winter and spring R_r was very low, even when GPP and soil temperature had significantly risen. We conclude that R_r and R_d have different response to the environment and that for R_r the substrate availability is a more important explaining variable than soil temperature.