Vertical Strata and Stem Carbon Dioxide Efflux in Cycas Trees.

Stem respiration is influenced by the vertical location of tree stems, but the influence of vertical location on stem respiration in a representative cycad species has not been determined. We quantified the influence of vertical strata on stem carbon dioxide efflux (E_s) for six arborescent Cycas L. species to characterize this component of stem respiration and ecosystem carbon cycling. The influence of strata on E_s was remarkably consistent among the species, with a stable baseline flux characterizing the full mid-strata of the pachycaulous stems and an increase in E_s at the lowest and highest strata. The mid-strata flux ranged from 1.8 μmol·m⁻²·s⁻¹ for Cycas micronesica K.D. Hill to 3.5 μmol·m⁻²·s⁻¹ for Cycas revoluta Thunb. For all species, E_s increased about 30% at the lowest stratum and about 80% at the highest stratum. A significant quadratic model adequately described the E_s patterns for all six species. The increase of E_s at the lowest stratum was consistent with the influence of root-respired carbon dioxide entering the stem via sap flow, then contributing to E_s via radial conductance to the stem surface. The substantial increase in E_s at the highest stratum is likely a result of the growth and maintenance respiration of the massive cycad primary thickening meristem that constructs the unique pachycaulous cycad stem. [ABSTRACT FROM AUTHOR]