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.