Stem Carbon Dioxide Efflux of Lignophytes Exceeds That of Cycads and Arborescent Monocots

Tree stem CO2 efflux (Es) can be substantial and the factors controlling ecosystem-level Es are required to fully understand the carbon cycle and construct models that predict atmospheric CO2 dynamics. The majority of Es studies used woody lignophyte trees as the model species. Applying these lignophyte data to represent all tree forms can be inaccurate. The Es of 318 arborescent species was quantified in a common garden setting and the results were sorted into four stem growth forms: cycads, palms, monocot trees that were not palms, and woody lignophyte trees. The woody trees were comprised of gymnosperm and eudicot species. The Es did not differ among the cycads, palms, and non-palm monocots. Lignophyte trees exhibited Es that was 40% greater than that of the other stem growth forms. The Es of lignophyte gymnosperm trees was similar to that of lignophyte eudicot trees. This extensive species survey indicates that the Es from lignophyte tree species do not align with the Es from other tree growth forms. Use of Es estimates from the literature can be inaccurate for understanding the carbon cycle in tropical forests, which contain numerous non-lignophyte tree species.

Perennial Trees Associating with Nitrogen-Fixing Symbionts Differ in Leaf After-Life Nitrogen and Carbon Release

Plants that enter symbiotic relationships with nitrogen (N)-fixing microbes contribute some of their N to the community through leaf litter decomposition and mineralization processes. The speed of these processes varies greatly among tree species. Mesocosm methods were used to determine the speed of N and carbon (C) release from Cycas micronesica, Intsia bijuga, and Serianthes nelsonii leaf litter. Microcosm methods were used to determine soil respiration traits in soils containing the leaf litter. The speed of leaf litter N and C release during decomposition occurred in the order C. micronesica < I. bijuga < S. nelsonii. Soil carbon dioxide efflux was increased by adding leaf litter to incubation soils, and the increase was greatest for S. nelsonii and least for C. micronesica litter. Ammonium, nitrate, total N, organic C, and total C were increased by adding litter to incubation soils, and the differences among the species converged with incubation duration. The rate of increases in available N and decreases in organic C were greatest for S. nelsonii and least for C. micronesica litter. These findings indicate that S. nelsonii litter released N and C rapidly, C. micronesica litter released N and C slowly, and the leaf economic spectrum accurately predicted the differences.

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 (Es) for six arborescent Cycas L. species to characterize this component of stem respiration and ecosystem carbon cycling. The influence of strata on Es was remarkably consistent among the species, with a stable baseline flux characterizing the full mid-strata of the pachycaulous stems and an increase in Es 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, Es increased about 30% at the lowest stratum and about 80% at the highest stratum. A significant quadratic model adequately described the Es patterns for all six species. The increase of Es at the lowest stratum was consistent with the influence of root-respired carbon dioxide entering the stem via sap flow, then contributing to Es via radial conductance to the stem surface. The substantial increase in Es 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.