Vertical Strata and Stem Carbon Dioxide Efflux in Cycas Trees.

Thomas E Marler,Murukesan V Krishnapillai

doi:10.3390/plants9020230

Thomas E Marler, Murukesan V Krishnapillai

Open Access

https://doi.org/10.3390/plants9020230

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Abstract

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.

Highlights

IntroductionInterpretations focus on respired carbon dioxide in root tissues that enters root xylem moves to stem tissues in sap flow
Carbon dioxide is among the greenhouse gases that are emitted by tree stems and the amounts are great enough for this source of carbon dioxide to influence regional and global carbon cycles [1,2].Vertical variations in stem respiration have been reported, with the stem surfaces directly above the root collar exhibiting greater carbon dioxide efflux (Es ) than stem surfaces at higher strata [3,4,5].Interpretations focus on respired carbon dioxide in root tissues that enters root xylem moves to stem tissues in sap flow
Stem surface temperature, and relative humidity were similar among the Midday air temperature, stem surface temperature, and relative humidity were similar among six locations of study

Summary

Introduction

Interpretations focus on respired carbon dioxide in root tissues that enters root xylem moves to stem tissues in sap flow. This root-derived carbon dioxide responds to a radial conductance gradient toward the free air at these basal stem strata and some estimates indicate that about half of the root-respired carbon dioxide may enter the atmosphere by way of Es [3]. Es in higher strata and is evinced in diel cycles of Es as the changes in sap flow directly modify the conductance of carbon dioxide toward the stem surface [6,7,8,9]. As a result of sap flow and localized storage or refixation, about 40% of the respired carbon dioxide in stems may not be emitted from stem surfaces closest to the site of respiration [10]

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