©2023 International Society of Arboriculture 142 for these pachychaulous trees may contrast with that for lignophyte trees. Understanding the carbon cycle has become crucial because of the increases in atmospheric greenhouse gases such as CO2. Accurate modelling of ecosystem respiration (e.g., Salomón et al. 2022) requires a full understanding of the tree and habitat characteristics that exert control over Es. The results of this study indi- cated that soil surface gas exchange beneath a tree’s canopy may be a key driver of tree stem CO2 e.flu. and therefore must be included in future attempts to model t.e influence o. trees on ecos.stem res.ir.tion. LITERATURE CITED Bloemen J, Agneessens L, Van Meulebroek L, Aubrey DP, McGuire MA, Teskey RO, Steppe K. 2014. Stem girdling affects the quantity of CO2 transported in xylem as well as CO2 e.flu. .rom soil. New Phytologist. 201(3):897-907. https://doi.org/10.1111/nph.12568 Bloemen J, McGuire MA, Aubrey DP, Teskey RO, Steppe K. 2013. Transport of root-respired CO2 via the transpiration stream affects aboveground carbon assimilation and CO2 e.flu. in trees. New Phytologist. 197(2):555-565. https://doi .org/10.1111/j.1469-8137.2012.04366.x .u..o.. .. .cost. .. ...eno.. .. .o.orn. .. ...el.. .. .0... .n.ironment.l ..ctors influencin. t.e rel.tions.i. .et.een stem CO2 e.flu. .nd s.. flo.. Trees. 29(2):333-343. https:// doi.org/10.1007/s00468-014-1113-z .z... .. .o.ton .. .ur.s .. .0.0. ..e com.le. issue o. ur..n trees—Stress factor accumulation and ecological service possi- bilities. Forests. 11(9):932. https://doi.org/10.3390/f11090932 ..erer .. .uc..olz .. .0... .e.r.min. ur..n ..ildli.e. to .ro- mote inclusive conservation science and practice. Biodiversity Conservation. 30:2255-2266. https://doi.org/10.1007/s10531 -021-02182-y Ford CR, Wurzburger N, Hendrick RL, Teskey RO. 2007. Soil ... u.t..e .nd ...tion in Pinus taeda seedlings and its C contribution to plant tissues and ectomycorrhizal fungi. Tree Physiology. 27(3):375-383. https://doi.org/10.1093/treephys/ 27.3.375 .eider ... .eluci. ... ..l.o.s.i ... .inzi ... .rime ... .r.ce .. Kana TM, LaRoche J, Long SP, Osborne BA, Platt T, Prentice IC, Raven JA, Schlesinger WH, Smetacek V, Stuart V, Sath- yendranath S, Thomas RB, Vogelmann TC, Williams P, Woodward FI. 2001. Primary productivity of planet earth: Biological determinants and physical constraints in terrestrial and aquatic habitats. Global Change Biology. 7(8):849-882. https://doi.org/10.1046/j.1365-2486.2001.00448.x Kunert N. 2018. A case study on the vertical and diurnal variation of stem CO2 e.flu.es in .n .m.zoni.n .orest tree. Trees. 32:913-917. https://doi.org/10.1007/s00468-018-1680-5 .unert .. .din.er .. .0... ..lem s.. flu. ...ects con.ention.l stem CO2 e.flu. me.surements in tro.ic.l trees. Biotropica. 47(6):650-653. Kuzyakov Y. 2006. Sources of CO2 e.flu. .rom soil .nd re.ie. of partitioning methods. Soil Biology and Biochemistry. 38(3):425-448. https://doi.org/10.1016/j.soilbio.2005.08.020 soil heterotroph community in urban spaces covered by hardscapes. Indeed, the muted gas exchange on the soil surfaces of hardscapes may create anaerobic conditions in a soil matrix where oxygen is consumed and CO2 is liberated. The hypoxia would become more acute for the heterotrophs if the CO2 was unable to escape through the tree xylem. In turn, this may lead to a feedback loop where the resulting absence of .ene.ci.l soil micro.iot. .ould le.d to declines in tree health in urban spaces where hardscapes occur. ..e m..nitude o. influence o. ..rdsc..es on d..- time Es was heterogeneous among the 8 tree species. For example, the M. zapota trees exhibited daytime Es that was 112% greater than night-time Es, but this metric was only 50% for the A. auriculiformis trees. Relative radial CO2 conductance in tree stems may differ among arborescent species and has been proposed as one factor that explains differences in Es (Wang et al. 2019). The differences in radial CO2 conductance from stem xylem to the stem surfaces may explain the differences in Es that were reported among the species in this study. The differences in Es between hardscape versus grass or mulch soil surfaces were not restricted to absolute amplitude, but also included time of day differences. An afternoon maximum in Es for the grass or mulch trees conformed to previous reports (e.g., Marler and Lindström 2020) and may be explained by a hysteresis effect of the time required for transfer of CO2 from root tissues into root xylem, transfer of CO2 to stem xylem, then radial diffusion of CO2 to the stem surfaces. In contrast, a morning maximum in Es for the hardscape trees was unique and may signify an overabundance of CO2 in the root-soil matrix by the end of the night period, which reduced the duration of hysteresis. The published reports of diel variation in tree Es are highly contrasting. For example, the greatest Es has been reported during the daytime for some tree species (..n. et .l. .0... .u..o.. et .l. .0... .unert .nd .din.er .0..). .n contr.st. t.e .re.test Es has been reported during the night-time for other species (Kunert 2018; Salomón et al. 2018; Tarvainen et al. 2018). The characteristics of the soil surfaces surrounding the experimental trees in these publications were not descri.ed. ..e .ndin.s indic.te t..t t.is de.cienc. in methods should be corrected in future publications that report tree stem respiration traits, as soil surface c..r.cteristics m.. e.ert . direct influence on Es. Trees with pachycaulous growth form exhibited reduced Es compared to lignophyte trees (Marler 2022). The influence o. soil sur..ce c..r.cteristics on diurn.l Es Marler: Hardscape Changes Stem Carbon Dioxide Flux
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