title: Stem compression reversibly reduces phloem transport in Pinus sylvestris trees.
abstract: Manipulating tree belowground carbon (C) transport enables investigation of the ecological and physiological roles of tree roots and their associated mycorrhizal fungi, as well as a range of other soil organisms and processes. Girdling remains the most reliable method for manipulating this flux and it has been used in numerous studies. However, girdling is destructive and irreversible. Belowground C transport is mediated by phloem tissue, pressurized through the high osmotic potential resulting from its high content of soluble sugars. We speculated that phloem transport may be reversibly blocked through the application of an external pressure on tree stems. Thus, we here introduce a technique based on compression of the phloem, which interrupts belowground flow of assimilates, but allows trees to recover when the external pressure is removed. Metal clamps were wrapped around the stems and tightened to achieve a pressure theoretically sufficient to collapse the phloem tissue, thereby aiming to block transport. The compression's performance was tested in two field experiments: a (13)C canopy labelling study conducted on small Scots pine (Pinus sylvestris L.) trees [2-3 m tall, 3-7 cm diameter at breast height (DBH)] and a larger study involving mature pines (∼15 m tall, 15-25 cm DBH) where stem respiration, phloem and root carbohydrate contents, and soil CO2 efflux were measured. The compression's effectiveness was demonstrated by the successful blockage of (13)C transport. Stem compression doubled stem respiration above treatment, reduced soil CO2 efflux by 34% and reduced phloem sucrose content by 50% compared with control trees. Stem respiration and soil CO2 efflux returned to normal within 3 weeks after pressure release, and (13)C labelling revealed recovery of phloem function the following year. Thus, we show that belowground phloem C transport can be reduced by compression, and we also demonstrate that trees recover after treatment, resuming C transport in the phloem.
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden.
authors.forename: Nils
authors.initials: N
authors.lastname: Henriksson
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden.
authors.forename: Lasse
authors.initials: L
authors.lastname: Tarvainen
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden.
authors.forename: Hyungwoo
authors.initials: H
authors.lastname: Lim
authors.affiliation: Department of Environmental Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand.
authors.forename: Pantana
authors.initials: P
authors.lastname: Tor-Ngern
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708, USA.
authors.forename: Sari
authors.initials: S
authors.lastname: Palmroth
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708, USA.
authors.forename: Ram
authors.initials: R
authors.lastname: Oren
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden Department of Forest, Rangeland, and Fire Sciences, University of Idaho, 875 Perimeter Drive, Moscow, ID 83844-1133, USA.
authors.forename: John
authors.initials: J
authors.lastname: Marshall
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden torgny.nasholm@slu.se.
authors.forename: Torgny
authors.initials: T
authors.lastname: Näsholm
mesh_terms: D001692:Biological Transport; D002244:Carbon / Q000378:metabolism*; D052585:Phloem / Q000378:metabolism*; D041605:Pinus sylvestris / Q000378:metabolism*; D018517:Plant Roots / Q000378:metabolism; D018547:Plant Stems / Q000378:metabolism*; D011312:Pressure; D014197:Trees / Q000378:metabolism
chemical_list: D002244:Carbon
keywords: belowground carbon transport; carbon partitioning; carbon-13; girdling; soil respiration; stem respiration