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 13C 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 13C 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
13C 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.
Manipulating tree belowground carbon (C) transport enables investigation of the ecological and physiological roles of tree rootsand their associated mycorrhizal fungi, as well as a range of other soil organisms and processes. Girdling remains the most reliablemethod 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 highcontent of soluble sugars. We speculated that phloem transport may be reversibly blocked through the application of an externalpressure on tree stems. Thus, we here introduce a technique based on compression of the phloem, which interrupts belowgroundflow of assimilates, but allows trees to recover when the external pressure is removed. Metal clamps were wrappedaround the stems and tightened to achieve a pressure theoretically sufficient to collapse the phloem tissue, thereby aiming toblock transport. The compression’s performance was tested in two field experiments: a 13C canopy labelling study conducted onsmall Scots pine (Pinus sylvestris L.) trees [2–3 m tall, 3–7 cm diameter at breast height (DBH)] and a larger study involvingmature pines (∼15 m tall, 15–25 cm DBH) where stem respiration, phloem and root carbohydrate contents, and soil CO2 effluxwere measured. The compression’s effectiveness was demonstrated by the successful blockage of 13C transport. Stem compressiondoubled 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, and13C labelling revealed recovery of phloem function the following year. Thus, we show that belowground phloem C transport canbe reduced by compression, and we also demonstrate that trees recover after treatment, resuming C transport in the phloem.
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