In situ assessment of the velocity of carbon transfer by tracing ¹³C in trunk CO₂ efflux after pulse labelling: variations among tree species and seasons

Abstract

Summary• Phloem is the main pathway for transferring photosynthates belowground. In situ 13 C pulse labelling of trees 8-10 m tall was conducted in the field on 10 beech (Fagus sylvatica) trees, six sessile oak (Quercus petraea) trees and 10 maritime pine (Pinus pinaster) trees throughout the growing season.• Respired 13 CO 2 from trunks was tracked at different heights using tunable diode laser absorption spectrometry to determine time lags and the velocity of carbon transfer (V). The isotope composition of phloem extracts was measured on several occasions after labelling and used to estimate the rate constant of phloem sap outflux (k P ).• Pulse labelling together with high-frequency measurement of the isotope composition of trunk CO 2 efflux is a promising tool for studying phloem transport in the field. Seasonal variability in V was predicted in pine and oak by bivariate linear regressions with air temperature and soil water content. V differed among the three species consistently with known differences in phloem anatomy between broadleaf and coniferous trees.• V increased with tree diameter in oak and beech, reflecting a nonlinear increase in volumetric flow with increasing bark cross-sectional area, which suggests changes in allocation pattern with tree diameter in broadleaf species. Discrepancies between V and k P indicate vertical changes in functional phloem properties.

Carbon flux to woody tissues in a beech/spruce forest during summer and in response to chronic O<sub>3</sub> exposure

“…2 and 3). Thus, slower C transfer in the phloem of gymnosperms compared to angiosperms could not be confirmed (Kuptz et al, 2011a;Dannoura et al, 2011). The fraction of labeled C (f E,new ) in the CO 2 efflux of beech stems was significantly reduced under 2 × O 3 (support of hypothesis I), indicating a higher dependency on C stores of the respiratory supply under 2 × O 3 (cf.…”

Seasonal variations of belowground carbon transfer assessed by in situ <sup>13</sup>CO<sub>2</sub> pulse labelling of trees

“…The study was conducted on three temperate forest stands (Dannoura et al, 2011): a 20 yr-old natural beech regeneration on a luvisol (Fagus sylvatica L., 48 • 40 N, 7 • 04 E, 300 m elevation), a 15 yr-old natural sessile oak regeneration on a gleyic luvisol (Quercus petraea Matt Liebl, 48 • 2 N, 02 • 47 E, 90 m elevation) and a 12 yr-old maritime pine plantation on a sandy podsol (Pinus pinaster Ait, 44 • 45 N, 0 • 42 W, 60 m elevation). Tree density was 1.6, 1.5 and 0.25 trees per m 2 for beech, oak and pine, respectively.…”

“…Evolution of [ 12 CO 2 ] and [ 13 CO 2 ] inside the chamber was monitored with a 12 CO 2 / 13 CO 2 infrared gas analyser (S710, SICK/MAIHAK, Reute, Germany, accuracy of 5 %). A total amount of 50.4 g of pure 13 CO 2 (99.299 atom %, Eurisotop, Cambridge Isotope Laboratory Inc., Andover, MA, USA) was progressively injected for 1 to 5 h at a flow rate adjusted to maintain the 13 CO 2 mixing ratio in the chamber between 300 and 400 µmol mol −1 , except in November and February for pine where higher 13 CO 2 mixing ratios (800-1000 µmol mol −1 ) were used to compensate a lower photosynthetic activity (see Dannoura et al, 2011;Plain et al, 2009 for more details). By only inserting the crown into the labelling chamber, we avoided contamination of the soil atmosphere by diffusion of 13 CO 2 into the soil pores that would later have back-diffused into the atmosphere leading to an artefact in soil CO 2 efflux isotopic composition (Subke et al, 2009).…”

“…Drought reduced the contribution of recent photosynthate to soil CO 2 efflux and the transfer of carbon into the microbial biomass in young potted beech (Ruehr et al, 2009). Both soil water content and temperature affected carbon allocation to microbial biomass in Ericaceae species (Gorissen et al, 2004) and the velocity of carbon transfer in the trunk of oak and pine (Dannoura et al, 2011). In the latter study, we demonstrated that pulse labelling of field-grown trees with 13 CO 2 and high frequency of measurement of 13 C in respiratory efflux during the chase period using laser-based infrared gas analysers is a powerful approach to study seasonal variations of carbon transfer (see also Bahn et al, 2009, on grassland).…”

“…13 CO 2 recovery in soil CO 2 efflux was recorded for 3 weeks by tuneable diode laser absorption spectrometers (TDLAS) after pulse labelling of 8-10 m tall trees and compared to 13 C recovery in both root and microbial biomass and their respective respiratory CO 2 . We hypothesised that (i) the transfer time would vary between broadleaved and coniferous species due to difference in anatomy that affect the velocity of carbon transport in the phloem (Dannoura et al, 2011) and growth rhythm (deciduous versus evergreen species), (ii) the amount of 13 C recovered in soil CO 2 efflux would vary along the season reflecting changes in carbon allocation related to phenology, temperature and soil water content, and (iii) 13 C would be first recovered in the roots and later in the microbial biomass.…”