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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

Dannoura, Masako, Maillard, Pascale, Fresneau, Chantal, Plain, Caroline, Berveiller, Daniel, Gerant, Dominique, Chipeaux, Christophe, Bosc, Alexandre, Ngao, Jérôme, Damesin, Claire, Loustau, Denis, Epron, Daniel
The new phytologist 2011 v.190 no.1 pp. 181-192
Quercus petraea, linear models, soil water content, absorption, isotopes, tree and stand measurements, trees, carbon, growing season, phloem, sap, seasonal variation, bark, conifers, Fagus sylvatica, Pinus pinaster, functional properties
• Phloem is the main pathway for transferring photosynthates belowground. In situ¹³C 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 ¹³CO₂ 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 (kP). • Pulse labelling together with high-frequency measurement of the isotope composition of trunk CO₂ 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 kP indicate vertical changes in functional phloem properties.