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Photosynthetic refixation varies along the stem and reduces CO2 efflux in mature boreal Pinus sylvestris trees

Journal article
Authors Lasse Tarvainen
Göran Wallin
Hyungwoo Lim
Sune Linder
Ram Oren
Mikaell Ottosson Löfvenius
Mats Räntfors
Pantana Tor-ngern
John Marshall
Published in Tree Physiology
Volume 38
Issue 4
Pages 558–569
ISSN 0829-318X
Publication year 2018
Published at Department of Biological and Environmental Sciences
Pages 558–569
Language en
Links https://doi.org/10.1093/treephys/tp...
Keywords CO2 recycling, corticular photosynthesis, forest carbon balance, Scots pine, stem photosynthesis, stem respiration
Subject categories Forest Science, Botany

Abstract

Trees are able to reduce their carbon (C) losses by refixing some of the CO2 diffusing out of their stems through corticular photosynthesis. Previous studies have shown that under ideal conditions the outflowing CO2 can be completely assimilated in metabolically active, young stem and branch tissues. Fewer studies have, however, been carried out on the older stem sections of large trees and, accordingly, the importance of refixation is still unclear under natural environmental conditions. We investigated the spatial and temporal variation in refixation in ~90-year-old boreal Scots pine (Pinus sylvestris L.) trees by utilizing month-long continuous measurements of stem CO2 efflux (Ec) made at four heights along the bole. Refixation rates were found to vary considerably along the bole, leading to a 28% reduction in long-term Ec in the upper stem compared with a negligible reduction at breast height. This vertical pattern correlated with variation in light availability, bark chlorophyll content and bark type. Analysis of the vertical and diurnal patterns in Ec further suggested that the influence of sap flow on the observed daytime reduction in Ec was small. The areal rates of corticular photosynthesis were much lower than previous estimates of photosynthetic rates per unit leaf area from the same trees, implying that the impact of refixation on tree-scale C uptake was small. However, upscaling of refixation indicated that 23–27% of the potential Ec was refixed by the bole and the branches, thereby significantly reducing the woody tissue C losses. Thus, our results suggest that refixation needs to be considered when evaluating the aboveground C cycling of mature P. sylvestris stands and that breast-height estimates should not be extrapolated to the whole tree.

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