Manipulating phloem transport affects wood formation but not local nonstructural carbon reserves in an evergreen conifer

• Published in: Plant, cell and environment Vol. 44; no. 8; pp. 2506 - 2521
• Main Authors: Rademacher, Tim, Fonti, Patrick, LeMoine, James M., Fonti, Marina V., Basler, David, Chen, Yizhao, Friend, Andrew D., Seyednasrollah, Bijan, Eckes‐Shephard, Annemarie H., Richardson, Andrew D.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01-08-2021; Wiley Subscription Services, Inc
• Subjects: allocation; Carbon; Carbon dioxide; Cell proliferation; Cell size; Compression; Compression tests; Decoupling; Earth and Related Environmental Sciences; Efflux; Geovetenskap och miljövetenskap; Girdling; growth; Natural Sciences; Naturgeografi; Naturvetenskap; Ontogeny; Phloem; Physical Geography; Pine needles; Pinus strobus; Reserves; respiration; Stems; Wood; wood anatomy; Xylem; xylogenesis
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1111/pce.14117

How variations in carbon supply affect wood formation remains poorly understood in particular in mature forest trees. To elucidate how carbon supply affects carbon allocation and wood formation, we attempted to manipulate carbon supply to the cambial region by phloem girdling and compression during the mid‐ and late‐growing season and measured effects on structural development, CO2 efflux and nonstructural carbon reserves in stems of mature white pines. Wood formation and stem CO2 efflux varied with a location relative to treatment (i.e., above or below the restriction). We observed up to twice as many tracheids formed above versus below the treatment after the phloem transport manipulation, whereas the cell‐wall area decreased only slightly below the treatments, and cell size did not change relative to the control. Nonstructural carbon reserves in the xylem, needles and roots were largely unaffected by the treatments. Our results suggest that low and high carbon supply affects wood formation, primarily through a strong effect on cell proliferation, and respiration, but local nonstructural carbon concentrations appear to be maintained homeostatically. This contrasts with reports of decoupling of source activity and wood formation at the whole‐tree or ecosystem level, highlighting the need to better understand organ‐specific responses, within‐tree feedbacks, as well as phenological and ontogenetic effects on sink‐source dynamics.
Wood formation and carbon allocation in mature forest trees are two important processes of the terrestrial carbon cycle, which both lack a mechanistic understanding. By girdling and temporarily compressing the phloem of mature white pine at Harvard Forest, we found that carbon supply variation due to these restrictions in phloem transport affected wood formation and stem respiration without substantially changing local nonstructural carbon reserves. We also observed large changes in the number of cells formed, correlating strongly with the presumed carbon supply gradient. In contrast, changes in cell‐wall thickness along the presumed carbon supply gradient were relatively small and cell size was not detectably affected. The presented carbon sensitivity of cambial cell division, cell elongation and cell‐wall thickening, as well as the effects on stem CO2 efflux and local nonstructural carbon reserves present significant advances in our understanding of wood formation and carbon allocation in forest trees with repercussions on the debate of source‐ versus sink‐limitation of growth in trees.