Manipulating phloem transport affects wood formation but not local
nonstructural carbon reserves in an evergreen conifer
Abstract
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 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 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 a 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 ontological effects
on sink-source dynamics.