Intra-annual dynamics of non-structural carbohydrates in the cambium of mature conifer trees reflects radial growth demands

The presence of soluble carbohydrates in the cambial zone, either from sugars recently produced during photosynthesis or from
starch remobilized from storage organs, is necessary for radial tree growth. However, considerable uncertainties on carbohydrate
dynamics and the consequences on tree productivity exist. This study aims to better understand the variation in different carbon
pools at intra-annual resolution by quantifying how cambial zone sugar and starch concentrations fluctuate over the season and
in relation to cambial phenology. A comparison between two physiologically different species growing at the same site, i.e., the
evergreen Picea abies Karst. and the deciduous Larix decidua Mill., and between L. decidua from two contrasting elevations, is
presented to identify mechanisms of growth limitation. Results indicate that the annual cycle of sugar concentration within the
cambial zone is coupled to the process of wood formation. The highest sugar concentration is observed when the number of cells
in secondary wall formation and lignification stages is at a maximum, subsequent to most radial growth. Starch disappears in
winter, while other freeze-resistant non-structural carbohydrates (NSCs) increase. Slight differences in NSC concentration between
species are consistent with the differing climate sensitivity of the evergreen and deciduous species investigated. The general
absence of differences between elevations suggests that the cambial activity of trees growing at the treeline was not limited by
the availability of carbohydrates at the cambial zone but instead by environmental controls on the growing season duration.