Mechanistic modelling of the influence of temperature on the wood anatomy of Scots pine

Despite its importance for the study of past climates, as well as its significance for carbon sequestration, we lack a mechanistic explanation for how temperature controls wood anatomy. A model of xylogenesis is presented and used to analyse observed tree ring anatomy-temperature relationships in Scots pine (Pinus sylvestris). The model treats the daily proliferation of new cells in the cambium and their subequent differentiation through expansion and secondary wall thickening phases. Control on size at division in the cambium follows recent work on the Arabidopsis shoot apical meristem, and cell enlargement rates in the cambium and enlargement zone are controlled by temperature. The duration of post-cambial enlargement is partially controlled by the rate at which cells pass through the enlargement zone, and partially by the size of this zone, which is controlled by daylength. This set of assumptions is sufficient to generate observed profiles of cell sizes across radial files, with characteristic transitions from earlywood to latewood. After they leave the enlarging zone, cells enter the wall thickening zone, the width of which is also dependent on daylength. A temperature-dependent rate of wall material deposition is insufficient to reproduce the observed gradient in mass density across the radial file, and fails to fully capture the observed seasonality of the correlation between maximum latewood density and temperature. Inclusion of a control on the rate of wall deposition from substrate (sugar) supply, diffusing from the phloem across the radial file, corrects these deficiencies. The resulting model provides a mechanistic explanation of temperature-tree ring relationships, and has the potential to underpin understanding of how climate and CO₂ interact in determining the amount of carbon sequestered in trees.