Interaction of two parenchyma ray types regulates redwood heartwood deposition.

Part of the longevity strategy of trees is the deposition of extractives, biocidal compounds within heartwood that slow decay long after tree death and are thus an important determinant of carbon residency time. The process of heartwood deposition is regulated by parenchyma rays, living tissues that store carbohydrates, are the sites of biosynthesis, and contain substantial quantities of extractives within mature heartwood. We sought to explore and predict heartwood deposition across the full axial gradient in Sequoia sempervirens by incorporating the plasticity of sapwood parenchyma rays into our models and testing the value of separating them into two size classes. We used synchrotron-based X-ray tomographic microscopy (microCT) for anatomical measurements and novel tissue-level densitometry of wood samples collected from up to 100 m above ground. We found that Sequoia has two distinct ray types, and the interaction of these short and tall rays strongly predicts variation in the extractive content of heartwood. Environmental context was crucial; separate anatomy-based models were needed for never-logged primary and recovering secondary Sequoia forests in the north and primary forests on the southern margin of the species’ range. Incorporating ray structure and distribution into future work on heartwood extractives could provide tools to quantify and monitor heartwood responses of Sequoia forests to management and climate based on the phenotypic plasticity of rays.