Abstract
Xylem and phloem need to maintain steady transport rates of water and carbohydrates to match the exchange rates of these compounds at the leaves. A major proportion of the carbon and nitrogen assimilated by a tree is allocated to the construction and maintenance of the xylem and phloem long distance transport tissues. This proportion can be expected to increase with increasing tree size due to the growing transport distances between the assimilating tissues, i.e., leaves and fine roots, at the expense of their growth. We formulated whole tree level scaling relations to estimate how xylem and phloem volume, nitrogen content and hydraulic conductance scale with tree size, and how these properties are distributed along a tree height. Xylem and phloem thicknesses and nitrogen contents were measured within varying positions in four tree species from Southern Finland. Phloem volume, nitrogen amount and hydraulic conductance were found to be concentrated toward the branch and stem apices, in contrast to the xylem where these properties were more concentrated toward the tree base. All of the species under study demonstrated very similar trends. Total nitrogen amount allocated to xylem and phloem was predicted to be comparable to the nitrogen amount allocated to the leaves in small and medium size trees, and to increase significantly above the nitrogen content of the leaves in larger trees. Total volume, hydraulic conductance and nitrogen content of the xylem were predicted to increase faster than that of the phloem with increasing tree height in small trees (<~10 m in height). In larger trees, xylem sapwood turnover to heartwood, if present, would maintain phloem conductance at the same level with xylem conductance with further increases in tree height. Further simulations with a previously published xylem-phloem transport model demonstrated that the Münch pressure flow hypothesis could explain phloem transport with increasing tree height even for the tallest trees.
Highlights
Structural-functional tree models describe formation and growth of new stem and branch axes as a function of their local environment and topological position within a tree (e.g., Sievänen et al, 2000)
We set out to determine allometric relations for the amounts of xylem and phloem, and their hydraulic conductances and nitrogen concentrations as a function of tree stem/branch diameters. We studied whether these relations were similar in four different species co-occurring in Southern Finland; birch (Betula pendula), aspen (Populus tremula), pine (Pinus sylvestris), and spruce (Picea abies)
Predictions can be made on how whole tree level properties scale with tree size assuming that the measured relationships do not change with tree height
Summary
Structural-functional tree models describe formation and growth of new stem and branch axes as a function of their local environment and topological position within a tree (e.g., Sievänen et al, 2000). This approach has been shown to be a powerful tool in reproducing realistic tree architectures (Prusinkiewicz, 2004). Scaling rules, such as the pipe model (Shinozaki et al, 1964), have been used to quantify secondary growth to analyse tree mass balance during growth (e.g., Perttunen et al, 1996). Understanding the functional implications of axial scaling patterns is an essential feature of structural-functional tree models
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