Scaling of xylem and phloem transport capacity and resource usage with tree size

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.

Dynamic variations in bark hydraulics – understanding whole tree processes and its linkage to bark hydraulic function and structure

“…Little is known, and much of the studies contradict one another about the variation in phloem conduit properties and flow conducting area with changing tree size and axial position . Studies have found that the phloem pressure gradient is constant (Windt et al 2006;De Schepper et al 2013a;Hölttä et al 2013), but may also increase with tree height (Mencuccini and Hölttä 2010;Dannoura et al 2011). The latter result contradicts Turgeon (2010), who suggested that phloem turgor pressure differences does not scale to tree size.…”

Hydraulic Anatomy and Function of Trees—Basics and Critical Developments

“…Here, the capacity to remobilize carbohydrates from storage may play a critical role [139] further supporting the notion that, ultimately, the functionality of water transport in xylem is coupled with carbohydrate transport in phloem [9,40]. In theory, this link is well described [38,140], yet in practice we lack studies that combine in-situ measurements of pressure and concentration gradients driving radial transport of water and carbohydrates.…”

Drivers of aggregation in a novel arboreal parasite: the influence of host size and infra-populations

“…Larger hosts are predominantly older individuals with larger external surface areas and accrue greater parasite intensities through time and space (see Poulin and Morand, 2000;Poulin, 2013). Tree size is strongly correlated with tree age (Bond, 2000), whilst phloem thickness strongly scales with tree diameter (Amman, 1969;Speights and Conway, 2010;Hölttä et al, 2013;Davis and Hofstetter, 2014). Phloem translocates nutritionally-rich photosynthates from the canopy to the roots (Högberg et al, 2001;Zwieniecki et al, 2004;Pompon et al, 2011) and is the sole foodstuff for A. virescens.…”

“…Beneath tree outer bark, phloem carries photosynthates from the canopy to the roots (Högberg et al, 2001;Zwieniecki et al, 2004;Pompon et al, 2011). Phloem thickness scales strongly with tree diameter (Amman, 1969;Speights and Conway, 2010;Hölttä et al, 2013;Davis and Hofstetter, 2014), is a nutrient-rich resource (Pompon et al, 2011), and the sole foodstuff of A. virescens. Conversely, the hydrostatic conductance of phloem is reduced in older, larger trees, indicating a decreased flow of photosynthates (Yoder et al, 1994;Bond, 2000;Hölttä et al, 2013).…”

“…Phloem thickness scales strongly with tree diameter (Amman, 1969;Speights and Conway, 2010;Hölttä et al, 2013;Davis and Hofstetter, 2014), is a nutrient-rich resource (Pompon et al, 2011), and the sole foodstuff of A. virescens. Conversely, the hydrostatic conductance of phloem is reduced in older, larger trees, indicating a decreased flow of photosynthates (Yoder et al, 1994;Bond, 2000;Hölttä et al, 2013). Whilst larger trees present a greater quantity of food, nutritional quality is likely reduced.…”