In trees, the vascular elements are known to increase in diameter from top to bottom of the crown. This is important for trees as according to today's knowledge, tapering conduits compensate for increased transport distance and have been claimed to maximize the conductance per carbon use or the difference between photosynthetic gains and wall construction costs, depending on the theoretical background. According to 'Murray's law', the change in cell size depends on the transporting distance to the 1/4 power. The results of studies performed with trunks of angiosperms trees have given support for this. However, in gymnosperms, xylem cells have both transporting and mechanical support functions, and thus more limited range in cell size is proposed. Moreover, properties of vascular elements may differ in below-ground compartments or change according to tree architecture, not just distance. Therefore, our objective is to quantify the change in size and density of vascular elements along the water pathway from root tips to leaf petioles, and to study the effect of tree architecture on this change.
Our study species were Scots pine (Pinus sylvestris), Norway spruce (Picea abies), and Silver birch (Betula pendula). We examined 9 root systems and crowns of trees with the mean age of 29 years and growing in three mixed forest stands, which formed a continuum of stand productivity. Three individuals per species were sampled in such a way that two pines and spruces from the site of lowest fertility, one of both from the most fertile site and one birch from each site. In each study tree, we excavated completely 1-3 roots down to the diameter of two millimetres and digitized the 3D structure of 5 branches from the crown shoot by shoot. From these sample roots and branches, we took 3 cross-cuttings from each existing branching order to measure vascular features.
According to our preliminary results, in the roots of each species and in the main pathway in the trunk of silver birch the vessels taper and their density increase with distance as expected, i.e. along the water pathway from root tips to trunk base, and thereon to the crown top. However, in conifers the tapering seems to be related with transporting distance in the below-ground parts only, while in the main trunk above-ground there is hardly any tapering. Most importantly, even in birch, when we follow another rode to a terminal unit beside the main trunk, that is along side branches, we found that conduit tapering with distance is not true anymore: it seems that both in birch and studied conifers here, the tapering occurs more as steps from one organ level to another (from trunk to side branches, and from side branches to petioles) rather than with distance.