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Xylem Functional Traits at Different Tree Position within Populus trichocarpa
Xylem is a complex vascular tissue that performs several functions related to the growth and survival of woody plants. These functions include the transport of water, biomechanical support, and the storage of water and nutrients. Trees are large organisms that have a highly branched growth form that is specialized to extract finite resources from the soil, move them to the shoot, and acquire limited carbon dioxide from the atmosphere. Because trees have large bodies that must transport water over long distances of changing environments, I hypothesized that their vascular system changes at different tree positions. I examined how xylem functional traits of water storage, strength, stiffness, and xylem density changed at different positions within the roots and shoots of the model tree species Populus trichocarpa (Torr. & Gray). I predicted the following: (1) xylem functional traits changed depending on tree position, most notably between root and shoot organs; (2) within organs, xylem traits differed depending upon organ diameter or position relative to the tree base. Because different parts of the vascular system may specialize in specific ways that balance the proficiency of one trait at the expense of another, I predicted (3) there would be functional trade-offs within the xylem, especially between biomechanical and storage functions. My hypotheses and predictions were tested by analyzing xylem samples from 6 juvenile P. trichocarpa trees grown in an irrigated plot at California State University, Bakersfield. For each tree, xylem was sampled at 1 m spaced positions from base to tip of the dominant root and leader shoot. My first prediction was supported; xylem density, xylem strength, and xylem stiffness were higher in shoots and lower in roots. Xylem water storage was higher in roots and lower in shoots. Support for the second prediction depended on organ and trait, where wider organ diameter and more basal positions positively associated with xylem density, xylem strength, and xylem stiffness in roots. In shoots, more apical positions and narrower organ diameters positively associated with xylem strength and stiffness, but shared no relationship with xylem density. Xylem water storage did not correlate with organ diameter or position within roots or shoots. I found that roots and shoots appear to be specialized, with roots having greater hydraulic storage with reduced biomechanical support, and shoots having biomechanically reinforced xylem with minimal water storage. These findings support the predicted trade-off of xylem traits across organs but not within them. My hypothesis was supported: the tree vascular system changes at different tree positions. Intra-organismal examinations can be used to find trade-offs and changes in vascular systems across plant bodies. Understanding how xylem functions differ throughout tree bodies is important in understanding how these terrestrial plants can maintain towering canopies and endure numerous environmental challenges over decades of growth.
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