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Xylem structure and hydraulic function in the roots and stems of chaparral shrub species that occur at a high and low elevation site in the southern Sierra Nevada mountains, California
The vascular transport system of most flowering plants is composed of vessels that are responsible for bulk axial water transport. Transport through vessels may become blocked by gas emboli following freeze-thaw events because gasses come out of solution in the vessels while freezing and these small gas bubbles may coalesce into larger emboli when stems thaw. Emboli may also form in vessels during water stress when gasses are pulled into vessels through a process termed air-seeding. Xylem vessels are not all equally vulnerable to the formation of emboli and there are specific traits that are associated with having more embolism resistant vessels. In general, the traits that confer resistance to embolism are also associated with reduced ability to efficiently transport water through the xylem. I examined and compared xylem structural and hydraulic traits of five chaparral shrub species occurring at two sites representing the lower and upper elevation distribution limits for these species along a steep transect in the southern Sierra Nevada mountains, California, USA. These sites represented the lower and upper ecotones of the chaparral community. The higher elevation site experiences more frequent freeze-thaw stress, lower temperatures, and receives more precipitation compared to the lower elevation site. I hypothesized that the environmental differences between the two sites would correlate to differences in xylem traits. I also compared traits among different organs (roots and stems) within species, as roots are buffered from freezing and thawing. Xylem vessel traits differed between species and between roots and stems but did not generally differ between the sites. Rather, the ratio of stem area to leaf area of shoots differed between sites, with plants growing more xylem tissue per leaf area at the lower site. Studies examining how traits may vary within individuals and within species have been relatively limited to date and knowledge of the structure and function differences between roots and stems is likely important in understanding whole-plant response to environmental stressors.
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