Graduate Thesis Or Dissertation
 

Xylem cavitation in newly planted western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings

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  • Reforestation of Pacific Northwest forest-lands with western hemlock has been hindered by unexplained plantation failures. The purpose of this thesis is to provide information that will help in determining the cause of seedling damage and mortality. The studies focused on the relationship between xylem water potential, xylem cavitation and the seedlings subsequent ability to transport water. The initial experiment illustrated that following planting and exposure to water stress newly planted western hemlock seedlings were unable to transport sufficient water to meet transpirational demand. Hydraulic conductance declined from 1.O1ug cm-2 s-1 MPa-1 to O.l6ug cm-2 s-1 MPa-1 over an 11 day period. In the second study xylem cavitation was detected almost immediately following planting in moist soils and in one case rapidly escalating diurnal cavitation rates ended in seedling mortality. There did not appear to be any correlation between cavitation rates and ambient temperature and relative humidity. In addition it was determined that xylem cavitation was initiated when xylem water potentials reached -1.5MPa. A peak rate of 90 events per minute was experienced at -3.6MPa declining thereafter as the number of water filled tracheids decreased. A similar pattern of cavitation rate and water potential was experienced when monitoring intact potted seedlings in a greenhouse. The cavitation rates followed a diurnal pattern however which declined with increasing xylem water potentials. The final experiment in this series measured the impact of xylem cavitation on hydraulic conductivity. A 30% loss in hydraulic conductivity was experienced following a water potential between -2.5 and -3.OMPa. A complete loss was experienced following water potentials of -4.OMPa. Following planting, western hemlock seedlings experience water stress due to the inability of the root system to supply sufficient water to meet transpirational demand. This water stress results in water potentials which are sufficiently negative to cause xylem cavitation and embolism formation. The embolized tracheids result in a decline in hydraulic conductance and subsequent adjustments in stomatal aperture and leaf area. Without adjustments to reduce transpirational demand the seedling can experience severe enough cavitation to result in mortality.
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