Growth and dieback of underplanted northern red oak seedlings under various light and moisture conditions
Tworkoski, Thomas James
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Survival, growth and dieback, endogenous auxin and total non-structural carbohydrate (TNC) relations of underplanted northern red oak (Quercus rubra L.) seedlings were investigated. Greenhouse-grown red oak seedlings were subjected to 0, 63, or 92% shade and soil water potential was permitted to reach -0.3 or -2.0 MPa before rewatering. In a field study, underplanted northern red oaks were exposed to three levels of canopy removal (0, 70, or 90% residual canopy). White oak (Q. alba L.) and eastern white pine (Pinus strobus L.) were included in the field experiment for comparative purposes. In the greenhouse, dieback was observed under full sunlight and 92% shade but not in intermediate shading. In the field, dieback was greatest following complete canopy removal; but dieback was usually followed by rapid sprout growth and, as a result, average seasonal height increases generally were unaffected by light availability. Seasonal height increases were small (1-4 cm) and survival usually exceeded 90% in both the field and greenhouse. Field-grown white oaks displayed similar growth and dieback as red oaks. White pines grew 3 cm and 10 cm under 0 and 90% residual canopy, respectively. Final TNC leves from red oaks grown in drier soils and full sunlight were greater than seedlings grown under full shade and higher soil moistures (27 and 17% of total dry weight, respectively). Under full sunlight, as occur with complete canopy removal, dieback was probably related to internal moisture/temperature stress. Dieback under 92% shade was likely induced by carbohydrate depletion and weak, succulent growth resulting from the chronically low light levels. Thus, oak dieback appears to be a survival mechanism in which an advantageous root/shoot ratio is maintained and nutrients can be conserved. High or low TNC was not strongly related to dieback or sprout growth in this study. However, decreased TNC concentrations associated with increased shade indicated that long-term low light intensities may adversely affect growth and survival of red oak. Quantification of endogenous auxin in oak stems with gas chromatography was impossible due to low IAA levels (less than 0.8 uM) and small amounts of available tissue. Large amounts of IAA were found to be lost to sublimation when IAA was subjected to vacuum.
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