Graduate Thesis Or Dissertation
 

Carbon and nitrogen allocation patterns of 2-0 Douglas-fir seedlings following nitrogen fertilization in the autumn

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  • Douglas-fir (Pseudotsuga menziesii Mirb. Franco) seedlings at a nursery in western Oregon were fertilized with nitrogen in October. Free amino acid (FAA) and total nitrogen concentrations in needles, stems and fine roots were followed from before fertilization until just prior to budbreak the following spring. Before budbreak in mid-March, the FAA and total nitrogen concentrations in the fertilized seedlings were significantly higher than the unfertilized seedlings. Fertilized seedlings showed significant depletion of non-structural carbohydrates (NSC) relative to the unfertilized seedlings. The reduction in carbohydrate reserves following fertilization probably reflects increased respiration associated with the synthesis and maintenance of higher levels of enzymes. The seedlings were lifted from the nursery bed and planted in a split plot design. The main treatment was the presence or lack of grass competition. Within each main plot, the previously fertilized and unfertilized seedlings were planted. Sucrose was applied to the soil around each seedling to limit the availability of nitrogen to tree roots. On the average, the fertilized seedlings broke bud ten days earlier than the unfertilized seedlings and produced more growth aboveground. The earlier budbreak was responsible for initial differences in growth response. Later harvests showed that fertilized seedlings also exhibited higher relative growth rates. Seedlings growing in the grass plots had predawn water potentials of -1.5 MPa by early August. By September 3, the unfertilized seedlings growing with grass showed significantly more predawn water stress than any of the other three treatments. Although the fertilized seedlings had higher FAA and total nitrogen concentrations than unfertilized seedlings when they were planted, by the end of one growing season the FAA arid total nitrogen concentrations had equalized. However, the fertilized seedlings contained more FAA and nitrogen because of their greater size. Grass competition affected both seedling nitrogen and non-structural carbohydrate chemistry. After one growing season, the fertilized seedlings showed a 3 cm increase in height increment; a 29% increase in the number of stem units on the terminal leader; a 44% increase in aboveground growth; a 25% increase in total seedling leaf area; a 23% increase in relative growth rate; and a 14% increase in production per unit nitrogen.
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