Abstract:
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.
