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spacings (10 and 12 ft) than at the closer spacings (4, 5, 6 and 8 ft). We considered three possible
explanations: (1) close spacing eventually retarded growth, (2) soil quality may be better in the
10- and 12-ft spacings than at closer spacings, and (3) tree spacing and soil quality combined
affected growth. To test these explanations, we measured and mapped several site factors
(topographic relief, depth to bedrock, and soil properties), and related these factors to tree and
stand growth. We infer from the strong correlation between spacing and soil variables that the
influence of soil and spacing cannot be separated; differences in soil depth and available water
capacity confound spacing effects and vice versa. Because soils in the wider spacings were
generally deeper and had more available water capacity than do soils in the closer spacings, we
conclude that some of the superior tree growth attained in the 10- and 12-ft spacings is due to
more favourable soil conditions. Visual comparisons of tree size, however, suggest that spacing is
probably the stronger factor affecting tree growth at this location.
399. Miller, R.E., D.L. Reukema and J.W. Hazard. 1996. Ammonium nitrate, urea, and biuret
fertilizers increase volume growth of 57-year-old Douglas-fir trees within a gradient of nitrogen
deficiency. Pacific-Northwest-Research-Station,-USDA-Forest-Service Research-Paper PNW-
RP-490. 12 p.
Keywords: planting operations
fertilization
growth
Abstract: Growth of dominant and codominant Pseudotsuga menziesii given 224 kg N/ha
as ammonium nitrate, urea or biuret (a slow-release N source) in a N-deficient plantation in SW
Washington was recorded over an 8-year period in relation to distance of the trees from a strip of
the plantation interplanted with N-fixing Alnus rubra. Adjusted mean volume growth of the
measured trees was increased by 22-28% on the east side of the mixed stand centreline and by 11-
14% on the west side, with no difference in response to the 3 fertilizers. Only biuret stimulated
growth within the mixed strip of the stand. Biuret had no visible toxic effect on competing
vegetation.
400. Miller, R.E., D.L. Reukema and T.A. Max. 1993. Size of Douglas-fir trees in relation to distance
from a mixed red alder - Douglas-fir stand. Canadian-Journal-of-Forest-Research 23(11): 2413-
2418.
Keywords: planting operations
growth
Abstract: Variation in diameter, height, and stem volume of 57-year-old Douglas fir
(Pseudotsuga menziesii var. menziesii) was studied in relation to distance of these trees from a 27
m wide strip in the same Douglas fir plantation that had been interplanted with red alder (Alnus
rubra) at a poor quality site in SW Washington. Measurements were made in 1983. Within the
interplanted strip, and despite its greater total stand density, stem volume of dominant and
codominant Douglas fir averaged 1.27 msuperscript 3 compared with 0.55, 0.45, 0.46, or 0.49
msuperscript 3, respectively, in trees 15, 30, 45, or 60 m distant from the edge of the mixed stand.
It is concluded that some positive influence of nitrogen-fixing red alder apparently extended
about 15 m beyond the edge of the mixed stand. It was inferred that similar ribbon-like
distributions of naturally regenerated red alder could be retained to improve growth of nearby
conifers on nitrogen-deficient sites.