- Six, recent clear cut areas were selected in the Douglas-fir
zone of the Western Cascade Range of Oregon to determine the
environmental and nutritional effects that snowbrush has on the establishment
and growth of Douglas-fir seedlings.
One thousand eighty milacre sample plots on the six clear cuts
were used to determine the survival, total height growth and browsing
damage of Douglas-fir seedlings. The nutrient status of the
seedlings growing in association with snowbrush and in the open was
also determined on the six clear cuts. Needles on the current year's
growth were analyzed at three times during the year, for nitrogen,
calcium, magnesium, potassium and phosphorus. The nutrient levels
of the seedlings were then related to their position relative to snowbrush.
The percent total nitrogen and the exchangeable calcium,
magnesium and potassium in the soil under snowbrush and in the open
Three of the six clear cuts were selected and soil moisture in
the open, along the edge and under snowbrush was measured with
Colman fiberglas moisture blocks and gravimetrically, In all, 33
moisture stations were used. Moisture was measured at 2, 6 and 24
inches. At each moisture station, the maximum soil surface
temperature was determined with a maximum reading mercury thermometer.
In addition to the mercury thermometers, six continuous
recording thermometers were used to determine the number of consecutive
days on which high soil temperature occurred.
Precipitation through the growing season, both in the open and
under snowbrush, was determined with home-made rain gauges.
Survival and growth of planted Douglas-fir seedlings were better
under snowbrush than in the open. The milacre stocking under snowbrush
was 48% greater than in the open. The total height was 2.5 feet greater on nine-year-old seedlings, whereas, the yearly height
growth was as much as 5.1 inches greater on seedlings associated
with snowbrush, than those in the open.
Browsing damage was greatly reduced by brush cover. Only
28.3% of the seedlings under snowbrush were damaged, compared to
69.0% and 73.5% damaged in the open and along the edge, respectively.
Throughout the growing season, soil moisture in the top six inches contained more moisture under snowbrush than in the open.
At 24 inches, less water was available under the brush than in the
open. The soil moisture consumption suggests that a newly planted
or germinating seedling will have more moisture available to it under
brush cover, than in the open, however, the seedling will eventually
have to compete with the brush for moisture as its roots penetrate
the soil to about 24 inches.
Lethal soil surface temperatures were encountered in the open.
Soil surface temperatures in the open reached levels high enough to
cause death to first year seedlings and at least some damage to
planted stock, whereas, under the brush, soil surface temperatures
never reached the lethal limit, even during the extremely hot, dry
summer of 1967,
Lower soil surface temperatures under snowbrush are probably
one of the most important factors in maintaining higher soil surface
moisture levels under the brush.
The concentration of nitrogen, calcium, magnesium and potassium
in current year's Douglas-fir needles is greater in seedlings
growing in association with snowbrush than in those growing in the
open. The concentration of phosphorus in needles, however, did not
appear to be related to snowbrush cover.
The concentration of all nutrients tested, except phosphorus,
declined as the growth rate increased. Seedlings growing in
association with snowbrush did not have as marked a decrease,
particularly in the case of nitrogen,as did seedlings in the open.
The effect of snowbrush on the declining nutrient content was more
marked on poorer sites, than on better sites. The effect of snowbrush
appeared most pronounced during periods of high nutrient
The nutrient content of the soil under a stand of snowbrush was
higher in terms of total nitrogen and exchangeable calcium, magnesium
and potassium than the soil in the adjacent opening. It would
appear that snowbrush is not only capable of increasing nitrogen
through fixation, but is also instrumental in maintaining more
cations in the surface soil, through re-cycling in the litter.
Snowbrush in the Western Cascades appears to be instrumental
in creating a more favorable microenvironment as well as a more
favorable nutrient regime for newly establishing Douglas-fir seedlings.