Abstract:
The ecological and silvicultural importance of snowbrush,
Ceanothus velutinus, in the areas west of the summit of the Oregon
Cascades was studied. Particular attention has been given its nitrogen-
fixing ability under both field and greenhouse conditions.
On west slopes of the Oregon Cascades snowbrush occurs above
2, 500 feet elevation, Contrary to its permanent presence in some
climax associations of the central Oregon, it may occur after an absence
of 300 to over 500 years in the area of this study. The germination
of the seed, which had accumulated during the brief lifetime
of snowbrush stands in the duff and upper layer of the mineral soil,
is stimulated by fire followed by winter stratification. Extensive
areas have been occupied by snowbrush following logging and slash
burning.
Twelve snowbrush stands ranging in age from three to 15 years
were selected for a biomass study in order to obtain some measure
of nitrogen fixation. Only fully stocked portions of these stands were
sampled. Fresh weights of snowbrush plants were obtained by removing
all above-ground portions and weights of roots were determined
by excavation of several root systems in each age class. Representative
samples of both were taken for dry weight determinations.
Litter and herbs were removed from one meter square areas,
All plant materials were dried at 70 degrees centigrade and nitrogen
determined by Kjeldahl analysis.
Total nitrogen in the upper two feet of soil was higher under
snowbrush than in the absence of cover; it is suggested that the difference
may have been caused by loss of nitrogen from open areas
rather than by nitrogen fixation. Total nitrogen in the upper 15 cm
of soil was not significantly different under snowbrush and under
other kinds of shrubs. It is suggested that various shrub species
may increase the total soil nitrogen under their canopies at the
expense of the open unvegetated areas.
Significantly higher amounts of nitrogen may be tied up in the
biomass of mature snowbrush stands than in stands of other shrubs
on the same site. The difference could be explained by nitrogen
fixation which on the average could range from zero to 20 kg/ha/year.
It would require over 35 years for snowbrush to ze-establish the
nitrogen lost in logging and slash burning to its original level. This points to the need for development of other methods of slash disposal
which would not destroy the organic matter.
Nodulated seedlings produced Z times the dry weight of non-nodulated
snowbrush plants after being grown for eight months in a
nitrogen-deficient soil in a greenhouse. It may be concluded that
61% of nitrogen in the nodulated seedlings has been fixed. Such fixation
perhaps may be reached on infertile soils but is very unlikely
on soils of medium or better fertility. There is some evidence that
in soils with increasing levels of organic matter the nodulation of
snowbrush may be delayed for several years.
Snowbrush may be beneficial or harmful to managed forests.
In general its desirability seems to be inversely proportional to the
fertility of the soil. Some benefit could be expected if coniferous
species were grown concurrently with snowbrush; however, no appreciable
benefit can be expected during the first eight years.
Results of the planting experiment suggest that the survival of
planted conifers is significantly higher on freshly-burned clearcuts
than under live or dead snowbruh, or in the openings in snowbrush
stands. Considering all treatments the survival of Douglas-fir was
significantly higher than of ponderosa pine, western hemlock, and
noble fir.
