- First year survival of Douglas-fir seedlings outplanted in areas
characterized by intense vegetative competition is heavily dependent
on available soil moisture. To test this hypothesis, five distinct
classes of Douglas-fir planting stock were planted on the south slope
of McCulloch Peak in McDonald Forest in February of 1975. The
stocking classes represented in this study are 2-2 transplants, 2-0
seedlings, one-year-old container-grown (plug) seedlings, 3-0 seedlings,
and 2-1 transplants. Four treatments were applied in two
replications: (1) a combination of irrigation and herbicidal control of
competing vegetation; (2) irrigation; (3) herbicidal control of
competing vegetation; and (4) no cultural treatment. The Scholander
pressure bomb technique was used to determine the timing of the
irrigation treatment. Whenever the average pre-dawn xylem
pressure potential of the seedlings fell below -20 bars, irrigation was
applied. The two replications corresponded to two distinct vegetative communities: a brush-dominant community and a, grass-dominant
community. To eliminate the variable of wildlife pressure,
every seedling was protected by a mesh animal exclosure. Seedling
mortality was tallied at intervals throughout the summer, and leader
elongation was measured in October of 1975.
The vegetation community in which a seedling was outplanted
was of overriding importance to the seedling's potential for survival.
Phenological development of the constituents of the vegetation community
greatly influenced the availability of soil moisture so critical
to seedling establishment. In turn, community structure determined
the favorability, or lack thereof, of the microenvironment in which a
seedling developed. In respect to both phenology and structure, the
community dominated by grasses was more adverse to the introduction
of Douglas-fir seedlings than was the community dominated by brush.
The importance of the type of vegetative cover was further
underscored by the response to the various cultural treatments.
the brush-dominant community, irrigation, herbicides, and the
combination of irrigation and herbicides proved equally effective as
measures of site preparation. This was in contrast to the results in
the grass-dominant community which showed that irrigation alone
could not ensure acceptable seedling survival. Due to their inherent
ability to disrupt the normal development of established vegetation,
herbicides emerged as an especially effective means of ameliorating adverse site conditions. In both communities, little additional
benefit was realized by coupling irrigation to the herbicide treatment.
As was expected, seedlings which received no cultural treatment
performed poorly regardless of type of vegetative cover.
In regards to the performance of the various age-classes, the
one-year-old container-grown seedlings showed a survival rate of
nearly 90% in the grass community. Unable to match this performance,
the 2-0, 2-1, 3-0, and 2-2 bare-rooted stock survived at the
following rates: 45%, 44%, 36%, and 33%, respectively.
The container-grown seedlings were not, however, superior in
the brush community. Both the 2-1 and 3-0 planting stock had higher
survival, 76 and 71%, respectively, than the container-grown seedlings
(70%) and the 2-2 transplants (68%). The 2-0 seedlings (56%
survival) performed poorly in the brushy area; although they had the
highest survival of the bare-rooted stock in the grass community.
Seedling morphological characteristics were meaningful to
survival in the case of the 3-0 seedlings in the grass community and
the container-grown seedlings in the brush community. In terms of
height, diameter, and weight, the smaller 3-0 seedlings adapted to
their new environment better than did larger 3-0 seedlings. For the
container-grown seedlings, larger stem diameters were correlated
with increasingly better survival. As a check on the various seedlings control of stomatal
aperture, leaf water conductance was measured with a null balance
diffusion porometer. Small seedlings tended to have higher rates of
transpiration than large seedlings, but total transpirational loss
under given environmental conditions was judged to be equivalent
regardless of seedling size.