- Profitable, even-aged forest management depends on the early establishment
and rapid growth of each new forest crop. These, in turn,
require that the young trees have access to an adequate supply of
native resources of moisture, nutrients and light. In Mediterranean-type
climates, competition for soil moisture is the most serious adverse
effect of weeds. Young trees respond with elevated moisture
stress levels throughout most of the summer, with reduced net assimilation
and potential for growth. If soil moisture depletion is severe
enough, dehydration and death ensue.
This dissertation explores the relationship between soil moisture
availability, as influenced by several herbaceous cover types, and
Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco.] response, mainly in
terms of tree moisture stress TMS) and growth; it seeks to link herbicide
treatment and tree response directly, and also indirectly, by its
effect on soil moisture availability and tree moisture stress.
Data for this study came mainly from field observation and experimentation
during the summer of 1970, and from stem analyses performed
in 1972 in a set of pm-existing herbicide trial plots. These were
located in a bentgrass (Agrostis tenuis Sibth) dominated meadow situated
about 18 miles west of Corvallis in the Oregon Coast Range. The
diverse vegetative covers of the plots in 1970 reflected histories of
up to three years herbicide. treatment. Four types of data were used:
total available soil moisture content; climatological records; survival
records; and height, diameter and volume measurements determined by
An early attempt was made to apply a complicated regression model
to individual tree TMS using various climatological, soil, and time
variables. Although approximately 80% Of the variation in TMS could
be explained by this, it was clear that this model had serious
Study of plots of TMS against Pacific daylight time (PDT), air
temperature, vapor pressure deficit VPD) and solar radiation led to
the development of a conceptual model of TMS based largely on postulated
light- and leaf temperature-triggered responses of the stomates, sensitized
and modulated by the availability of soil moisture, degree of
overnight recovery of tree turgor, and the atmospheric moisture demand.
A possible explanation of the so-called "freeze" damage of Douglas-fir
was suggested by this model.
Although abnormally favorable soil moisture conditions in the
summer of 1968 caused weed control that year to have no significant
effect on first year survival or height growth, first-year devegetation
did have significant or highly significant positive effects on height
and diameter growth in each of the subsequent four years. Likewise,
treatments in 1969 and 1970 also had the effect of setting the trees on
advanced growth trajectories. Of the three single treatment schedules,
treatment in the first year was chosen as the best because it combined
good growth response (82% increase in tree volume over control in five
years) with a normally highly positive influence on tree survival.
The multiple treatment schedules demonstrated that responses
(especially of height growth) to later treatments are maximized when
preceded by treatments which promote the accumulation of photosynthetic
reserves. Treatment in the first and second years was the optimum two treatment
schedule. It resulted in a 115% increase in tree volume. The
three-treatment schedule enhanced tree volume by 217% and was estimated
to shorten a 70 year rotation by two years, compared with controls.
Tree growth responses to chemical weed control were linked quantitatively
through the latter's positive effect on available soil moisture.
This in turn was shown to have a negative effect on tree moisture
stress, which had a positive effect on tree growth.
Art economic analysis based on projected treatment-induced height
differentials at stand closure indicated that for high site II areas,
even where weed control may not be necessary to obtain adequate survival,
the first-year-only and the three-consecutive-years treatment schedules
may be justifiable on the basis of the financial return from improved
growth alone; hut only if coupled with other forest management practices
designed to maximize yield, and stumpages of the order of $150
per mbf. The economics of weed control should be even more attractive
on less favorable growing sites, especially when the higher yield and
quality caused by better stocking are taken into account.