Effects of herbaceous weed control on young Douglas-fir moisture stress and growth

Abstract

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 stem analysis. 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 inadequacies. 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.