Summer soil moisture dynamics in a young Douglas-fir plantation as influenced by three herbaceous weed communities Public Deposited


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  • The use of chemicals such as the triazines (especially atrazine) and mixtures of the triazines and 2,4-D or 2,4,5-T has become almost standard practice in some parts of the world for selectively controlling grasses and other herbaceous weeds to conserve moisture for and/or prevent the smothering of newly-planted conifers. In the Pacific Northwest, where application is mainly to control grasses in forest and Christmas tree plantations of Douglas-fir [Pseudotsua menziesii (Mirb.) Franco.], a single such treatment dramatically increases first year survival and growth of the trees mainly because of increased moisture availability. However, this predisposes the site for heavy infestations of forbs and forbs plus annual grasses in the second and subsequent several years. 'To what extent can these be justifiably ignored?', is a question which this dissertation attempts to answer in terms of the effects of vegetation manipulation on soil moisture. Data for this study came from observations and experimentation mainly during the summer of 1970 in a series of already existing herbicide trial plots located in a grassy meadow in the Oregon Coast Range about 18 miles west of Corvallis. The vegetative covers of the plots reflected histories of zero to three years herbicide treatment and could be classified into: (1) non- or lightly vegetated with forbs, (2) moderate to dense pure forbs, (3) deuse forbs plus annual grasses, (4) heavy bent grass (Agrostis tenuis Sibth.). An attempt was made to develop regression models to describe soil moisture use as a function of cumulative open-pan evaporation, soil depth, standing biomass of herbaceous vegetation, fresh weight of trees, root distribution of the herbaceous vegetation, and vegetation type. This was only partially successful. The models described the dynamic characteristics and interrelations of the soil moisture profiles under the four vegetation types in a general way only, and they lacked sufficient predictive accuracy to make them of practical use in their present form. Response surfaces were developed from the data which portrayed the changes in specific soil moisture content, specific soil moisture used, specific available soil moisture content, and cumulative available soil moisture content with changes in depth, cumulative open-pan evaporation, and vegetation type. They demonstrated that bent grass made heavy demands on the soil moisture in the upper profile early in the season but only moderate demands on the lower profile, a pattern consistent with its aestivating and rooting characteristics. Moderate to dense pure orbs made relatively light early season demands on the upper profile, but came on strongly later in the season with heavy moisture use at all levels in the profile. The forb/annual grass mixture was the most demanding of all. It caused heavy, early season, upper profile moisture withdrawal coupled with sustained lower profile moisture depletion. Again, this was a pattern consistent with the phenological and rooting habits of the vegetation. There was evidence that a substantial amount of water, which because of weed control was not transpired, was eventually lost, probably through increased surface evaporation and unsaturated flow. Nevertheless, although the overall effect of weed control may have been to make available to the trees only a small amount of the moisture saved from transpirational loss, this component is of major importance in the relief of tree moisture stress. Using the conventional 15 bar estimate of permanent wilting point it was shown that the average rate of descent of the permanent wilting point front was about 0.17 in and 0.14 in per day for the bent grass and mixed forb/annual grass types, respectively. The late season rate of descent under pure forbs was even faster. These are rates which the root growth of new transplants or natural seedlings cannot match under field conditions so that their roots are sooner or later deprived of access to available soil moisture. It is apparent that bent grass, forbs, and forb/annual grass mixtures represent hostile ecosystems for the establishment of Douglas-fir in areas characterised by Mediterranean type summers. Pure forbs and forb/annual grass mixtures additionally make heavy demands on the lower profile, presumably causing elevated moisture stresses, and reduced photosynthesis and growth even in well-established trees. It would therefore seem desirable (and perhaps economically justifiable) to prolong the period of complete or semi-complete herbaceous weed control in young conifer plantations being established under these conditions.
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