Graduate Thesis Or Dissertation
 

The influence of seral Coast Range vegetation on the growth habit of juvenile Douglas-fir

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  • A detailed analysis was performed on the juvenile growth of six types of Douglas-fir stock under the influence of typical seral Coast Range vegetation. Objectives were to analyze competitive influences important to the growth of tree seedlings and evaluate differences between stock type characteristics and growth patterns in response to competition of several kinds. Results were also compared as far as possible with an earlier analysis of the same trees to investigate the changing competitive status of Douglas-fir of various sizes as their competitors grow. Initial seedling height was consistently important in explaining variation in the seven-year growth of tree seedlings associated with a variety of vegetation groups, regardless of stock type. Correlations of growth with stock type and site location were also significant, although the importance of stock type decreased with the increasing age of the trees, while the importance of site increased. In addition, there was more unexplained growth variation at seven years than at five years indicating that factors not included in regression models, such as microsite variation and/or increasing size of trees, are becoming increasingly more important in determining seedling growth. At six and seven years, vegetation on cutover sites explained only a limited portion of the variation in current growth. Vegetation effects on tree seedlings appear to be cumulative however, with past competitive stress highly correlated with the present status of tree seedlings. In addition, after four or five field seasons, the competitive status of juvenile tree seedlings has already been determined and there is little tendency for change either to a more favorable or less favorable competitive environment without interference. Seral vegetation is more competitive and hence has a greater influence on tree seedlings if it overtops them, as with tall hardwoods and sprouts which reduce available light, than when it encroaches on them from the side or below as with low shrubs and herbs. The advantages of using larger stock types in mesic to moist Coastal brushy areas are still obvious at seven years, even though seedling growth at this time is not significantly different between stock types, vegetation groups or sites. Larger seedlings maintain their initial height. advantage, are able to become 'established' (reach 4-1/2 ft.) up to three years before smaller seedlings and have higher survival rates than smaller seedlings. In addition, larger stock types are less susceptible to adverse site influences and damage than smaller stock types, at seven years. Intensive forest management is dependent upon reforestation success. Options which enhance the survival and growth of crop trees in the early years of a rotation allow for increased and more dependable yields in subsequent years. The use of larger stock types on brushy Coast Range sites is an effective alternative for meeting these reforestation goals.
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