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The Douglas-fir forest associations on Vancouver Island in their initial stages of secondary succession Mueller-Dombois, Dieter

Abstract

A study of the cut-over and immature forest associations in the Douglas-fir zone on eastern Vancouver Island was carried out as a continuation of earlier investigations (Krajina and Spilsbury 1950, Krajina 1952, Szczawinski 1953, McMinn 1957) of the associations in mature stands of Douglas-fir (Pseudotsuga menziesii). Information of the environmental characteristics of the associations has been extended and their vegetational aspects in the early stages of secondary succession after logging operations are described and evaluated. It is pointed out that a set of physiographic positions is typical for each association. However, similar physiographic positions may be occupied by different associations depending upon the configuration of the soil. The soils vary with type of parent material, geographical location and physiographic position within each association. In spite of this heterogeneity it is often possible to recognize a certain association through the soil, because of common pedological characteristics related to soil genesis, moisture regime and rhizosphere characteristics. Trends and some significant site correlations are demonstrated between soil reaction and humus horizon and soil reaction and lower solum, as well as between per cent organic matter of the humus and upper solum of the association type soils. A few representative samples from soil profiles preserved as monoliths give indications of a positive trend of replaceable calcium and magnesium with the B horizons of the association type soils, increasing along the moisture gradient of the ecological series of association from dry to wet. Forest stand statistics of the old-growth as well as new stands have been compared with conventional yield table values for Douglas-fir. Typical trends reflecting site quality are shown. The pattern of forest vegetation that has been used as a key for the separation of biologically equivalent localities among virgin stands of Douglas-fir can also be utilized on cut-over land after logging operations, since the remnant vegetation largely retains its diagnostic value in regard to ecological conditions. However, the moss association loses its original floristic features and is invaded by the more dominant and aggressive elements of the related salal and sword fern associations resulting in a floristic “hybrid" between the two. This may cause difficulties in a site appraisal by purely floristic measures, and the use of the described typical physiographic positions and megascopic edaphic characteristics in combination with the residual vegetation pattern for practical purposes of site identification is recommended. Depending upon the immediate history, whether an area has been logged only, or logged and burnt, the more prevalent remnant vegetation is reduced in relative dominance, but rarely destroyed. Hence, changes among the original vegetation are largely structural rather than compositional. However, considerable compositional changes in the cut-over plant communities as a whole are brought about by the influx of the local weed vegetation, whose relative dominance and masking effect over the residual vegetation is indicative of the kind, degree and time of disturbance as well as of the associations. But, their diagnostic value in regard to ecological criteria is generally low. Therefore, if the two major groups of vegetation that comprise the cut-over plant communities — the remnant virgin vegetation and the new weed vegetation — are understood as to the relative roles they play in the early secondary succession, the population pattern of the original and more permanent vegetation can still be usefully employed as a guide to the recognition of the various forest associations after they have been logged, with or without subsequent slash burning.

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