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Water relations in the Douglas-fir region on Vancouver Island McMinn, Robert Gordon

Abstract

In an evaluation of the role of water relations in forest distribution and growth in the Douglas-fir region on Vancouver Island, a section of the Nanaimo River Valley, lying from five to twelve miles inland from the east coast, was chosen as a suitable study area to exemplify conditions in the central mountains and on the eastern side of southern Vancouver Island. The study was conducted in mature stands typical of the principal forest associations found within the range of climates, topographies and soils represented in this area. The vegetation and soils of twenty-four, quarter-acre plots were analysed in order to characterize stands and relate such characteristics to the influence of water relations. The soil moisture regimes and microclimates of each plot were defined by measuring soil moisture levels, precipitation beneath the tree canopy, evaporation rates and soil and air temperatures. Variation in soil moisture contents was followed over a thirty-month period from July, 1951 to November, 1953, and precipitation and maximum/minimum temperatures at various open stations were measured at monthly intervals from June, 1951 until December, 1956 in order to delineate climatic variations within the study area. It was concluded that variation in soil moisture regimes was a most significant factor in the differentiation of sites. In moist, relatively nutritive soils Pseudotsuga menziesii so completely dominated Tsuga heterophylla that the latter species was restricted to the secondary canopy and formed only a small proportion of stand volume. In strongly leached soils, the growth of Pseudotsuga and Tsuga was impaired, so that trees of both species were smaller than in more nutritive soils. Where strongly leached soils were moist throughout the growing season, Tsuga could compete with Pseudotsuga on nearly equal terms and both species reached the upper tree canopy. In droughty, leached soils the growth of Tsuga was more impaired than the growth of Pseudotsuga. and Tsuga formed small to negligible proportions of stand volume. Thuja plicata appeared in the upper tree canopy only on moist to very wet, relatively nutritive sites. The dominant influence in the geographical location of the Douglas-fir region on Vancouver Island appears to be its rainshadow climate. Within this region of low summer rainfall Pseudotsuga can dominate other species on nearly all sites. In wetter climates, outside pronounced rainshadow areas, stands dominated by Pseudotsuga are evidently confined to moist, relatively nutritive sites. Within the different climatic areas encountered, topographic position and concomitant climatic and edaphic influences would seem largely responsible for site differentiation. Such site differentiation followed a definite catenary sequence, which regulated the sequence of forest associations on hillsides. Differences in local topography, aspect, parent material, soil texture and soil depth, however, may cause variation from the typical arrangements.

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