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Study of litterfall and forest floor accumulation in the spacing plantations of Douglas fir at the University of British Columbia Research Forest Woon, Chio-Yio

Abstract

Litterfall in the UBG Research Forest was collected from traps randomly-placed in each of five half-acre plots of twelve year old Coastal Douglas fir of spacings ranging from 3 x 3 to 15 x 15 feet. Collections were also made from a half-acre plot of 3 x 3 feet spaced eleven year old western hemlock, and from an older, thinned natural stand of western red cedar and western hemlock. Forest floor accumulations and soil samples from each plot were also collected and analysed. The amount of litterfall in the Douglas fir spacing plots showed that the denser the stand the greater was the litterfall. The percentage of litter from broadleaved species present was in increasing order from the 3 x 3 to the 15 x 15 feet Douglas fir plots. The amount of litterfall in the Douglas fir was about 3,000 kg/ha more than that in the western hemlock plot of the same spacing. The litterfall in the cedar-hemlock stand showed a large amount of cones and twigs, but was not greater than the litterfall collected from the denser plantations of Douglas fir. It should be noted that the 3x3 and 6x6 feet Douglas fir plots of this study gave a much greater litterfall than the values reported by other authors for this species. However, judging from the literature, comparable data for stands of this age and density are not available. Since the variation of litterfall within a plot was very great, the number of traps needed will have to be increased to attain a precision of ± 10 gm at 5% probability level for an annual collection. The amount of forest floor decreased as spacing widened. It was shown statistically that two groups of plots existed: one with associated vegetation (12 x 12 and 15 x 15 feet) and the other without (3x3 and 6x6 feet plots). The weight of forest floor of western hemlock was less than that of Douglas fir of the same spacing. The weights of forest floor obtained in this study were well below the figures given by other authors in this field. The index of forest floor turnover, calculated from the ratio of litter-fall/forest floor showed that the closer-spaced plantations had a higher ratio than the wider-spaced. This suggests that faster circulation of nutrient elements is going on in the stand with a full canopy than in a more widely spaced stand. The chemical contents of the litterfall and forest floor were determined on composite samples from each plot. Nutrient concentration in litterfall and forest floor did not vary with spacing in the Douglas fir plots. The ground vegetation layer found only under the wider Douglas fir spacings showed greater concentration of phosphorus and potassium than the litter or forest floor of all the spacings. The calcium concentration in the western hemlock and cedar-hemlock plots was higher than that found in the Douglas fir plots, but phosphorus was found to be lower. However, the total amount of nutrients in litterfall and in forest floor under differently spaced Douglas fir plantations followed a distinct pattern: a higher content was present in the denser and lower in the wider plots. No clear relationship between litterfall or forest floor accumulations and growth as measured so far was observed. The importance of soil physical properties in affecting growth should be considered more closely, because the plots were different in soil texture and probably in other physical characteristics.

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