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A least squares analysis of inventory data to compare yields of pure and mixed stands in British Columbia forest zones Yang, Richard C.


The author developed a statistical procedure to analyze irregular, unbalanced inventory data by the least squares principle. The method is found useful in forestry where data collected are often unbalanced in nature. It provides a unique means to incorporate qualitative as well as quantitative variables in forest yield analyses. Inventory data for three major species — Douglas-fir, spruce, and lodgepole pine were analyzed in connection with the study of growth and yield of pure (81% or more of the overstory is of a single species) and mixed stands in up to 12 B.C. forest inventory zones in which they occurred. More than 50% of Douglas-fir, spruce, and lodgepole pine stands occur naturally in pure stands. If there are any adverse effects on the establishment of pure stands, these should have been well reflected quantitatively in the data provided by the B.C. Forest Service. Estimates of site index from the inventory data might support that pure stands deteriorate soil conditions; however, the higher site indices in mixed stands may be attributed to the better site conditions when the stands were originally established. Mixed conifer stands tend to grow more trees per acre than pure or hardwood mixed type stands. Among the three species investigated, Douglas-fir required more growing space than the others. The relative stand density based on basal area per acre also indicates that stand density is higher in conifer mixed stands than in pure or hardwood mixed type stands. The mean annual increment is higher in hardwood mixed stands than in pure or conifer mixed ones. But stand age in hardwood mixed types is much less. The mean annual basal area increment of conifer mixed stands is consistently higher than that of the other two types. Zonal variations in the mean annual basal area growth are apparent. The mean annual volume increment follows a trend similar to that of the mean annual basal area increment. Douglas-fir stands growing on the Coast and in the Interior were compared. Mean annual volume growth is 84.00 cubic feet per acre for the Coast stands and 25.53 cubic feet for the Interior stands. The effect of species composition on net volume yield is significant in Interior Douglas-fir stands, but non-significant in Coast Douglas-fir, Interior spruce, and Interior lodgepole pine stands. That the effect of forest inventory zones is highly significant in the Interior Douglas-fir, spruce, and lodgepole pine stands justifies the zonation unless adjustments are made for stand density. Interactions for types and zones are significant in the Coast Douglas-fir, the Interior spruce, and the Interior lodgepole pine stands but are nonsignificant in the Interior Douglas-fir stands. The difference in yield in the Interior Douglas-fir stands is attributable to species composition types and forest inventory zones alone. The establishment of Douglas-fir conifer mixed type stands in the Interior effectively increases forest productivity by 21%. Interpretations of the interactions lead to the conclusion that the advantages of monocultural or multicultural practices cannot be over-generalized. Pure type stands are more productive in some zones but less in the others. The same is true for multicultural practices. Growth of forest trees is essentially site-dependent. Before a decision is reached on what species composition type to establish,, foresters should carefully investigate the local site quality and past yield history of various forest types to ensure that the maximum potential productivity of a particular site can be realized. Further analyses to test the hypothesis that no differences in volume yield exist among three species composition types for stands growing on same site conditions reveal that the effects for species types and inventory zones as well as interactions thereof are not significant for Coast Douglas-fir, however, for Interior Douglas-fir stands, the effect of species composition is significant. It is shown that on similar site conditions, Douglas-fir conifer mixed stands yield substantially more than pure or hardwood mixed stands in the Interior. The species composition effect is not significant in Interior spruce stands while zonal effects and interactions for types and zones are significant. In Interior lodgepole pine stands, effects of composition types, zones, and interactions thereof differ significantly. In all three species groups investigated, that the effects of hardwood mixed type consistently shows negative 'values implies that hardwood mixed type stands are the least desirable stand composition structure for these species in the Interior. Differences in volume between pure and mixed type stands result primarily from the inequality in basal area per acre. The variables, height x basal area and basal area are most important in yield table analyses. In addition, stand age, relative basal area, and forest inventory zone are all highly significant in contributing to the variations in volume yield of the Coast Douglas-fir stands. For Interior Douglas-fir, the most significant variables are, in addition to the above two variables, stand age, and relative stand density. Effects of species composition type and forest inventory zones are non-significant. For Interior spruce, the prominent variables in yield table analysis are height x basal area, basal area, species composition, stand age, height, and relative stand density. All variables being equal, pure spruce stands outyield stands of mixed spruce-hardwood and mixed spruce-conifer. The results provide good evidence that establishment of pure spruce stands is more desirable than of spruce and conifers or hardwood mixed stands. For lodgepole pine, the most significant variables in yield tables analysis are height x basal area, species composition types, and forest inventory zones. The high significance of zonal effects suggests that a separate yield table for lodgepole pine in each zone is warranted, unless appropriate adjustments are made for site index and stand density. Yield of pure lodgepole pine stands exceeds those of lodgepole pine conifer mixed type and lodgepole pine hardwood mixed types. Therefore, for high yields the establishment of pure lodgepole pine type stands is preferred. Application of these methods to the temporary sample plot data has clearly demonstrated the widespread distribution of pure stands and lack of substantial effects of monocultures on yield. Nevertheless, the fact that higher yields may result from some multicultures should encourage establishment of long term studies of spacing and mixtures of species.

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