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Investigation into the productivity of single- and mixed-species, second-growth stands of western hemlock and western redcedar Collins, D. Bradley

Abstract

In order to evaluate if mixed-species stands of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and western redcedar (Thujaplicata Donn ex D . Don) were more productive than single-species stands of either species, I: (1) reviewed the theory of positive plant interactions and integrated it with the silvical characteristics of hemlock and redcedar to determine if positive interactions are feasible among these species, (2) located even aged, unmanaged, second growth, single- and mixed-species stands of hemlock and redcedar on intermediate (fresh and nutrient medium) sites, within the perhumid mesofhermal climate of southern coastal British Columbia, (3) compared the productivity of each stand type using relative and absolute yield comparisons, and (4) evaluated the mixed-species stands to determine if increasing productivity was associated with physical separation of the species, as suggested by the findings in (1). Theory on positive plant interactions logically divides the mechanisms of these interactions into two components: competitive reduction, and facilitation. When applied to existing mixed-species stands of hemlock and redcedar, the mechanisms of competitive reduction and facilitation suggest that hemlock and redcedar may experience positive interactions through: (1) vertical canopy separation and a random spatial pattern of trees, and (2) preferential uptake of different nitrogen forms. The relative yield of hemlock and redcedar was lower in mixture, as compared to singlespecies stands, due to the effects of competition with each other and non-study species. Mean annual increment and wood volume production increased with increasing presence of hemlock. Basal area increased with increasing presence of redcedar, and the redcedar stand type was also the densest. Redcedar trees in the redcedar stand type were similar in height, diameter, and mean annual increment, relative to those in the hemlock-redcedar stand type. Hemlock trees in the hemlock stand type were taller, had larger diameters, and had higher mean annual increments, than those in the hemlock-redcedar stand type. These differences in hemlock and redcedar growth in each stand type are thought to be responsible for the differences in mean productivity detected among stand types. In the mixed-species stands, increasing productivity was associated with increasing vertical separation of hemlock and redcedar canopies, where the area of hemlock canopy overtopping that of redcedar, and crown depths of both species, were maximized. Increased productivity was not associated with increasing levels of randomness in stand spatial tree patterns, but was associated with decreasing stand density, and a tendency towards regularity in the stand spatial tree patterns. Maximum productivity in fully stocked hemlock-redcedar mixtures can be attained with low density stands in which hemlock does not experience an establishment lag. Given the choice between single- and mixed-species stands of hemlock and redcedar, hemlock stands should be established if maximum wood production is the management objective. If maintaining a mixture, or improving site nutrient status, is the management objective, then hemlock-redcedar stands should be established. Maximum wood production in a hemlock-redcedar stand will be achieved where stands are fully stocked, but established at low numbers of stems per hectare. These recommendations are for intermediate (blueberry) sites in the Submontane Very Wet Maritime Coastal Western Hemlock (CWHvml) variant, for stands less than 80 years old.

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