Effects of thinning overstory paper birch on survival and growth of interior spruce in British Columbia: implications for reforestation policy and biodiversity.

Growth and survival responses of 8 year-old interior spruce to reductions in paper birch density from 2500 to 1000, 50 and 0 overtopping stems ha[sup -1] were examined after five years on a single site in the Interior Cedar Hemlock biogeoclimatic zone of southern British Columbia. Stem diameter increased and height: diameter ratio decreased when birch density was reduced from 2500 to 50 stems ha[sup -1], but there were no further benefits to spruce growth when the entire birch overstory was removed. Spruce growth did not improve where birch was thinned to 1000 stems ha[sup -1], but its potential for release may have been confounded by the higher density of taller conifers in that treatment. Interspecific competition between spruce and paper birch appeared to be primarily for light because spruce responded to dramatic reductions in overstory density but not to reductions in understory sprout density. Spruce survival was unaffected by thinning treatments, indicating that birch density of 2500 stems ha[sup -1] was too low to induce mortality. Armillaria root disease was the main cause of mortality, and spruce leader weevil and Cooley's spruce gall adelgid were important damaging agents, but the behaviour of these organisms was unaffected by treatment. Stand structure was dramatically changed from a mixed coniferous-deciduous to a predominantly coniferous overstory when birch was thinned to 50 or 0 stems ha[sup -1], which may have negative implications for wildlife. If balancing maximum spruce growth with biodiversity is the primary objective, then between 50 and 1000 stems ha[sup -1] birch should be retained on sites similar to the one used in our experiment. This will require modification of the current legislation governing reforestation in British Columbia. [ABSTRACT FROM AUTHOR]