Long-term outcome of precommercial thinning on floristic diversity in north western New Brunswick, Canada

The Green River spacing trials were established between 1959 and 1961 to study the long-term growth and development of balsam fir (Abies balsamea (L.) Mill.) and red spruce (Picea rubens Sarg.) in response to precommercial thinning (PCT). Three nominal spacings (1.2 m, 1.8 m, 2.4 m) and an unthinned control were applied in a randomized complete block design with 5 replicates to regenerating stands, an average of 8 years after harvest. Our study examines floristic diversity associated with these treatments approximately four decades later. Floristic diversity was assessed with several alpha diversity indices as well as multivariate analysis to compare community composition. Specific a-priori contrasts compared plant diversity among a) control and average of the wider spacings (1.8 m and 2.4 m), b) control and the narrowest spacing (1.2 m), and c) the narrowest spacing and the widest spacing. Our results indicate that there were no appreciable differences among the treatments across all measures of plant diversity investigated. As such, we conclude that the forest understory, as represented by the unthinned plots, was analogous in the thinned plots at time of stand maturity. Vegetation response to PCT treatments is inconsistent in the published literature, but this can be attributed to differences in thinning intensities, recovery age or the type of forest ecosystem studied. We conclude that PCT is a variable silvicultural tool that could be used to attain both economic productivity and biodiversity conservation goals.