Effects of Intensification of Silviculture on Plant Diversity in Northern Temperate and Boreal Forests of Ontario, Canada

Abstract

This thesis is an investigation of the intensification of silviculture has long presented a conundrum to forest managers in the northern temperate and boreal forests of Ontario, Canada aiming to sustainably produce wood fibre and conserve biodiversity. Although intensive silviculture has long been viewed as a threat to biodiversity, it is also considered as a means to increase wood fibre production. In this thesis I determine the nature of the biodiversity–silviculture intensity relationships and improve our understanding of the mechanisms that underpin these relationships. More specifically, studied (i) compositional and functional biodiversity-silviculture intensity relationships, (ii) the relative influence of silviculture on species richness, (iii) effects of intensification of silviculture on functional response groups, and (iv) susceptibility to invasion. My results are based on fifth-year post-harvest NEBIE plot network data. Initiated in 2001, the NEBIE plot network is the only study in North America that provides a gradient of silviculture intensities in northern temperate and boreal forests. The NEBIE plot network is located in northern temperate and boreal forests near North Bay, Petawawa, Dryden, Timmins, Kapuskasing, and Sioux Lookout, Ontario, Canada. Data collections included plant diversity, soils, forest canopy structure, and forest floor structure. Response surface analyses, multiple regression, structural equation modelling, ordination, and fourth-corner analyses were used to analyze the data. Multiple response patterns, rather than a single pattern such as predicted by monoculture, intermediate disturbance or gradually reducing hypotheses, were observed. The effects of silviculture were marginal relative to climate, soils and biotic interactions. While the effects of silviculture systems were hierarchically associated with climate, soils, and historic fire regimes, the effects of silviculture intensity were independent of these factors. Fifty-six plant functional response groups (PFRGS) were formed based on specie’s individual persistence, competitive ability, propagule persistence, and landscape dispersal. These PFRGs were classified into 20 unique response patterns based on expected levels of abundance across the NEBIE gradient. The PFRGs associated with sensitive and recalcitrant species were observed in upwards of seven unique response patterns. The highest richness of exotic species was associated with basic silviculture. The propagule pressure and abiotic and biotic tolerance hypothesis provides a reasonable explanation for invasibility of northern forests.