Tree community structural development in young boreal forests: A comparison of fire and harvesting disturbance

Although natural disturbance-based management systems support the use of clearcut harvesting as a means of emulating fire disturbance in boreal forest, there are continual concerns and uncertainties over the degree to which clearcutting alters forest ecosystem dynamics relative to wildfire. Here, we sampled 56 and 47 post-fire and post-harvest stands, respectively, to examine early tree community structural development in young boreal mixedwood forests in central Canada. We find that tree species diversity is higher immediately following clearcut harvesting compared with stand-replacing crown fire. We attribute this primarily to protection of advanced regeneration, suitable regeneration substrates, and propagule availability of shade-tolerant conifer species following harvesting compared with fire. Although both disturbances promoted early successional species, harvesting resulted in higher regeneration densities of late-successional conifers (Picea mariana (Mill.) and Abies balsamea (L.) Mill.). Vertical height structure variation was higher after fire than harvesting, likely a result of spatial heterogeneity in burn severity. Although tree heights, corrected for stand age, did not differ between disturbances for Populus tremuloides Michx. and Pinus banksiana Lamb., they were shorter for Betula papyrifera Marsh. and taller for late-successional conifers following harvest. Higher tree species richness, regeneration densities, and tree heights of late-successional conifer species indicate that clearcut harvesting can potentially accelerate succession in boreal mixedwood forests; however, harvesting can reduce height structural variability. Furthermore, leaving Betula papyrifera to satisfy the policy requirement for green tree retention can negatively affect its post-harvest regeneration and growth.