Stand structural dynamics of boreal conifer, mixedwood, and broadleaf fire- and clearcut logging-origin stands in central Canada

Abstract

Stand structure has been linked to forest regeneration, nutrient cycling, wildlife habitat, biodiversity, and climate regulation. A review and synthesis of literature indicated that boreal forest stand structure is influence by time since stand-replacing fire (TSF), stand composition, non-stand-replacing disturbances (i.e., spruce budworm and blowdown), regional differences based on climate and surficial deposit variability, and forest management. My objectives were to examine the effects of TSF, overstory composition, and stand origin (fire versus clearcutting) on living tree and coarse woody debris (CWD) (defined as snags and downed woody debris (DWD)) dynamics. I sampled CWD in 69 stands representing typical upland conifer, mixedwood, and broadleaf stand types in the central boreal forest region of North America, ranging in age from 7 to 201 years since fire in the post-fire stands and 7 to 31 years since logging in the post-clearcut stands in northwestern Ontario, Canada. Living trees were only sampled in the 33 72 to 201 year-old fire-origin stands as most trees in younger stands did not meet my sampling criteria (diameter at breast-height (DBH) ^10 cm). Stand volume of trees differed with TSF with a U-shaped pattern in conifer stands, an inverse U-shaped pattern in broadleaf stands, but it did not change in mixedwoods. Tree density showed a U-shaped pattern with TSF for all stand types. For fire-origin stands, CWD volume showed a sigmoid pattern with stand age. The effect of stand type on CWD volume interacted significantly with stand age, showing that mixedwoods had the greatest CWD volume in 7 year-old stands, whereas conifer stands had highest CWD volume in 139 year-old stands. Contributions of snag and DWD volume to CWD volume also differed with stand age and stand type. As stand age increased from 25 to 201 years, species composition of CWD shifted from dominance by early successional tree species to dominance by later successional trees species, reflecting tree species successional trajectories. Composition of DWD decay status also varied with stand age and type. In post-clearcut stands, CWD volume was smaller, mostly noncommercial tree species, and more advanced status of decay compared to post-fire stands with similar age. Diameter at breast-height-based Shannon’s index, applied to tree and snag data from 72-201 years post fire indicated that mixedwood stands were most diverse followed by conifer and then broadleaf stands while no TSF effect was found. Height-based Shannon’s index showed that structural diversity peaked at a TSF of 139 years in broadleaf stands but was unaffected by TSF in conifer and mixedwood stands. While DBH-based coefficient of variation (CV) had a similar trend as height-based Shannon’s index, height-based CV indicated highest diversity at a TSF of 139 years regardless of stand type. The results of this study suggest that (a) forest management options for increasing structural diversity may include lengthening the harvest rotation and altering partialharvesting objectives, (b) by managing stand density in post-clearcut stands the dynamics of post-fire and post-clearcut stands may converge as stand age increases, and (c) leaving a greater proportion of coniferous residual trees may allow CWD to persist for longer in the developing stand.