Fire, Logging, and Overstory Affect Understory Abundance, Diversity, and Composition in Boreal Forest

Abstract

Understory vegetation plays a critical role in boreal ecosystems. Despite this, quantitative evaluation of the factors controlling understory vegetation abundance, diversity, and composition in the most diverse boreal forest region in North America is lacking. This study examined the dynamics of understory vegetation of stands of fire origin and tested effects of overstory composition and logging vs. fire on the understory vegetation dynamics in Ontario, Canada. Understory vegetation communities were sampled in 68 stands of conifer, mixed-wood, and deciduous overstory type ranging from 7 to 201 years postfire for stands of fire origin, and from 7 to 31 years for stands of logging origin. For stands of fire origin, total cover and species richness followed similar trends for the three overstory types and were highest in the intermediate-aged stands (72-90 years). Trends in cover and richness, however, differed significantly for vascular and nonvascular plant groups. Vascular cover and species richness were generally higher under deciduous stands, and lower on older stands, while nonvascular species richness was highest under conifer stands and increased with time since fire. Neither species richness nor compositional turnover was higher under mixed-wood stands; mixed-wood stands were compositionally intermediate to conifer and deciduous stands. Multivariate analysis using multiple-response permutation procedures indicated that understory communities were compositionally distinct for all overstory types and showed no convergence with increasing time since fire. Compared with postfire stands of similar ages, post-logged stands had similar total understory cover and richness. Vascular cover and richness, however, were higher on post-logged stands, and nonvascular cover and richness were lower. Stands of logging and fire origin were compositionally distinct for all overstory types and ages. Compositional differences appeared to be driven by higher preestablished rhizomatous species and few pyrophilic species on post-logged sites. Understory vegetation communities in the central boreal shield appear to support the intermediate disturbance hypothesis. Understory richness, however, was not negatively associated with high cover values as predicted by the intermediate disturbance hypothesis. Moreover, richness appears to be highest on sites with high light availability, suggesting that boreal understory communities are influenced more by plant tolerances for low resources, rather than by competition.

Dynamic old-growth forests? A case study of boreal black spruce forest bryophytes

“…The understory of boreal forests plays important roles in ecosystem functions including nutrient cycling, water cycling, and overstory succession (for a review see Hart and Chen 2008). Bryophytes are an appropriate group with which to examine the changes in coniferous boreal forests as they dominate the forest floor (Frego 1996), and independently from vascular plants have a significant impact on ecosystem processes such as nutrient and water cycling (Gower et al 1997, Price et al 1997, DeLuca et al 2002, and are indicators of ecosystem change (Fenton and Bergeron 2006).…”

Conifer Recruitment in Trembling Aspen (Populus Tremuloides Michx.) Stands along an East-West Gradient in the Boreal Mixedwoods of Canada

“…This low proportion could be attributed to the intensification of forest management activities that have been undertaken in recent decades. Indeed, logging activity has increased in the southern boreal forest to the extent that it has become a major stand-replacing disturbance [6]. In contrast, conifer recruitment was particularly low in the central part of the gradient (corresponding to MB and SK provinces) probably due to low annual precipitation (Appendix - Figure A1a) and the aridity being higher (Appendix - Figure A1b) in the region.…”

“…In boreal mixedwood stands, aspen usually establishes immediately after a severe fire event. Over time, successional changes shift stand composition from aspen dominated to mixed species and, finally, to stands dominated by conifers [2][3][4][5][6]. Different trajectories of canopy species replacement are possible during stand development [7,8], depending on the on-site climatic and edaphic conditions, the disturbance regime, regeneration potential, biotic interactions and the life histories of the species that are present [3,9].…”

“…The fire regime, which is influenced by regional climate [11], can significantly affect the processes of species regeneration [6] and the long-term development of forest stands [12]. Forest regeneration that is present in the understory can indicate the potential successional pathway that forest stand composition could take during its evolution [13].…”

“…BSP); and balsam fir or Bf (Abies balsamea (L.) Mill.). These late-successional species are commonly found in association with aspen in boreal mixedwood stands [6,14,20,33]. We only target aspen stands along the study gradient, thus controlling for the effect of forest cover.…”