No 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.

NCT00084136.

Antiretroviral drug resistance is relatively common among recently infected men who have sex with men in the United States. CXCR4-using strains were detected in a small number of these infections, which were all subtype B HIV-1.

Summary1. Charcoal plays an important role in soil function and carbon storage in fire-prone ecosystems. Charcoal is present in most boreal forest soils as a result of naturally recurring wildfires, which convert 0Á7-2% of biomass to charcoal. In boreal forests, charcoal represents 8-10% of soil carbon and 1 pg of carbon globally. 2. Charcoal is resistant to decay, representing a form of super-passive carbon, with half-lives one to two orders of magnitude greater than those of other soil organic matter. High concentrations of negative surface charges increase nutrient retention, impacting boreal soil function, productivity and species composition. 3. Due to a lack of soil mixing processes, charcoal in boreal soils is vulnerable to recombustion in recurring fires, inhibiting the accumulation of charcoal over time, unlike in other fire-prone ecosystems. 4. Boreal charcoal stocks are highly variable. Increased fire intensity results in greater charcoal formation, with stand-replacing crown fires resulting in much larger charcoal stocks than non-stand-replacing ground fires. Current estimates of carbon storage based on Scandinavian studies of non-stand-replacing fires may underestimate charcoal stocks by factors of 2-3. 5. Synthesis and applications. Charcoal contributes to boreal soil function, ecosystem productivity, nutrient retention and carbon cycling. In the absence of fire, charcoal loses many active properties, contributing to declining productivity with increasing time since fire. Incorporation of charcoal into ecosystem management using prescribed burns may contribute to sustainable management of boreal forests and maintaining global carbon cycles.