Slow degradation of compostable plastic carrier bags in a stream and its riparian area

There is no place on Earth where plastic debris could not be found. Impacts of plastics on aesthetics, biota and ecosystems are dependent on how long plastic items last, and what degradation products are released, in recipient environments. As bio-based plastics tend to replace petroleum-based plastics in everyday life, it is important to upgrade knowledge on the degradation of new polymers in natural environments. Single-use plastic carrier bags are nowadays made of bio-plastics certified as biodegradable and compostable. It is unclear, however, whether claims of biodegradability and compostability can be taken as evidence of rapid degradation of plastic bags outside recycling/composting facilities. This study sought to provide quantified information about the degradation of compostable plastic carrier bags in streams and riparian zones. We found that plastic samples enclosed in different types of mesh bags lost weight at extremely slow rates, albeit significant when submerged in a stream. 95% of initial plastic mass remained after 77 days spent in water whereas alder leaf litter allowed to decompose under the same condition had completely disappeared before the end of the study. Determination of respiration rate and invertebrate abundance in plastic samples showed a greater decomposer activity in the stream than in the riparian environment. However, biotically-mediated degradation by decomposers was probably overridden by dissolution processes in mediating plastic mass loss. Our findings suggest that mismanaged plastic carrier bags could impact recipient ecosystems even when they are claimed as biodegradable or compostable.

Fire spread probabilities for experimental beds composed of mixedwood boreal forest fuels

Although fuel characteristics are assumed to have an important impact on fire regimes through their effects on extinction dynamics, limited capabilities exist for predicting whether a fire will spread in mixedwood boreal forest surface fuels. To improve predictive capabilities, we conducted 347 no-wind, laboratory test burns in surface fuels collected from the mixedwood boreal forest of Saskatchewan. The beds were composed of single fuel types of contrasting characteristics, including feather moss, aspen leaf litter, aspen and alder leaf litter, and twigs. Shredded wood (i.e., excelsior) was included for comparison. An extinction index and logistic model from the literature that balances heat sources and sinks performed well for excelsior, a fuel used to develop the model, but poorly for forest fuels. As a result, we used logistic regression to develop a model for forest fuels finding that fire spread was largely determined by the heat sink, heat of combustion, and fuel bed depth. We found close correspondence between our model and fire spread in an independent sample of beds composed of mixtures of mixedwood fuels (N = 59). Our model can serve as a means of analyzing the relative importance of fuels and weather on extinction dynamics during mixedwood boreal forest fires.

The influence of mountain alder on the growth, nutrition, and survival of black spruce and Sitka spruce in an afforested heathland near Mobile, Newfoundland

The natural invasion of mountain alder (Alnuscrispa (Ait.) Pursh) into monoculture plantations of black spruce (Piceamariana (Mill.) B.S.P.) and Sitka spruce (Piceasitchensis (Bong.) Carr.) afforded an opportunity to assess the effects of mountain alder on soils, and on the growth, nutrient status, and survival of spruce in eastern Newfoundland. The plantations were established on ploughed Kalmia–Vaccinium heath in 1968–1969. Significant increases in the growth of spruce were associated with the presence of high densities of mountain alder in the two plantations studied. The increases began 3–6 years after the invasion and establishment of mountain alder. Estimated nitrogen (N) contributions to each plantation from mountain alder leaf litter fall were approximately 50 kg•ha−1•year−1. Spruce foliage N content was 10–15% higher in high-density mountain alder (HD-alder) plots than in low-density mountain alder (LD-alder) plots, but other foliar nutrient concentrations were low and possibly deficient in the HD-alder plots. Development of a litter fall–humus layer was observed in HD-alder plots but not in LD-alder plots; however, B-horizon soil-N values did not differ significantly between HD-alder and LD-alder plots. The mountain alder invasion did not reduce the densities (stems/ha) of spruce. Additional studies on the autecology of Alnus spp. native to Newfoundland, and on alder-spruce interaction are recommended to further the development of an adequate ecological basis for heathland and peatland afforestation in the region.