Stand age structural dynamics of conifer, mixedwood, and hardwood stands in the boreal forest of central Canada

To study the effects of stand development and overstory composition on stand age structure, we sampled 32 stands representing conifer, mixedwood, and hardwood stand types, ranging in ages from 72 to 201 years on upland mesic sites in northwestern Ontario. We defined the stages of stand development as: stem exclusion/canopy transition, canopy transition, canopy transition/gap dynamics, and gap dynamics. Stand age structure of conifer stands changed from bimodal, bimodal, reverse-J, and bimodal, respectively, through the stages of stand development. Mixedwood and hardwood stands revealed similar trends, with the exception of missing the canopy transition/gap dynamic stage in mixedwoods. Canopy transition/gap dynamic stage in hardwoods showed a weaker reverse-J distribution than their conifer counterparts. The results suggest that forest management activities such as partial and selection harvesting and seed-tree systems may diversify standard landscape-level age structures and benefit wildlife, hasten the onset of old-growth, and create desired stand age structures. We also recommend that the determination of old-growth using the following criteria in the boreal forest: 1) canopy breakdown of pioneering cohort is complete and stand is dominated by later successional tree species, and 2) stand age structure is bimodal, with dominating canopy trees that fall within a relatively narrow range of age and height classes and a significant amount of understory regeneration.

Spatial Distribution and Seasonal Variations of Heavy Metal Contamination in Surface Waters of Liaohe River, Northeast China

Heavy metal pollutants are a worldwide concern due to slow decomposition, biocondensation, and negative effects on human health. We investigated seasonal and spatial variations of the five heavy metals and evaluated their health risk in the Liaohe River, Northeast China. A total of 324 surface water samples collected from 2009 to 2010 were analyzed. Levels(high to low) of heavy metals in the Liaohe River were: zinc(Zn) > chromium(Cr) > copper(Cu) > cadmium(Cd) > mercury(Hg). Spatial and seasonal changes impacting concentrations of Cu and Zn were significant, but not significant for Cr, Cd and Hg. The highest concentrations of heavy metals were: Hg at Liuheqiao, Cu at Fudedian, Zn at Tongjiangkou, Cr at Mahushan, and Cd at Shenglitang. The highest concentrations of Hg and Cr were found in the wet period, Cu and Cd in the level period, and Zn in the dry period. The surface water of a tributary was an important accumulation site for heavy metals. Health risks from carcinogens and non-carcinogens increased from upstream to downstream in the mainstream of the Liaohe River. The total health risk for one person in the Liaohe River exceeded acceptable levels. The total health risk was the greatest during the wet period and least in the dry period. Among the five heavy metals in the Liaohe River, Cr posed the greatest single health risk.

Cryosphere as a temporal sink and source of microplastics in the Arcticregion

Microplastics(MPs)pollution has become a serious environmental issue of growing global concern due to the increasing plastic production and usage.Under climate warming,the cryosphere,defined as the part of Earth’s layer characterized by the low temperatures and the presence of frozen water,has been experiencing significant changes.The Arctic cryosphere(e.g.,sea ice,snow cover,Greenland ice sheet,permafrost)can store and release pollutants into environments,making Arctic an important temporal sink and source of MPs.Here,we summarized the distributions of MPs in Arctic snow,sea ice,seawater,rivers,and sediments,to illustrate their potential sources,transport pathways,storage and release,and possible effects in this sentinel region.Items concentrations of MPs in snow and ice varied about 1-6 orders of magnitude in different regions,which were mostly attributed to the different sampling and measurement methods,and potential sources of MPs.MPs concentrations from Arctic seawater,river/lake water,and sediments also fluctuated largely,ranging from several items of per unit to>40,000 items m^(-3),100 items m^(-3),and 10,000 items kg^(-1) dw,respectively.Arctic land snow cover can be a temporal storage of MPs,with MPs deposition flux of about(4.9-14.26)×10^(8) items km^(-2) yr^(-1).MPs transported by rivers to Arctic ocean was estimated to be approximately 8-48 ton/yr,with discharge flux of MPs at about(1.65-9.35)×10^(8) items/s.Average storage of MPs in sea ice was estimated to be about 6.1×10^(18) items,with annual release of about 5.1×10^(18) items.Atmospheric transport of MPs from long-distance terrestrial sources contributed significantly to MPs deposition in Arctic land snow cover,sea ice and oceanic surface waters.Arctic Great Rivers can flow MPs into the Arctic Ocean.Sea ice can temporally store,transport and then release MPs in the surrounded environment.Ocean currents from the Atlantic brought high concentrations of MPs into the Arctic.However,there existed large uncertainties of estimation on the storage and release of MPs in Arctic cryosphere owing to the hypothesis of average MPs concentrations.Meanwhile,representatives of MPs data across the large Arctic region should be mutually verified with in situ observations and modeling.Therefore,we suggested that systematic monitoring MPs in the Arctic cryosphere,potential threats on Arctic ecosystems,and the carbon cycle under increasing Arctic warming,are urgently needed to be studied in future.