Decline of an Ecotone Forest: 50 Years of Demography in the Southern Boreal Forest

Variation in tree recruitment, mortality, and growth can alter forest community composition and structure. Because tree recruitment and mortality events are generally infrequent, long‐time scales are needed to confirm trends in forests. We performed a 50‐yr demographic census of a forest plot located on the southern edge of the Canadian boreal forest, a region currently experiencing forest die‐back in response to direct and indirect effects of recent severe droughts. Here, we show that over the last 30 yr biomass, basal area, growth, and recruitment have decreased along with a precipitous rise in mortality across the dominant tree species. The stand experienced periods of drought in combination with multiple outbreaks of forest tent caterpillar (Malacosoma disstria) and bark beetles. These insect disturbances interacted to increase mortality rates within the stand and decrease stand density. The interaction of endogenous and exogenous factors may shift forests in this region onto novel successional trajectories with the possibility of changes in regional vegetation type

Size-Resolved Fluorescence Underscores Negligible Interaction of Dissolved Organic Matter During Conservative Mixing in a Large Boreal River

Although river mixing occurs widely in nature, the corresponding evolution of dissolved organic matter (DOM) composition remains poorly understood. Here, surface water samples were collected at multiple transects in the lower Athabasca River (LAR) under base-flow conditions. Asymmetric flow field-flow fractionation (AF4) coupled to online excitation-emission measurements (EEMs) and parallel factor analysis (PARAFAC) were utilized to investigate the size distribution of fluorescent DOM components during river mixing and the corresponding variation in size-resolved fluorescence. The majority of fluorescent components occurred at 0.810 and 1.170 kDa, reflecting the small size of the DOM molecules with maximum fluorescence. The loadings of fluorescence normalized to absorbance at 254 nm (A254) were highest for most terrestrial humic-like components, followed by the microbial humic-like component, and the protein-like components. Differences in size-resolved fluorescence were observed between DOM in humic-rich tributaries and in the mainstem of the LAR upstream of tributary inputs. The trend of variations in the A254-normalized PARAFAC loadings of terrestrial humic-like components also illustrates conservative mixing of aromatic-rich terrestrial DOM across size fractions in the LAR. From a molecular point of view, the mixing of fluorescent DOM occurred linearly and simultaneously across sizes without any evidence of aggregation, sedimentation, or changes in the fluorescence or concentration of any size fraction over the >60 km required for complete mixing of the river and its tributaries. Overall, this study provides insights into the size characteristics of fluorescent components of DOM and their conservative mixing behavior in large boreal rivers

Peat Bogs Document Decades of Declining Atmospheric Contamination by Trace Metals in the Athabasca Bituminous Sands Region

Peat cores were collected from five bogs in the vicinity of open pit mines and upgraders of the Athabasca Bituminous Sands, the largest reservoir of bitumen in the world. Frozen cores were sectioned into 1 cm slices, and trace metals determined in the ultraclean SWAMP lab using ICP-QMS. The uppermost sections of the cores were age-dated with ²¹⁰Pb using ultralow background gamma spectrometry, and selected plant macrofossils dated using ¹⁴C. At each site, trace metal concentrations as well as enrichment factors (calculated relative to the corresponding element/Th ratio of the Upper Continental Crust) reveal maximum values 10 to 40 cm below the surface which shows that the zenith of atmospheric contamination occurred in the past. The age-depth relationships show that atmospheric contamination by trace metals (Ag, Cd, Sb, Tl, but also V, Ni, and Mo which are enriched in bitumen) has been declining in northern Alberta for decades. In fact, the greatest contemporary enrichments of Ag, Cd, Sb, and Tl (in the top layers of the peat cores) are found at the control site (Utikuma) which is 264 km SW, suggesting that long-range atmospheric transport from other sources must be duly considered in any source assessment

Iron Regulation of the Major Virulence Factors in the AIDS-Associated Pathogen Cryptococcus neoformans

Iron overload is known to exacerbate many infectious diseases, and conversely, iron withholding is an important defense strategy for mammalian hosts. Iron is a critical cue for Cryptococcus neoformans because the fungus senses iron to regulate elaboration of the polysaccharide capsule that is the major virulence factor during infection. Excess iron exacerbates experimental cryptococcosis and the prevalence of this disease in Sub-Saharan Africa has been associated with nutritional and genetic aspects of iron loading in the background of the HIV/AIDS epidemic. We demonstrate that the iron-responsive transcription factor Cir1 in Cr. neoformans controls the regulon of genes for iron acquisition such that cir1 mutants are “blind” to changes in external iron levels. Cir1 also controls the known major virulence factors of the pathogen including the capsule, the formation of the anti-oxidant melanin in the cell wall, and the ability to grow at host body temperature. Thus, the fungus is remarkably tuned to perceive iron as part of the disease process, as confirmed by the avirulence of the cir1 mutant; this characteristic of the pathogen may provide opportunities for antifungal treatment

Peat bogs in northern Alberta, Canada reveal decades of declining atmospheric Pb contamination

Peat cores were collected from six bogs in northern Alberta to reconstruct changes in the atmospheric deposition of Pb, a valuable tracer of human activities. In each profile, the maximum Pb enrichment is found well below the surface. Radiometric age dating using three independent approaches (14C measurements of plant macrofossils combined with the atmospheric bomb pulse curve, plus 210Pb confirmed using the fallout radionuclides 137Cs and 241Am) showed that Pb contamination has been in decline for decades. Today, the surface layers of these bogs are comparable in composition to the "cleanest" peat samples ever found in the Northern Hemisphere, from a Swiss bog ~ 6000 to 9000years old. The lack of contemporary Pb contamination in the Alberta bogs is testimony to successful international efforts of the past decades to reduce anthropogenic emissions of this potentially toxic metal to the atmosphere

Animal density and track counts: understanding the nature of observations based on animal movements.

Counting animals to estimate their population sizes is often essential for their management and conservation. Since practitioners frequently rely on indirect observations of animals, it is important to better understand the relationship between such indirect indices and animal abundance. The Formozov-Malyshev-Pereleshin (FMP) formula provides a theoretical foundation for understanding the relationship between animal track counts and the true density of species. Although this analytical method potentially has universal applicability wherever animals are readily detectable by their tracks, it has long been unique to Russia and remains widely underappreciated. In this paper, we provide a test of the FMP formula by isolating the influence of animal travel path tortuosity (i.e., convolutedness) on track counts. We employed simulations using virtual and empirical data, in addition to a field test comparing FMP estimates with independent estimates from line transect distance sampling. We verify that track counts (total intersections between animals and transects) are determined entirely by density and daily movement distances. Hence, the FMP estimator is theoretically robust against potential biases from specific shapes or patterns of animal movement paths if transects are randomly situated with respect to those movements (i.e., the transects do not influence animals' movements). However, detectability (the detection probability of individual animals) is not determined simply by daily travel distance but also by tortuosity, so ensuring that all intersections with transects are counted regardless of the number of individual animals that made them becomes critical for an accurate density estimate. Additionally, although tortuosity has no bearing on mean track encounter rates, it does affect encounter rate variance and therefore estimate precision. We discuss how these fundamental principles made explicit by the FMP formula have widespread implications for methods of assessing animal abundance that rely on indirect observations

Sample output from three combinations of simulated daily travel paths and densities.

<p>Box plots with outliers are shown; each data point represents the numbers of intersections per transect (500 iterations) across five arbitrary levels of travel path tortuosity.</p

Empirical daily movements dispersed randomly in simulation space.

<p>Image capture (1∶50 000) shows a single iteration of simulation runs at 2 km⁻² density for (A) gemsbok and (B) steenbok. Approximately half of the randomly oriented transect (black) appears diagonally, underlying travel paths (grey). Note that both gemsbok and steenbok have similar daily travel distances but display different tortuosity in their movements, resulting in different spatial use.</p