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In the Arctic, the ground is frozen most of the year. Only the top layer of soil thaws each summer. This frozen ground, called permafrost, contains a lot of frozen water (ice). There are many small lakes in the Arctic, in low spots formed from melted ice. But melting ice does not just create lakes, it can destroy them too. Melting permafrost can create gullies that let the water drain out of a lake. Most lakes in the Arctic are far from where people live, so we watch them using pictures taken from satellites. Recently, we have seen the water drain out of many lakes, which can affect plants and animals. We measure the number and size of drained lakes caused by thawing permafrost to understand how the Arctic is changing.

How do you tell if someone is not well? You take the person’s temperature. If it is too warm, something is not quite right. We care about how the weather and climate are changing in Alaska’s national parks, so we continuously take their temperatures. We have dozens of weather stations in remote locations in the northern Alaska parks that run continuously, powered by the sun. Over the past several years, we found that the air and ground temperatures have been warmer than normal. Plants, animals, and people get used to living in their environments. They thrive within an expected temperature range. Things get out of whack when the environment that organisms are accustomed to changes. In northern Alaska, warming of just a few degrees can cause ice to melt and formerly frozen ground to thaw. Once the ground thaws, the ice changes to water and the landscape changes.

Climate change is impacting both the distribution and abundance of vegetation, especially in far northern latitudes. The effects of climate change are different for every plant assemblage and vary heterogeneously in both space and time. Small changes in climate could result in large vegetation responses in sensitive assemblages but weak responses in robust assemblages. But, patterns and mechanisms of sensitivity and robustness are not yet well understood, largely due to a lack of long-term measurements of climate and vegetation. Fortunately, observations are sometimes available across a broad spatial extent. We develop a novel statistical model for a multivariate response based on unknown cluster-specific effects and covariances, where cluster labels correspond to sensitivity and robustness. Our approach utilizes a prototype model for cluster membership that offers flexibility while enforcing smoothness in cluster probabilities across sites with similar characteristics. We demonstrate our approach with an application to vegetation abundance in Alaska, USA, in which we leverage the broad spatial extent of the study area as a proxy for unrecorded historical observations. In the context of the application, our approach yields interpretable site-level cluster labels associated with assemblage-level sensitivity and robustness without requiring strong a priori assumptions about the drivers of climate sensitivity.

Arctic sea-ice is declining in extent, leaving coastlines exposed to more storm-wave events. There is an urgent need to understand how these changes affect geomorphic processes along Arctic coasts. Here we describe spatial and temporal patterns of shoreline changes along two geomorphologically distinct, storm-wave dominated reaches of the Chukchi Sea coastline over the last 64 years. One study area encompasses the west- to southwest-facing, coarse-clastic shoreline and ice-rich bluffs of Cape Krusenstern (CAKR). The other covers the north-facing, sandy shorelines on barrier islands, ice-rich bluffs, and the Cape Espenberg spit in the Bering Land Bridge National Park (BELA). Both study areas lie within the zone of continuous permafrost, which exists both on and offshore and outcrops as ice-rich bluffs along the BELA coast. We mapped changes in coastal geomorphology over three observation periods: 1950–1980, 1980–2003, and 2003–2014 using aerial and satellite imagery. We then compared these geomorphic changes to changes in sea-ice coverage, which declined ~10 days per decade between 1979 and 2016 in the southern Chukchi Sea. Changes in coastal geomorphology in both BELA and CAKR exhibited high spatial variability over the study period. Between 2003 and 2014, shorelines of barrier islands in BELA exhibited the highest mean rates of change, −1.5 m yr−1, while coarse, clastic barrier beaches in CAKR showed only minimal change. Overall, shorelines in both BELA and CAKR became more dynamic (increasing erosion or increasing accumulation) after ca. 2003, with spatial variability in shoreline changes roughly doubling between the first period of observation (1950–1980) and the last (2003–2014). This increase in coastal dynamism may signal a transitional period leading to new state of geomorphic equilibria along these ice-affected coastlines.

Lakes are an important component of high-latitude regions, providing habitat for fish and wildlife and playing a critical role in biogeochemical and global carbon cycles. High-latitude lakes are sensitive to climate change, in part due to their development within permafrost soils. Considerable heterogeneity exists across arctic and subarctic landscapes, yet little is known about how this landscape variability influences chemical and physical attributes of lakes. We investigated the physical and chemical limnology of 617 lakes in Alaska's boreal forest and boreal-arctic transition zone. We categorized lakes into 10 basin types based on parent material, topography, genesis, and permafrost characteristics. Physical parameters varied across lake basin types, with the deepest lakes occurring in ice-poor glacial deposits and ice-rich terrain, while the shallowest lakes were observed in floodplain deposits and coastal lowlands. Dissolved inorganic nitrogen (N) and phosphorous (P) concentrations were generally low across all landscapes, whereas total N and P were highest in lakes underlain by ice-rich Pleistocene loess. Total N and P concentrations were significantly correlated with chlorophyll a, indicating a possible colimitation of primary productivity in these systems. Base cation concentrations helped elucidate lake basin hydrology and the relative influence of shallow versus deep groundwater inputs to surface water. Using these results, we developed a simple conceptual model for each lake and landscape type based on differences in physical and chemical parameters. Overall, we expect that the vulnerability of lake ecosystems to climate change will vary across lake basin types and will be mediated by spatial patterns in permafrost characteristics and subsurface hydrology.

Recent climate change in the Arctic is driving permafrost thaw, which has important implications for regional hydrology and global carbon dynamics. Permafrost is an important control on groundwater dynamics and the amount and chemical composition of dissolved organic matter (DOM) transported by high-latitude rivers. The consequences of permafrost thaw for riverine DOM dynamics will likely vary across space and time, due in part to spatial variation in ecosystem properties in Arctic watersheds. Here we examined watershed controls on DOM composition in 69 streams and rivers draining heterogeneous landscapes across a broad region of Arctic Alaska. We characterized DOM using bulk dissolved organic carbon (DOC) concentration, optical properties, and chemical fractionation and classified watersheds based on permafrost characteristics (mapping of parent material and ground ice content, modeling of thermal state) and ecotypes. Parent material and ground ice content significantly affected the amount and composition of DOM. DOC concentrations were higher in watersheds underlain by fine-grained loess compared to watersheds underlain by coarse-grained sand or shallow bedrock. DOC concentration was also higher in rivers draining ice-rich landscapes compared to rivers draining ice-poor landscapes. Similarly, specific ultraviolet absorbance (SUVA254, an index of DOM aromaticity) values were highest in watersheds underlain by fine-grained deposits or ice-rich permafrost. We also observed differences in hydrophobic organic acids, hydrophilic compounds, and DOM fluorescence across watersheds. Both DOC concentration and SUVA254 were negatively correlated with watershed active layer thickness, as determined by high-resolution permafrost modeling. Together, these findings highlight how spatial variations in permafrost physical and thermal properties can influence riverine DOM.

We monitored the growth of 22 retrogressive thaw slumps (RTS), dramatic erosion features associated with thaw of permafrost, in the Noatak Valley of northern Alaska using high-resolution structure-from-motion digital photogrammetry. We created time-series of 3–6 Digital Elevation Models (DEMs) and orthophoto mosaics during the time period from 2010–2016 at each slump, using high-resolution digital single-lens reflex (SLR) photographs taken from airplanes or helicopters. DEMs created using airborne GPS camera locations were adequate to detect elevations changes as small as 10 to 15 cm. Measurements made on these DEMs and orthophotographs showed slump growth rates of up to 38 m yr−1, with the fastest rates on slumps with scarps of moderate height (1 to 4 m) exposing Pleistocene glacial ice. Most of the slumps grew at constant or declining rates during the study, apparently as a result of the slumps encountering more gentle topography as they expanded upslope. Sedimentation was predominantly on the slump floor within 40 m of the active scarp, and the zone of accumulation migrated upslope with the scarp, away from adjacent water bodies. This study demonstrates that low-cost cameras coupled with airborne GPS and no ground control are suitable for monitoring geomorphic change on the order of decimeters and are a powerful tool for monitoring in remote settings.

Climate warming affects arctic and boreal ecosystems by interacting with numerous biophysical factors across heterogeneous landscapes. To assess potential effects of warming on diverse local-scale ecosystems (ecotypes) across northwest Alaska, we compiled data on historical areal changes over the last 25–50 years. Based on historical rates of change relative to time and temperature, we developed three state-transition models to project future changes in area for 60 ecotypes involving 243 potential transitions during three 30-year periods (ending 2040, 2070, 2100). The time model, assuming changes over the past 30 years continue at the same rate, projected a net change, or directional shift, of 6 % by 2100. The temperature model, using past rates of change relative to the past increase in regional mean annual air temperatures (1 °C/30 year), projected a net change of 17 % in response to expected warming of 2, 4, and 6 °C at the end of the three periods. A rate-adjusted temperature model, which adjusted transition rates (±50 %) based on assigned feedbacks associated with 23 biophysical drivers, estimated a net change of 13 %, with 33 ecotypes gaining and 23 ecotypes losing area. Major drivers included shrub and tree expansion, fire, succession, and thermokarst. Overall, projected changes will be modest over the next century even though climate warming increased transition rates up to 9 fold. The strength of this state-transition modeling is that it used a large dataset of past changes to provide a comprehensive assessment of likely future changes associated with numerous drivers affecting the full diversity of ecosystems across a broad region.

Multiple ablation technologies are used to treat atrial fibrillation during cardiac operations. All such ablation technologies use locally induced temperature extremes (>50°C or −20°C) to kill tissue and create a lesion pattern in the atria which blocks activation pathways that initiate and sustain atrial fibrillation. The technologies used to heat tissue have included radiofrequency (RF), microwave, high-intensity focused ultrasound, and infrared laser. RF accounts for more than 95% of the heating-based ablation technology used by cardiac surgeons. Energy delivery with RF is easier to control than with some other technologies, the heating produced by the energy source is well understood, and manufacturing costs are not excessive. Whichever heating technology is used, control of energy delivery is required to ensure both safe and effective heating of the targeted tissue. All targeted tissue needs to be heated above 50°C to achieve cell death. However, the targeted tissue should not be heated above 100°C, as this can cause perforation due to a steam pop. In addition, adjacent noncardiac tissues must not be damaged during the ablation procedure. The best method to achieve this control uses direct measurement of tissue temperature, because the tissue temperature defines both the safe and effective limits for the ablative process.

For a comprehensive bird inventory of Yukon-Charley Rivers National Preserve, Alaska, we stratified the 1 million hectare study area by large, physiographically defined regions known as ecological units. Point-count data from the bird inventory were used to test the ability of the ecological units to differentiate bird assemblages, and compare the effectiveness of ecological units to fine-scale vegetation types. The ecological units were a synthesis of geology, landforms, soils, and vegetation mapped at a scale of 1:250,000; the vegetation types were based on vegetation within 50 m of the sample points. Nonparametric multivariate statistical tests showed that ecological units and vegetation types had similar success in differentiating bird assemblages, despite their different scales and conceptual bases. Analyses of individual bird species showed that both ecological units and vegetation types provide useful and complementary information about bird habitat selection. Ecological units have several advantag...

We sampled shrub canopy volume (height times area) and environmental factors (soil wetness, soil depth of thaw, soil pH, mean July air temperature, and typical date of spring snow loss) on 471 plots across five National Park Service units in northern Alaska. Our goal was to determine the environments where tall shrubs thrive and use this information to predict the location of future shrub expansion. The study area covers over 80,000 km2 and has mostly tundra vegetation. Large canopy volumes were uncommon, with volumes over 0.5 m3/m2 present on just 8% of plots. Shrub canopy volumes were highest where mean July temperatures were above 10.5°C and on weakly acid to neutral soils (pH of 6 to 7) with deep summer thaw (>80 cm) and good drainage. On many sites, flooding helped maintain favorable soil conditions for shrub growth. Canopy volumes were highest where the typical snow loss date was near 20 May; these represent sites that are neither strongly wind-scoured in the winter nor late to melt from deep snowdrifts. Individual species varied widely in the canopy volumes they attained and their response to the environmental factors. Betula sp. shrubs were the most common and quite tolerant of soil acidity, cold July temperatures, and shallow thaw depths, but they did not form high-volume canopies under these conditions. Alnus viridis formed the largest canopies and was tolerant of soil acidity down to about pH 5, but required more summer warmth (over 12°C) than the other species. The Salix species varied widely from S. pulchra, tolerant of wet and moderately acid soils, to S. alaxensis, requiring well-drained soils with near neutral pH. Nearly half of the land area in ARCN has mean July temperatures of 10.5 to 12.5°C, where 2°C of warming would bring temperatures into the range needed for all of the potential tall shrub species to form large canopies. However, limitations in the other environmental factors would probably prevent the formation of large shrub canopies on at least half of the land area with newly favorable temperatures after 2°C of warming.

backslopes is shallow to moderately deep due to gelifluction and slope movement but deep toward toeslopes. Charcoal particles, commonly central Canada. The watershed is representative of found within the soil profiles, attest to frequent fire events in the past. non-glaciated upland headwater stream basins found Fire has the greatest impact on soil properties on south aspect slopes throughout interior Alaska. Forest communities in the because the organic horizons are thin and dry, and easily destroyed by watershed are representative of the boreal forest zone fire. The underlying mineral horizons often develop hydrophobicity in Alaska. Forest fire is a dominant ecological factor in resulted from moderately and severe burn. Slope, aspect, and slope this region (Kasischke and Stocks, 2000) and there has gradient are major controlling factors for the contrasting soil types along the catena sequences in the watershed. been wide interest in the effects of wildfires on forest ecosystem but little on soils. Quantitative, site-specific baseline soils data are essential for comparing pre- and J enny’s functional-factoral soil formation model postfire changes within and below the forest floor. Rieger (Jenny, 1941) identifies the major components of et al. (1972) provide descriptions and maps (at a scale pedogenesis to be parent material, organisms, topogra- of 1:31 680) for seven soil series within the watershed; phy, climate, and time. These factors tend to be highly however, little quantitative data are provided. Viereck interdependentanddifficulttoisolateinnaturalsettings. etal. (1983) addressed, in detail, the soils, vegetation, and Landscape topography dictates microclimate, water dy- their relationship along a toposequence at the Bonanza namics, and material redistribution processes that criti- Creek Long-Term-Ecological-Research site 90 km to the callyinfluencethenatureofsoilsandvegetationmosaics southwest. The soils at Bonanza Creek developed on (Hole and Campbell, 1985; Hunckler and Schaetzl, 1997). deep loess deposits associated with the nearby Tanana Slope gradient and aspect strongly affect soil morpho- River and are not representative of the soils formed logicalcharacteristicsandgeochemicalprocessesinmidin the Yukon-Tanana Uplands metamorphic complex.

Aim To understand the similarities and differences between the taiga floras of far north-eastern Asia and north-western North America in the light of their Tertiary and Quaternary histories. Does the taiga flora follow the tundra pattern (Asian–American commonality of species as a result of continuity through the Quaternary), the temperate forest pattern (distinct species because of late Tertiary disjunction), a combination of these two patterns, or some pattern unique to the taiga? Location The taiga regions of interior Alaska and the Yukon in North America (the ‘Alaskan taiga’), and the Kolyma and eastern Indigirka River basins in Russia (the ‘Kolyma taiga’). The study areas include both forested and unforested habitats below elevational treeline. The two regions have similar climate and topography and were linked via the Bering Land Bridge in the Tertiary and for several extended periods during Quaternary cold periods. Methods Systematic comparison of the vascular floras of the two regions from published sources; and review of palaeoecological literature for the region. Results Of the 796 species found in the study areas, 27% occur only in the Alaskan taiga, 35% occur only in the Kolyma taiga, and 38% occur in both the regions. The following subsets of species show a high proportion of species in common between the study areas (subsets are not mutually exclusive): plants that occur on the tundra and the taiga, non-flowering plants, abundant taiga understory plants, and wetland and aquatic plants. A lower proportion of shared plants was noted for warm, south-facing steppe communities. No tree species are common to both areas. Main conclusions The Bering Strait region in the Quaternary has acted as a biogeographical filter for taiga plants. Significant divergence between northeast Asia and northwest North America has developed among the more southerly ranging fraction of the flora (e.g. trees), while the more cosmopolitan and the most cold-adapted elements of the taiga flora are common to both areas. Many plants in the former group have been disjunct between Asia and North America for millions of years, while many plants in the latter group have probably maintained continuity between the study areas via the Bering Land Bridge through much of the late Tertiary and Quaternary periods. Repeated extirpation of the less cold-adapted species from both study areas during Pleistocene cold periods has probably enhanced floristic differences between the two regions.

Limnological reconnaissance data were collected during summers 1992–93 and 1995 from 16 major lakes within Gates of the Arctic National Park and Preserve, Alaska (GAAR) located above the Arctic Circle. In GAAR the southern lakes (~67°N) and those in the Brooks Range foothills are in watersheds with taiga and tundra vegetation. The northern lakes (~68°N) are at higher elevations in the Brooks Range in watersheds that lie wholly beyond the treeline. Average sum of the cations for all lakes matches the world average for fresh waters and the average for drainage from tundra and taiga landscapes. Local lithography explains the measured decrease in calcium equivalents and silica, and an increase in magnesium with altitude because of more calcareous rock in the southern basins at low altitude and shale/conglomerate in northern mountain catchments. In most low altitude lakes Secchi depth was located in sub-surface algal peaks at temperatures averaging ~8.5 C cooler than the surface, typically with double...

Background Endocardial mapping using standard electrode catheters is often technically limited in ventricular tachycardia and constitutes a major obstacle to successful ablation. We wished to examine the utility of a basket-shaped multielectrode mapping catheter (MMC) in the mapping and ablation of ventricular tachycardia. Methods and Results This study of sustained monomorphic ventricular tachycardia (SMVT) was conducted in two phases in the postinfarction pig model. In the first phase, the utility of the MMC in providing adequate localization of potential ablation site(s) of SMVT by different techniques (presystolic potentials, pace mapping, and concealed entrainment) was assessed in 21 pigs. In the second phase, ablation of induced SMVT was attempted in 10 pigs. Mapping of SMVT was performed after percutaneous introduction of the MMC to the LV. Comprehensive mapping was performed in 90 episodes of SMVT and required 2.0 to 25 seconds. Diastolic potentials were recorded during 86 episodes; good or identical pace maps (≥9 of 12 paced surface ECG leads identical to ventricular tachycardia surface ECG leads) were obtained in 25 of 31 maps, and entrainment was achieved during 28 of 42 SMVTs. In 10 pigs, 10 SMVTs were recorded at least twice and were considered for radiofrequency ablation. An 8-mm tip ablation catheter was advanced to potential ablation sites with a specially designed “homing” device, requiring a median time of 120 seconds. In these 10 pigs, either identical pace map (≥11 of 12, 6 SMVTs) or concealed entrainment (4 SMVTs) guided the ablation procedure. After ablation, 8 of 10 SMVTs were rendered noninducible, while 2 pigs died during energy application of degeneration of SMVT to ventricular fibrillation. Conclusions The MMC allows rapid, comprehensive, and reliable endocardial mapping during SMVTs, which facilitates successful ablation in the porcine post–myocardial infarction model.

Ablation of typical atrial flutter using radiofrequency energy is limitted by difficulties in creating a continuous line of block across the istmus between the tricuspid anulus and the inferior vena cava. Recent studies from our laboratory have shown that a novel infusion ablation technique can create large endocardial lesions in a safe and predictable fashion. The purpose of our study was to assess the feasibility of transcatheter subendocardial infusion of a mixture of 95% enthanol, iodinated contrast and glycerin to create atrial lesions resulting in complete ablation of the critical isthmus mediating typical atrial flutter.

Background Identification of critical areas within the ventricular tachycardia circuit is a prerequisite for catheter ablation. Currently, mapping during ventricular tachycardia, usually performed with standard catheters, is difficult and time-consuming and can be used only in patients with hemodynamically stable tachycardia. Methods and Results A total of 43 pigs underwent closed-chest induction of myocardial infarction. A basket-shaped catheter carrying 64 electrodes was deployed in the left ventricle during normal sinus rhythm. Unipolar pacing at 3 mA was successful in 78% of the basket catheter electrodes, demonstrating good electrode-tissue contact. Hemodynamic and echocardiographic measurements did not reveal any significant interference with myocardial or valvular function during or after catheter deployment. One hundred eighteen episodes of monomorphic ventricular tachycardia were induced in 28 pigs through right ventricular stimulation, 81 of which were mapped and analyzed. Ventricular tachycardia mapping was rapid, requiring only several beats and Conclusions The multielectrode catheter enables rapid and safe percutaneous endocardial mapping of ventricular tachycardia in the swine model. Exploration of the clinical potential of the multielectrode catheter seems warranted.

The depth to permafrost, thickness of the surface organic layer, and strength of redoximorphic features are closely correlated in soils on low-relief bedrock hills and colluvial slopes near Hughes, Alaska. Soils can be arrayed along a morphological gradient from warm-dry-mineral to cold-wet-organic, and the ranks of soils along this gradient correlate with topographic parameters and mineral soil texture. The warm-dry-mineral soils tend to occur on coarse-textured materials, convex slopes, steep slopes, and south-facing slope aspects, while the cold-wet-organic soils tend to occur on fine-textured materials, concave slopes, gentle slopes, and north-facing aspects. Slope shape and mineral soil texture are the geomorphic factors most closely associated with the ranking of soils along the morphological gradient from warm-dry-mineral to cold-wet-organic.

This study analyzed the influence of electrode geometry, tissue-electrode angle, and blood flow on current density and temperature distribution, lesion size, and power requirements during radio-frequency ablation. The authors used validated three-dimensional finite element models to perform these analyses. They found that the use of an electrically insulating layer over the junction between electrode and catheter body reduced the chances of charring and coagulation. The use of a thermistor at the tip of the ablation electrodes did not affect the current density distribution. For longer electrodes, the lateral current density decreased more slowly with distance from the electrode surface. The authors analyzed the effects of three tissue-electrode angles: 0, 45, and 90/spl deg/. More power was needed to reach a maximal tissue temperature of 95/spl deg/C after 120 s when the electrode-tissue angle was 45/spl deg/. Consequently, the lesions were larger and deeper for a tissue-electrode angle of 45/spl deg/ than for 0 and 90/spl deg/. The lesion depth, volume, and required power increased with blood flow rate regardless of the tissue-electrode angle. The significant changes in power with the tissue-electrode angle suggest that it is safer and more efficient to ablate using temperature-controlled RF generators. The maximal temperature was reached at locations within the tissue, a fraction of a millimeter away from the electrode surface. These locations did not always coincide with the local current density maxima. The locations of these hottest spots and the difference between their temperature and the temperature read by a sensor placed at the electrode tip changed with blood flow rate and tissue-electrode angle. >

Quaking aspen (Populus tremuloides Michx.) is a valuable species that is declining in the Blue Mountains of northeastern Oregon. This publication is a compilation of over 20 years of aspen management experience by USDA Forest Service workers in the Blue Mountains. It includes a summary of aspen biology and occurrence in the Blue Mountains, and a discussion of aspen conservation and management techniques such as fencing, conifer removal, and artificial propagation. Local data on bird use of aspen stands, insects and diseases in aspen, and genetic studies of aspen are also included. An aspen community classification developed from over 200 sample plots is presented, with plant species composition and cover, environment and soils, and management considerations.

Late Pleistocene Beringia had herb-dominated vegetation with abundant grasses (Poaceae), and it was inhabited by an impressive assemblage of large grazing mammals. This paper reconstructs the list of most probable late Pleistocene Beringian lowland grass species from biogeographical evidence. Late Pleistocene eolian sediments and buried soils indicate that large areas of the Beringian lowlands had nutrient-rich, silty soils that occurred over ice-rich permafrost but were generally not waterlogged. A list of likely grasses was compiled from all species that have been recorded on similar fine-grained, mesic-to-dry lowland soils (i.e., presumed refugia) and are distributed at least sporadically across the whole region today. Grasses from 13 genera met these criteria, including most of the taxa that have been identified as late Pleistocene fossils from the study area. Most of these grasses are high-latitude species of genera that are also common in temperate latitudes (e.g., Elymus , Festuca , and Poa ). This diverse group of plants has a wide range of adaptations today, suggesting that grasses would have been available to occupy a variety of habitats through Pleistocene climatic fluctuations. Among these grasses are a number of highly productive forage species.

The structure of peatland vegetation in Minnesota is affected by trophic conditions, the soil-water regime, and the disturbance history. Trophic conditions have a major effect on the species composition. Structurally, this results in the exclusion of most tall shrubs, hardwood trees, and forbs from ombrotrophic sites and a greater biomass of mosses and ericad shrubs on those sites than on minerotrophic sites. Classification of peatland vegetation into physiognomic groups provides reasonable estimates of total aboveground biomass. Trophic conditionsper se, however, have a minor effect on aboveground biomass; on both ombrotrophic and minerotrophic peatlands, biomass increases from less than 10 Mg ha−1 on the wettest sites to over 100 Mg ha−1 on the driest, undisturbed sites. Peatland sites that have comparatively dry, well-aerated soils support dense forests if they have not recently been disturbed; shrubs dominate on those sites after disturbance. Excessively wet sites are dominated by mosses (particularly on ombrotrophic peatlands) or graminoids (on minerotrophic peatlands). On ombrotrophic sites, the relatively dry conditions that lead to higher tree and total biomass also lead to lower pH resulting in an inverse relationship between biomass and pH.

Radiolabeled deoxyglucose (FDG) has been advocated as a marker of viability of reperfused myocardium during acute infarction. However, data for such recommendation are few. We investigated cardiac deposition of C-14 deoxyglucose (C-14 DG) and of Thallium -201 (T1–201) in rabbits subjected to coronary occlusion (15, 30, 60 or > 100 min) and reperfusion (75 min and 24 h). Measured myocardial concentrations of C-14 DG and T1–201 in macroautoradiograms were quantitatively correlated in a 24 h reperfusion group with presence of myocardial necrosis evaluated by light microscopy. The major finding in this investigation was that with 30 min or 60 min of ischemia followed by reperfusion there were myocardial regions with significant hypoperfusion (T1–201) and histologic necrosis. However, in the same myocardial areas, the deposition of C-14 DG was not correlated with the extent of necrosis (r = 0.27). Also, the deposition of C-14 DG in acute myocardial infarction was higher than that of T1–201 (P = 0.05 by paired T test and by nonparametric Wilcoxon's test). It was also demonstrated that when the occlusion time was varied (15–130 min) and early reperfusion was provided for 75 min or omitted altogether, the myocardial accumulation of T1–201 was variable and that myocardial sequestration of C-14 DG was higher than perfusion in central and peripheral portions of the areaat-risk. These observations do not support a role for the use of radiolabeled deoxyglucose for the detection of myocardial viability in recently infarcted cardiac muscle.

Lichens in the Arctic play important ecological roles. They also face the threats of increasing fire and shrub and tree expansion, exacerbated or caused by climate change. These forces may lead to changes not only in lichen community composition but also in the abundance, diversity and distribution of lichen functional traits. We sought to connect landscape-scale patterns of lichen community composition and traits to environmental gradients to both monitor lichen communities and clarify community-trait-environment relationships. We measured lichens throughout one of the largest and most remote U.S. National Parks within the Arctic. We then analyzed lichen community composition and species richness within ecologically informative lichen trait groups along environmental and vascular vegetation gradients. Macrolichen species richness in 0.4 ha plots averaged 41 species with a total landscape level observed gamma diversity of 262 macrolichen species. Jackknife estimators placed the landscape level ma...

The most striking feature of Alaska’s boreal soils, compared to those from most other biomes, is the lack of significant soil development even though much of interior Alaska has not been glaciated for millions of years. Soils in the boreal region support forest ecosystems that account for nearly half of the land area of Alaska. Worldwide, boreal forests store nearly a third of the terrestrial carbon (Apps et al. 1993). Hence, changes in boreal soils could greatly impact the global carbon balance (Chapter 19). Another striking feature of boreal soils is their great local variation due to slope and aspect. Compared to the well-developed, colorful soils of the temperate and tropical regions, boreal soils generally have ochric (yellowish brown) colors in uplands and dark-colors in lowlands due to organic matter accumulations. These lowlands account for 85% of the wetland inventory in the United States (Bridgham et al. 2001). Soil is a mixture of geological parent material and organic matter altered by weathering and the action of living organisms and conditioned by topography over time. Jenny (1941) defined the major factors that influence soil formation as parent material, organisms, topography, climate, and time. When only one of these factors varies, soil characteristics can be treated as a function of that factor (Chapter 1). Soils are also responsive to anthropogenic changes, such as agriculture and forestry, that alter interactive controls at the local scale (Moore and Ping 1989). Interior Alaska consists of several broad, nearly level lowlands with elevations mostly below 500 m and rounded mountains with elevations up to about 2000 m (Wahrhaftig 1965). The Interior Highlands Ecoregion (Chapter 2) includes the Kuskokwim Highlands and the Interior Highlands. The Interior Bottomlands Ecoregion includes the Koyukuk-Innoko Lowland, the Kanuti Flats, and the Tanana- Kuskokwim Lowlands (Rieger et al. 1979). The Copper River Basin south of the Alaska Range has a climate and vegetation similar to that of interior Alaska. The Copper River Basin has fine-textured subsoils of lacustrine origin. Soils from the Alaska Range, the Kuskokwim, and the Interior Highlands generally form in glacial deposits and residual materials and have abundant rock fragments.

Permafrost, or perennially frozen ground, covers 9×105km2 or 55% of the landmass of Alaska. The continuous zone of permafrost encompasses the physiographic subdivisions of the Arctic Coastal Plain, the Arctic Foothills, the northern Norton Sound Highlands, and the Brooks Range. The zone of discontinuous permafrost encompasses the physiographic subdivisions of the Interior Alaska Lowlands, the Interior Alaska Highlands, the Alaska Range, the Copper River Plateau, the Kuskokwin Highlands, the southern Norton Sound Highlands, and the Western Alaska Coastal Plains and Deltas (Wahrhaftg, 1965; Pewe, 1975) (see Figure 2.2.1).

We tested the performance of a multielectrode RF ablation system during in vivo procedures in the right atrium. A total of 75% of the RF applications resulted in complete lesions at the gaps between electrodes and 90% of them were considered potentially therapeutic, as they were transmural and expended along the intended contour. The average total procedure and fluoroscopy times were 83.4 and 5.9 min, respectively. Average power varied between 15 and 114 W. No sudden impedance rise occurred and no char was found by post-ablation inspection. The average impedance was 105/spl plusmn/16 /spl Omega/, with one electrode on, and 75/spl plusmn/12 /spl Omega/ when two electrodes were simultaneously on. The average maximal lesion length and width were 59.2/spl plusmn/11.7 and 7.1/spl plusmn/1.2-mm.

Long and contiguous lesions may cure AF. We analyzed a multielectrode steerable catheter capable of creating such lesions. For optimal temperature control purposes, based on analytical and experimental data, one temperature sensor was placed at each end of the long coil electrodes. In vitro ablations were performed to analyze the catheter performance. At set temperatures of 70/spl deg/C, 85% of the lesions were contiguous. All lesions created at set temperatures of 80 and 90/spl deg/C were contiguous. No sudden impedance rises occurred. The lesion length increased linearly with the number of electrodes. Power to reach set temperature was larger and the impedance decreased as more electrodes were connected in the ablation circuit.

The locations of current density maxima can provide information about the locations of temperature maxima. Our goals were: (1) to find the optimal place of the temperature sensor used for temperature-controlled ablation; (2) to assess at which locations char is more likely to occur. Using finite element modeling, we analyzed the current density distribution for a 8Fr/5 mm straight and a 8Fr/8 mm valve ablation electrodes. The maximal current densities occurred at the tip of the electrodes. Hence, it is best to place the temperature sensor, which provides input for the temperature control, at the electrode tip. Also, the analysis showed that using an electrically insulating coating layer at the junction between the electrode and the catheter body significantly reduced the local current density, therefore the local temperature. This avoids the formation of charring. >

We performed in vitro experiments to compare lesion sizes after ablating with five electrode geometries positioned at either 0/spl deg/ (flat) or 90/spl deg/ (end-on) with respect to the endocardial surface. The RF power was adjusted to maintain a constant thermistor temperature of 70/spl deg/C. The maximum lesion lengths, depths, and widths were measured for each lesion and compared to those corresponding to a 8Fr/4 mm straight control electrode. In the end-on catheter tip orientation, the primary determinant of lesion size was the diameter of the tip. This was consistent except for the 8Fr/8 mm valve electrode which required more input power to maintain a 70/spl deg/C thermistor temperature because of the larger surface area. The 8Fr/8 mm valve electrode had a larger surface area with which to conduct heat from the tissue and to be convectively cooled by the surrounding saline. In the flat orientation, the surface area was the main predictor of lesion size. The 8Fr/8 mm valve electrode was the only electrode in the flat orientation to produce significantly different lesion size than the control. None of the electrode geometries or orientations produced charring. >

The significant changes in power with tissue-electrode angle suggested that the lesions were more predictable when ablating with temperature-based RF generators, rather than non-temperature systems. The maximal tissue temperature was reached at locations within the tissue a fraction of a millimeter away from the ablation electrode surface. The errors in tissue temperature prediction introduced by temperature sensors placed inside the electrode changed with blood flow rate and tissue-electrode angle. This study shows that to reduce tissue temperature prediction errors, during temperature-controlled ablation it was better to use a temperature sensor placed at the tip rather than at the center of the ablation electrode.

We developed a temperature-controlled radiofrequency (RF) system which can ablate by delivering energy to up to six 12.5 mm long coil electrodes simultaneously. Temperature feedback was obtained from temperature sensors placed at each end of coil electrodes, in diametrically opposite positions. The coil electrodes were connected in parallel, via a set of electronic switches, to a 150 W 500 kHz temperature-controlled RF generator. Temperatures measured at all user-selected coil electrodes were processed by a microcontroller which sent the maximum value to the temperature input of the generator. The generator adjusted the delivered power to regulate the temperature at its input within a 5 degrees C interval about a user-defined set point. The microcontroller also activated the corresponding electronic switches so that temperatures at all selected electrodes were controlled within a 5 degrees C interval with respect to each other. Physical aspects of tissue heating were first analysed using finite element models and current density measurements. Results from these analyses also constituted design input. The performance of this system was studied in vitro and in vivo. In vitro, at set temperatures of 70 degrees C, 85% of the lesions were contiguous. All lesions created at set temperatures of 80 and 90 degrees C were contiguous. The lesion length increased almost linearly with the number of electrodes. Power requirements to reach a set temperature were larger as more electrodes were driven by the generator. The system impedance decreased as more electrodes were connected in the ablation circuit and reached a low of 45.5 ohms with five coil electrodes in the circuit. In vivo, right atrial lesions were created in eight mongrel canines. The power needed to reach 70 degrees C set temperature varied between 15 and 114 W. The system impedance was 105+/-16 ohms, with one coil electrode in the circuit, and dropped to 75+/-12 ohms when two coil electrodes were simultaneously powered. The length and the width of the lesion set varied between 17.6+/-6.1 and 59.2+/-11.7 mm and 5.9+/-0.7 and 7.1+/-1.2 mm respectively. No sudden impedance rises occurred and 75% of the lesions were contiguous. From the set of contiguous lesions, 90% were potentially therapeutic as they were transmural and extended over the entire target region. The average total procedure and fluoroscopy times were 83.4 and 5.9 min respectively. We concluded that the system can safely perform long and contiguous lesions in canine right atria.