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Your Yukon is a weekly column about environmental issues and research, from Canada's Yukon. This column discusses how climate change, especially drought stress, may be affecting the boreal forest.--Includes text from publisher's website.
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Unprecedented rates of climate warming over the past century have resulted in increased forest stress and mortality worldwide. Decreased tree growth in association with increasing temperatures is generally accepted as a signal of temperature-induced drought stress. However, variations in tree growth alone do not reveal the physiological mechanisms behind recent changes in tree growth. Examining stable carbon isotope composition of tree rings in addition to tree growth can provide a secondary line of evidence for physiological drought stress. In this study, we examined patterns of black spruce growth and carbon isotopic composition in tree rings in response to climate warming and drying in the boreal forest of interior...
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Owing to large climatic and orographic variation, British Columbia covers a variety of ecosystems extending from temperate rainforests on the Pacific coast to boreal forests in the north-east. The aim of this study is to investigate the spatial variation of trends in wildfire activity and their relationship to summer drought for the entire province of British Columbia. Time series of annual wildfire extent and occurrence, summer self-calibrating Palmer Drought Severity Index and summer Aridity Index were derived from spatially explicit data. Sixteen landscape regions according to the provincial Biogeoclimatic Ecosystem Classification system served as spatial reference. The regional series for 1920-2000 were subjected...
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Increment cores from the boreal forest have long been used to reconstruct past climates. However, in recent years, numerous studies have revealed a deterioration of the correlation between temperature and tree growth that is commonly referred to as "divergence". In the Brooks Range of northern Alaska, studies of white spruce (Picea glauca) revealed that trees in the west generally showed positive growth trends, while trees in the central and eastern Brooks Range showed mixed and negative trends during late 20th century warming. The growing season climate of the eastern Brooks Range is thought to be drier than the west. On this basis, divergent tree growth in the eastern Brooks Range has been attributed to drought...
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High latitude trees are increasingly showing mixed growth responses to climate warming. Recent studies of white spruce (Picea glauca ) in the Brooks Range of Alaska have revealed that trees in the west have generally shown positive growth responses, while trees in the central and eastern Brooks Range have shown mixed and negative growth responses, respectively. An earlier study suggested that the growing season climate of the eastern Brooks Range is warmer and drier than in the west. On this basis, growth declines in the eastern Brooks Range were attributed to drought stress, caused by rising temperature and increasing evaporative demand. I investigated the hypothesis that drought stress can explain white spruce...
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Climate has warmed substantially in interior Alaska and several remote sensing studies have documented a decadal-scale decline in the normalized difference vegetation index (NDVI) termed a ‘browning trend’. Reduced summer soil moisture due to changing climatic factors such as earlier springs, less snowpack, and summer drought may reduce boreal productivity and NDVI. However, the relative importance of these climatic factors is poorly understood in boreal interior Alaska. In this study, I used the remotely sensed peak summer NDVI as an index of boreal productivity at 250 m pixel size from 2000 to 2014. Maximum summer NDVI was related to last day of spring snow, early spring snow water equivalent (SWE), and a...
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Trends of summer precipitation and summer temperature and their influence on trends in summer drought and area burned in British Columbia (BC) were investigated for the period 1920–2000. The complexity imposed by topography was taken into account by incorporating high spatial resolution climate and fire data. Considerable regional variation in trends and in climate–fire relationships was observed. A weak but significant increase in summer temperature was detected in northeastern and coastal BC, whereas summer precipitation increased significantly in all regions—by up to 45.9 %. A significant decrease in province-wide area burned and at the level of sub-units was strongly related to increasing precipitation, more...
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The northern treeline is generally limited by available warmth. However, in recent years, more and more studies have identified drought stress as an additional limiting factor for tree growth in northern boreal forests and at treelines. Three growth responses to warming have been identified: increase in growth, decrease in growth, and nonsignificant correlation of tree growth with climate. Here we investigate the effect of drought stress on radial growth of white spruce at northern treelines along a longitudinal gradient spanning the entire Brooks Range in Alaska. We systematically sampled 687 white spruce at seven treeline sites. Where possible, we sampled three site types at a given site: high-density forest,...
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Algae are important to many of the processes that characterize wetland ecosystems. Despite their importance, we know relatively little about the factors that regulate algal communities in wetlands. This is particularly true for northern boreal regions where wetlands are abundant and are considered to be extremely vulnerable to disturbances associated with climate change. My dissertation research investigates how nutrients, grazing, light, and hydrology regulate algal primary production and taxonomic structure in high latitude wetlands. I documented spatial and temporal variability in algal structure and function in six northern boreal wetlands in interior Alaska to determine the contribution of algal primary production...
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Global vegetation models predict that boreal forests are particularly sensitive to a biome shift during the 21st century. This shift would manifest itself first at the biome?s margins, with evergreen forest expanding into current tundra while being replaced by grasslands or temperate forest at the biome?s southern edge. We evaluated changes in forest productivity since 1982 across boreal Alaska by linking satellite estimates of primary productivity and a large tree-ring data set. Trends in both records show consistent growth increases at the boreal?tundra ecotones that contrast with drought-induced productivity declines throughout interior Alaska. These patterns support the hypothesized effects of an initiating...
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Wildfires are a common experience in Alaska where, on average, 3,775 km^sup 2^ burn annually. More than 90% of the area consumed occurs in Interior Alaska, where the summers are relatively warm and dry, and the vegetation consists predominantly of spruce, birch, and cottonwood. Summers with above normal temperatures generate an increased amount of convection, resulting in more thunderstorm development and an amplified number of lightning strikes. The resulting dry conditions facilitate the spread of wildfires started by the lightning. Working with a 55-year dataset of wildfires for Alaska, an increase in the annual area burned was observed. Due to climate change, the last three decades have shown to be warmer than...
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Climate change is an increasing concern for Yukon and its communities. Many observable changes have occurred across the Territory over the past fifty years, especially an increase in the annual temperature and precipitation of the Western Arctic. The potential outcomes of changing climate have raised subsequent concerns for Yukon residents including a shifting distribution of country foods, the thaw of permafrost, changing landscape conditions, drought, and a host of other regional vulnerabilities. In response to this growing concern about climate change, the Northern Climate ExChange submitted a proposal to the Northern Strategy Trust in 2007 to develop and implement adaptation plans in three Yukon communities:...
Mast-seeding conifers such as Picea glauca exhibit synchronous production of large seed crops over wide areas, suggesting climate factors as possible triggers for episodic high seed production. Rapidly changing climatic conditions may thus alter the tempo and spatial pattern of masting of dominant species with potentially far-reaching ecological consequences. Understanding the future reproductive dynamics of ecosystems including boreal forests, which may be dominated by mast-seeding species, requires identifying the specific cues that drive variation in reproductive output across landscape gradients and among years. Here we used annual data collected at three sites spanning an elevation gradient in interior Alaska,...
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To determine the influence of fire and thermokarst in a boreal landscape, we investigated peat cores within and adjacent to a permafrost collapse feature on the Tanana River Floodplain of Interior Alaska. Radioisotope dating, diatom assemblages, plant macrofossils, charcoal fragments, and carbon and nitrogen content of the peat profile indicate approximately 600 years of vegetation succession with a transition from a terrestrial forest to a sedge-dominated wetland over 100 years ago, and to a Sphagnum-dominated peatland in approximately 1970. The shift from sedge to Sphagnum, and a decrease in the detrended tree-ring width index of black spruce trees adjacent to the collapse coincided with an increase in the growing...
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Boreal forests play critical roles in global carbon, water and energy cycles. Recent studies suggest drought is causing a decline in boreal spruce growth, leading to predictions of widespread mortality and a shift in dominant vegetation type in interior Alaska. We took advantage of a large set of tree cores collected from random locations across a vast area of interior Alaska to examine long-term trends in carbon isotope discrimination and growth of black and white spruce. Our results confirm that growth of both species is sensitive to moisture availability, yet show limited evidence of declining growth in recent decades. These findings contrast with many earlier tree-ring studies, but agree with dynamic global...
map background search result map search result map Future Histories of Whitehorse: Scenarios of Change Climate sensitivity of reproduction in a mast-seeding boreal conifer across its distributional range from lowland to treeline forests Precipitation-driven decrease in wildfires in British Columbia Stable carbon isotope analysis reveals widespread drought stress in boreal black spruce forests Impacts of drought on forest growth and regeneration following fire in southwestern Yukon, Canada Drought-induced stomatal closure probably cannot explain divergent white spruce growth in the Brooks Range, Alaska Carbon isotope discrimination in tree ring alpha-cellulose indicates that drought stress cannot explain white spruce growth declines in the eastern Brooks Range, Alaska Relationship between fire, climate oscillations, and drought in British Columbia, Canada, 1920–2000 Remote sensing of interannual boreal forest NDVI in relation to climatic conditions in interior Alaska Wetland succession in a permafrost collapse; interactions between fire and thermokarst Response of plant community structure and primary productivity to experimental drought and flooding in an Alaskan fen Longitudinal variation of radial growth at Alaska's northern treeline; recent changes and possible scenarios for the 21st century Regulation of benthic algal structure and function in northern boreal wetlands Spatial variation of trends in wildfire and summer drought in British Columbia, Canada, 1920-2000 The sensitivity of carbon fluxes to spring warming and summer drought depends on plant functional type in boreal forest ecosystems [electronic resource] Reduced growth of Alaskan white spruce in the twentieth century from temperature-induced drought stress Drought stress changing the Takhini landscape Limited evidence of declining growth among moisture-limited black and white spruce in interior Alaska Regulation of benthic algal structure and function in northern boreal wetlands Wetland succession in a permafrost collapse; interactions between fire and thermokarst Response of plant community structure and primary productivity to experimental drought and flooding in an Alaskan fen Future Histories of Whitehorse: Scenarios of Change Impacts of drought on forest growth and regeneration following fire in southwestern Yukon, Canada Reduced growth of Alaskan white spruce in the twentieth century from temperature-induced drought stress Climate sensitivity of reproduction in a mast-seeding boreal conifer across its distributional range from lowland to treeline forests Limited evidence of declining growth among moisture-limited black and white spruce in interior Alaska Stable carbon isotope analysis reveals widespread drought stress in boreal black spruce forests Drought-induced stomatal closure probably cannot explain divergent white spruce growth in the Brooks Range, Alaska Carbon isotope discrimination in tree ring alpha-cellulose indicates that drought stress cannot explain white spruce growth declines in the eastern Brooks Range, Alaska Longitudinal variation of radial growth at Alaska's northern treeline; recent changes and possible scenarios for the 21st century Remote sensing of interannual boreal forest NDVI in relation to climatic conditions in interior Alaska The sensitivity of carbon fluxes to spring warming and summer drought depends on plant functional type in boreal forest ecosystems [electronic resource] Precipitation-driven decrease in wildfires in British Columbia Relationship between fire, climate oscillations, and drought in British Columbia, Canada, 1920–2000 Spatial variation of trends in wildfire and summer drought in British Columbia, Canada, 1920-2000