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Ryan K. Danby

Bio: Ryan K. Danby is an academic researcher from Queen's University. The author has contributed to research in topics: Tundra & Climate change. The author has an hindex of 16, co-authored 27 publications receiving 1000 citations. Previous affiliations of Ryan K. Danby include Wilfrid Laurier University & University of Alberta.
Topics: Tundra, Climate change, Ecotone, Vegetation, Shrub

Papers
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Journal ArticleDOI
TL;DR: In this article, the authors examined recent tree line dynamics at six topographically different, but climatically similar, sites in south-west Yukon, Canada and found that tree line elevation and stand density increased significantly during the early to mid 20th century.
Abstract: Summary 1 Boundaries between forest and tundra ecosystems, tree lines, are expected to advance in altitude and latitude in response to climate warming. However, varied responses to 20th century warming suggest that in addition to temperature, tree line dynamics are mediated by species-specific traits and environmental conditions at landscape and local scales. 2 We examined recent tree line dynamics at six topographically different, but climatically similar, sites in south-west Yukon, Canada. Dendroecological techniques were used to reconstruct changes in density of the dominant tree species, white spruce (Picea glauca), and to construct static age distributions of willow (Salix spp.), one of two dominant shrub genera. Data were analysed to identify periods and rates of establishment and mortality and to relate these to past climate. 3 Tree line elevation and stand density increased significantly during the early to mid 20th century. However, this change was not uniform across sites. Spruce advanced rapidly on south-facing slopes and tree line rose 65–85 m in elevation. Tree line did not advance on north-facing slopes, but stand density increased 40–65%. Differences observed between aspects were due primarily to the differential presence of permafrost. Additional variability among sites was related to slope and vegetation type. Results were less conclusive for willow, but evidence for an advance was found at two sites. 4 Increases in stand density were strongly correlated with summer temperatures. The period of rapid change coincided with a 30-year period of above average temperatures, beginning in 1920. The highest correlations were obtained using a forward average of 30–50 years, supporting the hypothesis that tree line dynamics are controlled more by conditions influencing recruitment than by establishment alone. 5 The changes observed at several sites are suggestive of a threshold response and challenge the notion that tree lines respond gradually to climate warming. Overall, the results provide further evidence to support the idea that the pattern and timing of change is contingent on local, landscape, and regional-scale factors, as well as species’ biology.

334 citations

Journal ArticleDOI
TL;DR: In this paper, the authors experimentally warmed 40 large, naturally established, white spruce seedlings at alpine treeline in southwest Yukon, Canada, using passive open-top chambers (OTCs) distributed equally between opposing north and south-facing slopes.
Abstract: From 2001 to 2004 we experimentally warmed 40 large, naturally established, white spruce [Picea glauca (Moench) Voss] seedlings at alpine treeline in southwest Yukon, Canada, using passive open-top chambers (OTCs) distributed equally between opposing north and south-facing slopes. Our goal was to test the hypothesis that an increase in temperature consistent with global climate warming would elicit a positive growth response. OTCs increased growing season air temperatures by 1.8°C and annual growing degree-days by one-third. In response, warmed seedlings grew significantly taller and had higher photosynthetic rates compared with control seedlings. On the south aspect, soil temperatures averaged 1.0°C warmer and the snow-free period was nearly 1 month longer. These seedlings grew longer branches and wider annual rings than seedlings on the north aspect, but had reduced Photosystem-II efficiency and experienced higher winter needle mortality. The presence of OTCs tended to reduce winter dieback over the course of the experiment. These results indicate that climate warming will enhance vertical growth rates of young conifers, with implications for future changes to the structure and elevation of treeline contingent upon exposure-related differences. Our results suggest that the growth of seedlings on north-facing slopes is limited by low soil temperature in the presence of permafrost, while growth on south-facing slopes appears limited by winter desiccation and cold-induced photoinhibition.

217 citations

Journal ArticleDOI
TL;DR: In this article, the authors surveyed plant communities and measured key abiotic variables across forest-tundra ecotones in six alpine valleys, each with a north and a south-facing slope, in two mountain ranges of southwest Yukon.
Abstract: Questions How does treeline community composition vary between elevations, aspects and slope angles in the alpine subarctic and what are the specific abiotic factors governing this variability? How do species richness and rates of community turnover vary from low to high elevation across the forest-tundra ecotone? What do the results indicate about future vegetation change? Location: Kluane Region, southwest Yukon, Canada. Methods We surveyed plant communities and measured key abiotic variables across forest-tundra ecotones in six alpine valleys, each with a north and a south-facing slope, in two mountain ranges of southwest Yukon. We used non-metric multidimensional scaling to identify patterns in plant community composition and infer which abiotic variables drive these patterns. We calculated species richness and community dissimilarity at regular elevational intervals to assess trends in richness and rates of community turnover within the ecotone. Results Plant communities varied more with aspect and slope angle than they did with elevation. Aspect-related differences were driven by warmer soil temperatures and deeper active layers on south compared to north-facing slopes, while differences related to slope angle occurred only on north-facing slopes and were driven by soil moisture. Species richness increased with elevation on north-facing slopes and showed no trend with elevation on south-facing slopes. Rates of community turnover were higher on south-facing than north-facing slopes. Conclusions Plant community composition within the forest-tundra ecotone is driven primarily by soil temperature and, to a lesser extent, soil moisture, both of which vary more in relation to aspect and slope angle than they do between forest and tundra elevations. We recommend that models of vegetation change in subarctic alpine regions address the possibility of change occurring at different rates and in different directions depending on the topographic characteristics of each slope. This article is protected by copyright. All rights reserved.

67 citations

Journal ArticleDOI
09 Dec 2009-Arctic
TL;DR: In this article, small-scale vertical aerial photographs taken in 1947 and 1948 covering 200 km2 of the Kluane Ranges, southwest Yukon, were compared with corresponding photos taken in 1989 for the purpose of characterizing changes in the distribution and abundance of white spruce at the alpine treeline.
Abstract: Small-scale vertical aerial photographs taken in 1947 and 1948 covering 200 km2 of the Kluane Ranges, southwest Yukon, were compared with corresponding photographs taken in 1989 for the purpose of characterizing changes in the distribution and abundance of white spruce ( Picea glauca (Moench) Voss) at the alpine treeline. Digital photogrammetry, including orthorectification and on-screen interpretation, was supplemented by stereoscopic inspection of the original prints. Qualitative assessment of change across nine image pairs was accompanied by quantitative analysis of changes in spruce density and elevation using 1 hectare plots and 100 m wide elevational belt transects, respectively, superimposed on the orthorectified images. Significant changes were observed over the 41 years, but the degree of change varied throughout the study area. The most common changes were an increase in canopy size of individual trees and an increase in stand density resulting from the establishment of new individuals. Several instances of treeline advance were also observed. An absence of major natural disturbances or widespread land use change indicates that treeline change is attributable to climate. Results from concurrent dendroecological studies indicate that these dynamics represent only part of the total extent of change to occur during the 20th century.

63 citations

Journal ArticleDOI
TL;DR: In this article, the relationship between remotely-sensed data, using the normalized difference vegetation index (NDVI), and annual ring-width indices obtained from trees and shrubs in the boreal mountains of southwest Yukon was examined.

51 citations


Cited by
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Journal ArticleDOI
TL;DR: As all tree species will be suffering lags, interspecific competition may weaken, facilitating persistence under suboptimal conditions, and species with small populations, fragmented ranges, low fecundity, or suffering declines due to introduced insects or diseases should be candidates for facilitated migration.
Abstract: Species distribution models predict a wholesale redistribution of trees in the next century, yet migratory responses necessary to spatially track climates far exceed maximum post-glacial rates. The extent to which populations will adapt will depend upon phenotypic variation, strength of selection, fecundity, interspecific competition, and biotic interactions. Populations of temperate and boreal trees show moderate to strong clines in phenology and growth along temperature gradients, indicating substantial local adaptation. Traits involved in local adaptation appear to be the product of small effects of many genes, and the resulting genotypic redundancy combined with high fecundity may facilitate rapid local adaptation despite high gene flow. Gene flow with preadapted alleles from warmer climates may promote adaptation and migration at the leading edge, while populations at the rear will likely face extirpation. Widespread species with large populations and high fecundity are likely to persist and adapt, but will likely suffer adaptational lag for a few generations. As all tree species will be suffering lags, interspecific competition may weaken, facilitating persistence under suboptimal conditions. Species with small populations, fragmented ranges, low fecundity, or suffering declines due to introduced insects or diseases should be candidates for facilitated migration.

1,719 citations

Journal ArticleDOI
11 Sep 2009-Science
TL;DR: These rapid changes in terrestrial, freshwater, and marine systems, presage changes at lower latitudes that will affect natural resources, food production, and future climate buffering, and highlight areas of ecological research that deserve priority as the Arctic continues to warm.
Abstract: At the close of the Fourth International Polar Year, we take stock of the ecological consequences of recent climate change in the Arctic, focusing on effects at population, community, and ecosystem scales. Despite the buffering effect of landscape heterogeneity, Arctic ecosystems and the trophic relationships that structure them have been severely perturbed. These rapid changes may be a bellwether of changes to come at lower latitudes and have the potential to affect ecosystem services related to natural resources, food production, climate regulation, and cultural integrity. We highlight areas of ecological research that deserve priority as the Arctic continues to warm.

1,157 citations

Journal ArticleDOI
TL;DR: This article used repeat photography, long-term ecological monitoring and dendrochronology to document shrub expansion in arctic, high-latitude and alpine tundra.
Abstract: Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra

1,153 citations

Journal ArticleDOI
TL;DR: Diffuse treelines may be more responsive to warming because they are more strongly growth limited, whereas other treeline forms may be subject to additional constraints.
Abstract: Treelines are temperature sensitive transition zones that are expected to respond to climate warming by advancing beyond their current position. Response to climate warming over the last century, however, has been mixed, with some treelines showing evidence of recruitment at higher altitudes and/or latitudes (advance) whereas others reveal no marked change in the upper limit of tree establishment. To explore this variation, we analysed a global dataset of 166 sites for which treeline dynamics had been recorded since 1900 AD. Advance was recorded at 52% of sites with only 1% reporting treeline recession. Treelines that experienced strong winter warming were more likely to have advanced, and treelines with a diffuse form were more likely to have advanced than those with an abrupt or krummholz form. Diffuse treelines may be more responsive to warming because they are more strongly growth limited, whereas other treeline forms may be subject to additional constraints.

1,003 citations

Journal ArticleDOI
TL;DR: To understand the potential negative and positive contributions of soil microbes to land–atmosphere carbon exchange and global warming requires explicit consideration of both direct and indirect impacts of climate change on microorganisms.
Abstract: There is considerable interest in understanding the biological mechanisms that regulate carbon exchanges between the land and atmosphere, and how these exchanges respond to climate change. An understanding of soil microbial ecology is central to our ability to assess terrestrial carbon cycle–climate feedbacks, but the complexity of the soil microbial community and the many ways that it can be affected by climate and other global changes hampers our ability to draw firm conclusions on this topic. In this paper, we argue that to understand the potential negative and positive contributions of soil microbes to land–atmosphere carbon exchange and global warming requires explicit consideration of both direct and indirect impacts of climate change on microorganisms. Moreover, we argue that this requires consideration of complex interactions and feedbacks that occur between microbes, plants and their physical environment in the context of climate change, and the influence of other global changes which have the capacity to amplify climate-driven effects on soil microbes. Overall, we emphasize the urgent need for greater understanding of how soil microbial ecology contributes to land–atmosphere carbon exchange in the context of climate change, and identify some challenges for the future. In particular, we highlight the need for a multifactor experimental approach to understand how soil microbes and their activities respond to climate change and consequences for carbon cycle feedbacks.

884 citations