Determining the ecological mechanisms of forest encroachment within the aspen parkland of western Canada

The encroachment of woody species into grassland and savanna ecosystems has been well document since the early 1800s. Within the parkland ecoregion of western Canada, trembling aspen (Populus tremuloides Michx.) has been one of the key tree species increasing in dominance. Aspen encroachment is best...

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Main Author: Lastra, Rod
Other Authors: Kenkel, Norm (Biological Sciences)
Published: 2011
Subjects:
Online Access:http://hdl.handle.net/1993/4828
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spelling ndltd-LACETR-oai-collectionscanada.gc.ca-MWU.1993-48282014-03-29T03:43:48Z Determining the ecological mechanisms of forest encroachment within the aspen parkland of western Canada Lastra, Rod Kenkel, Norm (Biological Sciences) Ford, Bruce (Biological Sciences) Lobb, David (Soil Science) Wilmshurst, John (Biological Sciences) Lieffers, Vic (University of Alberta, Department of Renewable Resources) woody encroachment trembling aspen aspen parkland browsing intraspecific variation clone vigor The encroachment of woody species into grassland and savanna ecosystems has been well document since the early 1800s. Within the parkland ecoregion of western Canada, trembling aspen (Populus tremuloides Michx.) has been one of the key tree species increasing in dominance. Aspen encroachment is best explained not by single mechanism, but rather by a number of interacting ecological factors. In this study I examined the ecological consequences of the clonal biology in aspen as a means to explain persistence and observed tree-grass ratios within grassland savannas of western Canada. Results suggest that aspen stands cycle between a “stable” phase characterized by a dense mature canopy, and an “unstable” phase characterized by canopy breakup and increased regeneration from root suckers. It is during this unstable phase that clonal encroachment is likely to occur. Within these mature stands, different-aged ramets promote persistence by maximizing developmental variation. Such a mechanism overcomes the functional phenotypic uniformity of ramets within a single age-structured stand. Results from my study indicate that physiological integration is beneficial to the growth and survivorship of regenerating and encroaching aspen ramets. My results also suggest that the benefits of physiological integration are greatest in more stressful environments, and in recently established post-fire ramets. Finally, the consequences of variation in adaptive ecological relevant traits among individuals was examined by determining differences in vigor among aspen clones in relation to the production of secondary compounds (phenolic glycosides). My results demonstrate a high degree of variation in leaf phenolic glycosides production among clones. A significant amount of this variation was accounted for by differences in clone vigor (within population: individual susceptibility hypothesis), with a smaller amount related to environmental differences (among populations). In all instances, vigorous clones were significantly higher in levels of phenolic glycosides compared with dieback clones, suggesting that some individuals may be predisposed to undergo density-independent mortality. This has important ecological implications, because it implies that one of the key mechanisms regulating population dynamics, community interactions and biodiversity may be related to intrinsic adaptive differences in susceptibility among individuals. 2011-09-02T14:24:36Z 2011-09-02T14:24:36Z 2011-09-02 http://hdl.handle.net/1993/4828
collection NDLTD
sources NDLTD
topic woody encroachment
trembling aspen
aspen parkland
browsing
intraspecific variation
clone vigor
spellingShingle woody encroachment
trembling aspen
aspen parkland
browsing
intraspecific variation
clone vigor
Lastra, Rod
Determining the ecological mechanisms of forest encroachment within the aspen parkland of western Canada
description The encroachment of woody species into grassland and savanna ecosystems has been well document since the early 1800s. Within the parkland ecoregion of western Canada, trembling aspen (Populus tremuloides Michx.) has been one of the key tree species increasing in dominance. Aspen encroachment is best explained not by single mechanism, but rather by a number of interacting ecological factors. In this study I examined the ecological consequences of the clonal biology in aspen as a means to explain persistence and observed tree-grass ratios within grassland savannas of western Canada. Results suggest that aspen stands cycle between a “stable” phase characterized by a dense mature canopy, and an “unstable” phase characterized by canopy breakup and increased regeneration from root suckers. It is during this unstable phase that clonal encroachment is likely to occur. Within these mature stands, different-aged ramets promote persistence by maximizing developmental variation. Such a mechanism overcomes the functional phenotypic uniformity of ramets within a single age-structured stand. Results from my study indicate that physiological integration is beneficial to the growth and survivorship of regenerating and encroaching aspen ramets. My results also suggest that the benefits of physiological integration are greatest in more stressful environments, and in recently established post-fire ramets. Finally, the consequences of variation in adaptive ecological relevant traits among individuals was examined by determining differences in vigor among aspen clones in relation to the production of secondary compounds (phenolic glycosides). My results demonstrate a high degree of variation in leaf phenolic glycosides production among clones. A significant amount of this variation was accounted for by differences in clone vigor (within population: individual susceptibility hypothesis), with a smaller amount related to environmental differences (among populations). In all instances, vigorous clones were significantly higher in levels of phenolic glycosides compared with dieback clones, suggesting that some individuals may be predisposed to undergo density-independent mortality. This has important ecological implications, because it implies that one of the key mechanisms regulating population dynamics, community interactions and biodiversity may be related to intrinsic adaptive differences in susceptibility among individuals.
author2 Kenkel, Norm (Biological Sciences)
author_facet Kenkel, Norm (Biological Sciences)
Lastra, Rod
author Lastra, Rod
author_sort Lastra, Rod
title Determining the ecological mechanisms of forest encroachment within the aspen parkland of western Canada
title_short Determining the ecological mechanisms of forest encroachment within the aspen parkland of western Canada
title_full Determining the ecological mechanisms of forest encroachment within the aspen parkland of western Canada
title_fullStr Determining the ecological mechanisms of forest encroachment within the aspen parkland of western Canada
title_full_unstemmed Determining the ecological mechanisms of forest encroachment within the aspen parkland of western Canada
title_sort determining the ecological mechanisms of forest encroachment within the aspen parkland of western canada
publishDate 2011
url http://hdl.handle.net/1993/4828
work_keys_str_mv AT lastrarod determiningtheecologicalmechanismsofforestencroachmentwithintheaspenparklandofwesterncanada
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