Background and Rational
In Canada the boreal mixedwood forest, composed mainly of Populus tremuloides (aspen) and Picea glauca (white spruce), is the most widespread forest type, but in Alberta silvicultural operations and regeneration rules tend to lead to the establishment of relatively pure stands. More recently concern has risen over single species management reducing biodiversity and changing natural forest processes (Man and Lieffers 1999).
Underplanting involves planting a shade tolerant species (such as white spruce) in the understory of a mature stand (generally a shade intolerant species such as aspen). Underplanting white spruce in aspen-dominated stands was introduced in Alberta, Manitoba, and Ontario in the 1960s (Duffy and Nemeth 1965, Duffy and Nemeth 1967, Wang and Horton 1968, Dyck 1994) but was abandoned in the 1970s and 80s in favor of clearcutting and managing regeneration of the two species separately. In the 1990s underplanted was reintroduced as an alternative to single species management to address issues of competing interests on the landbase, white spruce regeneration difficulties after clearcutting, and a public demand for more ecologically based silvicultural systems (DeLong 1997). Underplanting white spruce in aspen-dominated stands mimics natural forest succession in the boreal, where white spruce regenerates in the understory of aspen stands after the aspen canopy starts opening up from self thinning, creating a mixedwood stand when the white spruce mature. While some studies looking at the survival and growth of white spruce underplanted in aspen-dominated stands have been conducted (DeLong 1997, Comeau et al. 2004, Comeau et al. 2009), the impact to the understory environment and vegetation has not been studied.
Differences between the understory environment and vegetation of broadleaf, mixedwood and conifer forests have been fairly well studied; canopy cover and composition have been shown to be important factors determining the composition of understory plant communities. Mixedwood and conifer forests are more similar to one another than to broadleaf forests; differences observed between mixedwood/conifer-dominated forests and broadleaf-dominated forests have been in canopy cover, litter depth, soil nitrogen, soil temperature, shrub cover, and herb and shrub richness and diversity (Macdonald and Fenniak 2007). Canopy trees can influence the understory through competition for resources and direct effects to environmental and edaphic conditions.
Underplanting involves planting a shade tolerant species (such as white spruce) in the understory of a mature stand (generally a shade intolerant species such as aspen). Underplanting white spruce in aspen-dominated stands was introduced in Alberta, Manitoba, and Ontario in the 1960s (Duffy and Nemeth 1965, Duffy and Nemeth 1967, Wang and Horton 1968, Dyck 1994) but was abandoned in the 1970s and 80s in favor of clearcutting and managing regeneration of the two species separately. In the 1990s underplanted was reintroduced as an alternative to single species management to address issues of competing interests on the landbase, white spruce regeneration difficulties after clearcutting, and a public demand for more ecologically based silvicultural systems (DeLong 1997). Underplanting white spruce in aspen-dominated stands mimics natural forest succession in the boreal, where white spruce regenerates in the understory of aspen stands after the aspen canopy starts opening up from self thinning, creating a mixedwood stand when the white spruce mature. While some studies looking at the survival and growth of white spruce underplanted in aspen-dominated stands have been conducted (DeLong 1997, Comeau et al. 2004, Comeau et al. 2009), the impact to the understory environment and vegetation has not been studied.
Differences between the understory environment and vegetation of broadleaf, mixedwood and conifer forests have been fairly well studied; canopy cover and composition have been shown to be important factors determining the composition of understory plant communities. Mixedwood and conifer forests are more similar to one another than to broadleaf forests; differences observed between mixedwood/conifer-dominated forests and broadleaf-dominated forests have been in canopy cover, litter depth, soil nitrogen, soil temperature, shrub cover, and herb and shrub richness and diversity (Macdonald and Fenniak 2007). Canopy trees can influence the understory through competition for resources and direct effects to environmental and edaphic conditions.
Figure 2 shows the differences in herb diversity (Shannon-Weiner) at the plot scale (alpha diversity) (a) and beta diversity (Whittaker's) among plots (b) for broadleaf, mixedwood and conifer stands in the Alberta boreal forest based upon data from Macdonald and Fenniak (2007). This study showed that while herb diversity (and shrub diversity, not shown in the figure) was highest in the broadleaf stands and decreased with the addition of white spruce, the beta diversity actually increased with the addition of spruce. Beta diversity is a measure of "turnover" diversity, indicating how the species composition changes over a spatial gradient. This increase in beta diversity with the addition of white spruce indicates that the species composition changes throughout the stands, creating a more heterogeneous understory in the mixedwoods and conifer stands. The boreal is well known for this heterogeneity and underplanting may assist in creating this heterogeneity in a way that has been naturally observed in the boreal ecosystem during succession.
Research Objectives
The purpose of this study is to try to determine the role underplanted white spruce play in the understory community, and determine if white spruce is an ecosystem engineer. The objectives of this study are to:
1. determine if underplanting white spruce in aspen-dominated stands changes the understory environment and subsequently the understory vegetation; and
2. if changes are observed:
a) are the changes more strongly observed closer to the base of the underplanted tree and less pronounced with increasing distance outward from the base of the tree?; and
b) does the spatial extent and strength of the effects increase with time passed since underplanting?
1. determine if underplanting white spruce in aspen-dominated stands changes the understory environment and subsequently the understory vegetation; and
2. if changes are observed:
a) are the changes more strongly observed closer to the base of the underplanted tree and less pronounced with increasing distance outward from the base of the tree?; and
b) does the spatial extent and strength of the effects increase with time passed since underplanting?
Fig. 3: Stands underplanted in various years in Calling Lake and Lac La Biche, Alberta (photos taken in 2009).
Expected Results
Based upon previous research (mentioned above) showing differences in the understory between broadleaf, mixedwood and conifer stands, white spruce appears to have an important role in understory development. There should be a change in the understory environment and vegetation from underplanting but the timing of these changes and the spatial extent to which changes may occur are not certain.
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