Background
The boreal zone covers 552 million hectares in Canada (Brandt et al. 2013). It provides numerous goods and services to Albertans such as: timber, fresh water, climate regulation, water purification, recreation, primary production, and nutrient cycling (Hassan et al. 2005). These goods and services make the boreal zone important to the province’s resource-based economy. The boreal is also very important to biodiversity. A mosaic of mixedwood stands and pure forest stands interspersed with wetlands provides numerous important habitats for many different types of wildlife species (Downing & Pettapiece 2006).
There are several natural disturbances that occur on large and small scales in the boreal, including: fire, windthrow, insect and disease outbreaks. Fire is the most important disturbance in the boreal zone (Brandt et al. 2013). It is the major stand-renewing agent and plays an essential role in regulating the effects of insects and diseases (Weber & Flannigan 1997). Fire also shapes boreal ecosystems by influencing species composition, age structure, productivity, and biodiversity (Weber & Flannigan 1997). Many tree species in the boreal mixedwood are adapted to stand renewing fires that commonly take place in the region. The dominating tree species on the landscape are trembling aspen, jack pine (Pinus banksiana Lamb.), black spruce, and white spruce (Downing & Pettapiece 2006). Aspen are well adapted to disturbance because they have a high capacity for vegetative reproduction (Greene et al. 1999). Root suckers are the main form of regeneration after disturbance; however stump sprouts can also be produced (Greene et al. 1999). Jack pine and black spruce both hold aerial seed banks in the form of serotinous and semi-serotinous cones, enabling them to respond quickly to disturbances (Greene et al. 1999). Other late successional species, such as white spruce and balsam fir, rely on seed dispersal from surviving individuals after a disturbance (Greene et al. 1999). Overall, tree species in the boreal mixedwood are able to cope well with natural disturbances on the landscape, whether it is an immediate or delayed response. Previous models of post-fire tree regeneration at the stand level are often developed after experimental fires which may not experience the same levels of burn severity as wildfires (e.g. Weber et al., 1987; de Groot et al., 2004). Other studies use chronosequences of forest development to model post-fire regeneration and succession but fire severity in these studies is often uncertain (e.g. Arseneault, 2001; Greene and Johnson, 1999), and the assumptions for using chronosequences are often not met (Johnson and Miyanishi, 2008). Therefore, from a forest land management perspective, this study will allow for a better understanding of the range of possible successional trajectories and future forest compositions after fire. |
Objectives
The goal of this study is to determine the effects of fire severity and stand characteristics on the regeneration of boreal forest tree species after fire. The research questions of this study include:
- Does fire severity affect the regeneration of aspen stands?
- Does habitat type (i.e. bog or upland) have an effect on regeneration of black spruce stands?
- Do stand characteristics (i.e. age, height, basal area, competition) have an effect on regeneration in aspen and black spruce stands?
- Does fire severity and habitat type have an effect on the regeneration of ground cover vegetation?
Hypothesis
I hypothesize that fire severity will have an effect on aspen regeneration. Fire severity should impact root suckering because in order for a tree to produce root suckers, there must be parent tree mortality and an intact root system. Therefore, low severity burns may not produce many suckers because not all parent trees would experience mortality. On the other hand, high severity burns will cause mortality in all the trees, but could damage the roots causing less suckering. Therefore, I hypothesize that moderate severity burns will encourage the highest sucker response. There will be a high percentage of mortality and root systems should remain intact. This, however, could be proven wrong if higher fire severities do not damage roots as much as I am predicting or if low severity fires cause mortality in more trees than I am expecting.
Habitat type will likely have an effect on black spruce regeneration. In order for germination of black spruce seeds to be successful mineral soil must be exposed. Since bogs have large buildups of organic material, it is unlikely that much mineral soil will be exposed after fire. Therefore, I expect there to be less black spruce regeneration on bog black spruce sites. This could be proven wrong if fire is able to consume more organic material than expected in the bog stands. Greater amounts of exposed mineral soil in upland stands could also facilitate the regeneration of other vegetation cover, which will compete with black spruce seedlings, causing less regeneration on these sites, proving my hypothesis wrong.
I expect stand characteristics to effect the regeneration of both aspen and black spruce stands. Larger and older trees should produce more propagules compared to smaller, younger stands. Therefore, older stands with a greater basal area should have highest regeneration numbers. Higher amounts vegetative ground cover will likely result in lower amounts of tree regeneration, as the seedlings will have to compete with other vegetation for resources. I suspect grass to have the greatest negative impact on seedling growth because they have very dense root systems and can also form dense above ground patches, blocking out light. These hypotheses could be proven wrong if there is a threshold level for the number of aspen suckers produced in a single area. Perhaps aspen seedlings can only regenerate at a maximum density, therefore having higher pre-fire stand basal areas may not have any significant impacts because there are not enough resources to facilitate more root suckering. My hypotheses regarding vegetaion cover as competition could be proven wrong if fire is able to reduce the amount of understory vegetation enough that trees do not need to compete for resources, therefore understory vegetation cover will have no effect.
I believe fire severity and habitat type should both have an effect on the amount of understory vegetation returning after fire. Higher fire severities will consume more topsoil and the seedbank found within it, compared to lower severity fires. Different habitats have different productivity levels. I suspect that habitat types with higher productivity levels of early successional species will have the greatest amount of ground cover. This hypothesis could be proven wrong if fire severities do not have as great as an effect on seedbanks as expected. Fires are very common to the boreal landscape, and seeds may be more resilient to high severity fires than expected.
Habitat type will likely have an effect on black spruce regeneration. In order for germination of black spruce seeds to be successful mineral soil must be exposed. Since bogs have large buildups of organic material, it is unlikely that much mineral soil will be exposed after fire. Therefore, I expect there to be less black spruce regeneration on bog black spruce sites. This could be proven wrong if fire is able to consume more organic material than expected in the bog stands. Greater amounts of exposed mineral soil in upland stands could also facilitate the regeneration of other vegetation cover, which will compete with black spruce seedlings, causing less regeneration on these sites, proving my hypothesis wrong.
I expect stand characteristics to effect the regeneration of both aspen and black spruce stands. Larger and older trees should produce more propagules compared to smaller, younger stands. Therefore, older stands with a greater basal area should have highest regeneration numbers. Higher amounts vegetative ground cover will likely result in lower amounts of tree regeneration, as the seedlings will have to compete with other vegetation for resources. I suspect grass to have the greatest negative impact on seedling growth because they have very dense root systems and can also form dense above ground patches, blocking out light. These hypotheses could be proven wrong if there is a threshold level for the number of aspen suckers produced in a single area. Perhaps aspen seedlings can only regenerate at a maximum density, therefore having higher pre-fire stand basal areas may not have any significant impacts because there are not enough resources to facilitate more root suckering. My hypotheses regarding vegetaion cover as competition could be proven wrong if fire is able to reduce the amount of understory vegetation enough that trees do not need to compete for resources, therefore understory vegetation cover will have no effect.
I believe fire severity and habitat type should both have an effect on the amount of understory vegetation returning after fire. Higher fire severities will consume more topsoil and the seedbank found within it, compared to lower severity fires. Different habitats have different productivity levels. I suspect that habitat types with higher productivity levels of early successional species will have the greatest amount of ground cover. This hypothesis could be proven wrong if fire severities do not have as great as an effect on seedbanks as expected. Fires are very common to the boreal landscape, and seeds may be more resilient to high severity fires than expected.