The height of the spruce in centimeters is a good proxy for the overall viability of the stocktypes. It's an easily measurable characteristic with a limited margin of error that allows us to see the impact of the forb on its companion spruce, and compare it to the control. Overall there is a very apparent trend between sow date and spruce height (Figure 7). Spruce height increases with the later sow dates, which is what would be expected. The earlier sow dates introduce enough competition on the spruce to hinder its overall height.
For the aster 512A ten and twelve weeks, and the 615A eight and ten weeks, spruce height was not significantly different than the control (p-values of 0.9862, 0.5603, 0.9958 and 0.1072 respectively). In the aster 615A twelve weeks, spruce height was actually significantly greater than the control (p-value of <0.0001). This increase in spruce growth relative to the control is likely due to the bigger stock size allowing more development in the plug, versus an effect of forb presence.
The fireweed 512A twelve weeks, and 615 ten weeks treatments had a spruce height that was not significantly different than the control (p-values of 0.9758 and 0.7767 respectively). The 615A twelve week treatment for fireweed had a spruce height that was significantly greater than the control (p-value of <0.0001); however this again is likely due to the larger stock size allowing more development in the plug.
Figure 7. Spruce height in centimeters by treatment. Error bars represent the standard error of the mean.
Now that we have determined that we have viable stocktypes in terms of spruce height, the next goal was to achieve significant forb cover onsite. Aster cover was generally low in the background at 5% while increasing to around 15% on average in the aster stocktypes (Figure 8). All aster stocktypes had relatively the same level of cover with the exception of the six week, 615A stocktype. It had significantly increased cover relative to the other stocktypes, which is logical as it had the biggest plug and the earliest head start of the aster stocktypes. The stocktypes deemed viable based on spruce height (512A ten and twelve weeks and 615A eight, ten and twelve weeks) all had significantly greater forb cover than the control (p-values of 0.0002366, 0.0022, 0.002854, 0.002186 and 0.001046 respectively).
Figure 8. Percent aster cover in a 0.5 meter by 0.5 meter quadrant centered on the hitchhiker plant by treatment. Error bars represent the standard error of the mean.
When we switch to look at fireweed cover we see that it is more pronounced in the background (Figure 9), but that the fireweed stocktypes still have more cover than the aster and control stocktypes . No individual fireweed stocktype has significantly more fireweed cover than any other. The stocktypes deemed viable based on spruce height (512A twelve weeks and 615A ten and twelve weeks) all had significantly greater forb cover than the control (p-values of 0.01398, 0.01054 and 0.001051 respectively).
Figure 9. Percent fireweed cover in a 0.5 meter by 0.5 meter quadrant centered on the hitchhiker plant by treatment. Error bars represent the standard error of the mean.
The final research question was whether or not the forb cover could reduce the cover of undesirable non-native herbaceous species. The results were highly variable between all stocktypes (Figure 10) with some stocktypes showing a significant reduction, while most were not significantly different than the control. The eight week fireweed in the 615A stocksizes is the only treatment that showed significantly lower non-native herbaceous species cover (p-value =0.0099). This was one of the stocktypes deemed viable based on spruce height and forb cover, but the result is concerning as there is no apparent trend as would have been expected.
Figure 10. Percent non-native herbaceous species cover in a 0.5 meter by 0.5 meter quadrant centered on the hitchhiker plant by treatment. Error bars represent the standard error of the mean.
When looking at all the factors overall, the ten and twelve week sow dates for either forb species in either stock size are the preferred choices, as they allow for a comparable spruce while still significantly increasing forb cover. The ten week, 615A fireweed stocktype does show the potential to reduce weeds, but the lack of an overall trend suggests there may be some bias. Between the 512A and 615A stocksizes there is a tradeoff between reduced costs for the smaller stocksize, versus the 12 week in 615A allows for a potentially larger spruce to be grown relative to the control. These tradeoffs would need to be considered on a site by site basis.
The fact that we have been able to create a viable stocktype that allows for an unaffected spruce seedling to be grown, and significant forb cover to be achieved, is already sufficient proof that this method should be implemented by industry. To further show its importance we decided to compare the costs. For a standard 100m by 100m wellsite the costs for using hitchhiker plants is the same as planting trees and seeding forbs themselves, which therefore shows that our method is more effective for the same cost. Waiting for natural encroachment does have the potential to save on purchasing any forb seed, however encroachment takes multiple years and the cost of adding forbs to create hitchhiker plans is approximately the same as one year's lease on a site. This again favors hitchhiker planting as the most cost effective method with the highest success rate for revegetating wellsites, as well as potentially being an extremely effective method for revegetating large scale oil sands mines.
References:
Alberta Environment and Sustainable Resource Development (ESRD) (2013) 2010 Reclamation criteria for wellsites and associated facilities for forested lands. Edmonton, Alberta
R Core Team, 2015. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria.
Schoonmaker A, Sobze J-M, Fraser E, Marenholtz E, Smreciu A, Powter C.B., Mckenzie M (2014) Alternative Native Boreal Seed and Plant Delivery Systems for Oil Sands Reclamation.