Forest floor soil pH was found to not be significantly affected by an interaction between treatment and year (F = 1.2048, p = 0.3371). Treatment type was found to have no significant effect on soil pH (F = 2.0756, p = 0.2049), while year was found to have a significant effect. (F = 40.8609, p < 0.001). When comparing pH trends across years, regardless of treatment, the soil pH in the pre-treatment and treatment year was not significantly different. Soil pH increased in alkalinity 1 year post-treatment compared to pre-treatment and treatment years. This trend of increasing alkalinity continued to rise significantly 16 years post-treatment, resulting in the most basic forest floor compared to all other years.

Figure 1. Interval plot depicts mean forest floor soil pH across treatment years, and between mountain pine beetle (MPB) treatment types. Error bars denote 95% Confidence Intervals (CI), and letters A-B denote significant difference in pH between single treatment type across years (alpha = 0.05). Letters X-Z denote significant difference in overall soil pH between years (alpha = 0.05). Data was collected in the summers of 2008, 2009, 2010, and 2025 in lodgepole pine stands near Robb, Alberta, were soil samples were taken and analyzed back at the University of Alberta.  

My findings suggest that mountain pine beetle attacks cause the forest floor to become more basic overall across years, regardless of MPB treatment type. I observed a significant increase in soil pH between the pre-treatment and treatment years, compared to 1 year post-treatment and 16 years post-treatment, with the 16-year post-treatment value being the most basic. Despite the increasing alkalinity of the soil, the overall pH remains well within the range expected of Albertian pine forests. Long-term monitoring of jack pine (Pinus banksiana Lamb) forests by the Wood Buffalo Environmental Association (WBEA) in Fort McMurray, Alberta, showed that the mean forest floor soil is between 4.46 and 4.61 for 2012 through 2024 (Unreleased data). Additionally, a previous study in 15-57 year old lodgepole pine forests near Robb, Alberta, around where this study took place, found that the average pH of those forest stands was 4.03 (McIntosh et al 2012). Given that the soil pH in this study was on average between 3.58 and 3.95 for 2008 through 2025, the increase in pH across years did not cause the soil to surpass the normal range for pine forest soil pH. 

Previous research surrounding the effects of bark beetles on soil pH has been limited, and is often reported in conjunction with other measures of soil health, such as carbon and nitrogen ratios, and concentration of other soil ions. Nonetheless, past studies have demonstrated mixed results on bark beetle infestations affecting soil pH. One study in Mexico looked at montezuma pine (Pinus montezumae) infected with a bark beetle of the Dendroctonus genus, and found that soil pH did not differ between pines infected with the beetle and those not infected (Vázquez-Ochoa et al 2021). These montezuma pine results are in contrast with Griffin et al 2011 chronosequence study of bark beetle-attacked lodgepole pine forests in Yellowstone, where it was found that soil was more alkaline in the red and gray phase of attack compared to the control and 30 years after attack, where the soil was more acidic. Additionally, Kopáček et al 2023 found that in Norway spruce (Picea abies) infested by Ips typographus bark beetles, the forest floor pH had a significant increase in pH from before the attack to roughly 20 years following the initial attack phase. Looking at my findings, I am unable to draw a connection between the increase in alkalinity and MPB attack, since the control and salvage also had a pH increase. If the MPB treatments did have an effect on the pH, I would have expected the control pH to stay consistent with previous years, and there to be a difference in the 50% and 100% kill only. However, this was not the case for my findings, which leads to the conclusion that MPB does not affect soil pH, or at least no effect that can be seen 16 years post-treatment. 

Regarding the salvage treatment, previous studies suggest that my results do not follow the normal response of forest floor pH after whole tree harvesting. Thiffault et al 2011 review of harvesting effects on the boreal forest determined that soil becomes more acidic in response to harvesting. Similarly, Brais et al 1995 study in Quebec found an increase in soil acidity following harvesting in balsam fir, white birch, and black spruce-dominated forests. Another study in the mixedwood forests in the Northwest Territories, Canada, found that pH did not differ between control and clear-cut forests (Bock and Van Rees 2002). These three studies contrast my results for salvage forest floor pH, which suggest increasing alkalinity over time, not soil acidification. Having the salvage plot increase in pH in conjunction with the other treatments also seeing a similar increase, leads to the thought that something else may be affecting the soil pH, rather than the treatments applied. 

Climate change could be a proposed option for why the soil has shifted to be more alkaline. As proposed effects of increasing temperatures and higher atmospheric carbon dioxide levels have suggested that soil would undergo acidification, rather than alkalization (Rengel 2011; Kinnunen et al 2013). However, other long-term measures of Alberta forests have not found such an increase (WBEA 2025), as well as my own data suggesting the opposite trajectory for boreal forest floor pH. One reason that the soil could have shifted to become more basic is due to the cold temperatures of the region slowing decomposition of the organic litter accumulated (Holden et al 2015; Allison and Treseder 2011). Since pine needle decomposition increases the acidity of the soil, a decrease in the rate of decomposition could lead to the litter holding onto the ions necessary for soil acidification. Litter decomposition could explain why the two post-treatment years were more basic compared to the pre-treatment and treatment years, since the litter accumulation from the beetles hasn’t occurred. Following this thought of litter decomposition, this could also explain the control and salvage stands also had an increase in soil pH. Given the control stands, advanced age increasing surface litter, and the salvage plots' amount of fallen biomass, they could have more biomass on top of the soil, holding onto the acidifying ions. Additionally, both Kopáček et al 2023 and Griffin et al 2011 found shifts in the carbon:nitrogen ratios and also noted an increase in alkalinity in their sites affected by MPB. While the increase in alkalinity aligns with what was seen in my own MPB treatment sites, I also had an increase in the non-MPB treatment sites. Similar to the litter decomposition reason for increased pH, the increase could have also happened in the control due to the advanced age, causing C:N ratio shifts similar to the MBP sites. Furthermore, the removal of the trees and the subsequent growth of new ones in the salvage could have also led to similar shifts in the ratio of C:N seen in Kopáček et al 2023 and Griffin et al 2011 studies.

I did have some limitations that might have affected the results of my study. The first being that I did have to use a different pH meter compared to what was used in the first three years of this project. While pH probes are meant to get the same reading regardless of the one that was used if calibrated properly, there is always the chance the differences in pH probes could have shifted the results. Secondly, I was unsuccessful in getting the repeated measures aspect of my statistics to work, meaning that I had to use the average pH from each year rather than raw pH values for each individual point that was sampled each year.