Mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins), is a native bark beetle to the western side of the Rocky Mountains that primarily infect lodgepole pines (Pinus contorta Dougl. ex Loud. var. latifolia), in addition to other species in the Pinus genus (Cullingham 2011). MPB typically only attacks stressed trees during low population levels, however, during rapid erupting populations massive amounts of healthy trees are also affected; making MPB the most significant insect induced mortality in North American pine forests (Pec et al 2015; Axelson et al 2018). Colonization of a lodgepole pine by MPB starts with the adult beetle burrowing into the tree to gain access to the phloem, where they lay eggs, and feed in all life stages (Bentz and Powell 2014). In addition to the initial ‘green phase’ of MPB colonization, where the trees still look healthy with only the eggs in the tree, there are also the red and grey phases (Biology of... [date unknown]). During the red phase, the beetles have successfully killed the tree and the needles start to turn red, then the grey phase is marked by the tree having lost all its needles (Safranyik and Carroll 2006). 

Historically, MPB outbreaks have been limited in Alberta due to the harsher climate, causing cold-induced mortality, and/or a lack of heat accumulation to allow full beetle lifecycles (Sambaraju et al 2012). However, in the last two decades, MPB outbreaks have extended into novel parts of Alberta’s forests with the help of climate change (Bentz and Powell 2014). Seasonal temperature warming now allows the beetle to inhabit places that have been historically too cold, as these areas do not experience temperatures needed to kill off the beetles. Combine multiple consecutive years of warm and dry weather with weakening vigor in large trees, and populations of MPB are able to drastically increase in novel areas like Alberta (Bassil et al. 2025). 

Insect outbreaks effects are not limited solely to the trees they infect, but can also indirectly affect the soil chemistry. As the insects attack, nutrients held within the plant are leached out or deposited as leaf or woody litter onto the top soil (Maynard et al. 2014). With all the new nutrients present from the attacked trees now in the soil, many aspects of the soil can change, including pH. Soil pH is referred to as the ‘master soil variable’ due to its effect on a multitude of biogeochemical processes needed for plant growth and microbial processes (Neina 2019). In regards to plant process pH can control the solubility of trace elements needed for plant growth and development, in addition to affecting the mineralization of carbon and nitrogen (Niena 2019). Not only is soil pH vital for plant processes, but it is needed for nitrification within the soil as well. Nitrification is the process by which nitrogen is changed from its inorganic form to the organic form, which is usable by plants. A study conducted in boreal forests of Quebec, Canada, found that soil pH explained 92% of the variation between high and low nitrification potential among the sites (Ste-Marie and Paré 1999). Additionally, soil pH plays a role in the distribution of key microbes found within forest soil. Near Fort McMurray, Alberta, Canada, soil pH, along with the presence of woody debris were vital factors in explaining microbial community composition (Dimitriu et al. 2010). In an earlier study by Dimitriu and Grayston 2009, found that soil pH explained 34% of the phylogenetic variance in community structure, and 14-18% of the taxon-based dissimilarities. The effect that pH has on microbial communities' diversity can have reaching implications on the processes performed by those microbes, such as decomposition of organic matter, nutrient assimilation into soil, and inorganic element transformations (Holguin et al. 2024). Ultimately, these processes influence the nutrients available in the soil for plants, which can in turn affect the plant community composition. 

Looking at data before the MPB treatment was applied, pH regardless of treatment was lower than in post-treatment years. During the treatment year the salvage plots had a higher forest floor pH than the control and 50% kill, with the 100% kill plots being an intermediate in pH. On year post treatment the control has the lowest forest floor pH compared to the other treatments. Along with the 50% kill and 100% having an intermediate pH between the control and the salvage treatments (McIntosh and Macdonald 2013).

This study aims to answer the following questions: How is lodgepole pine forest floor soil pH impacted sixteen years after a simulated mountain pine beetle attack across the four treatment types? How has the forest floor soil changed since the initial red attack phase of the mountain pine beetle? 

Based on the previous data from McIntosh and Macdonald (2013), showing that the 50% kill shifted from more acidic initially to slightly more basic after treatment, I hypothesize that the continual death of the lodgepole pines will have continued to shift the 50% kill pH to be more basic. Additionally, given the 16-year growth period, I hypothesize that the salvage pH will shift to be more similar to the control pH as the forest has started to regenerate.