Introduction

Mountain pine beetle outbreak in Alberta

Mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) outbreaks have killed millions of hectares of lodgepole pine (Pinus contorta var. latifolia) forests across western Canada. This has had a big effect on the timber supply and has caused economic losses for the forest industry (Corbett et al. 2016), as well as changes to wildlife habitats (Saab et al. 2014) and the carbon cycle (Kurz et al. 2008). In extreme cases, such outbreaks can convert large forested landscapes from carbon sinks to carbon sources (Kurz et al. 2008).

Beginning in 2005, the mountain pine beetle experienced a range expansion, spreading eastward across the Rocky Mountains as a result of warmer temperatures that created optimal conditions for epidemic levels, infesting extensive stands of lodgepole pine in Alberta (Cullingham et al. 2011). Following a peak in the province's population in 2019, a 98% MPB population decline has been observed in more recent years, thereby shifting the epidemiological status from an epidemic to an endemic state (Government of Alberta 2024).

Mountain Pine Beetle attacked canopy in the red stage seen from top of the Kennedy Siding Flux Tower in Interior BC. Photo by Matthew Brown, UBC.

Following a MPB infestation, attacked trees die through inoculation of blue-stain fungi as beetles bore through the bark, disrupting transpiration and inducing desiccation. Needles typically turn red after one year of attack, marking the onset of tree mortality (Fig. 1; Safranyik and Carroll 2006). Over the following three to five years, the tree progressively loses its needles, reaching the grey attack stage once defoliation is complete (Chojnacky et al. 2000).

Figure 1. Stages of trees killed by mountain pine beetle infestation.

Attack severity varies across outbreaks, with mortality ranging from 10 to 100% of overstory pine trees in mature lodgepole pine forests (Axelson et al. 2018). In Alberta stands experiencing ≥50% MPB-induced mortality, studies have documented low pine regeneration and declines in pine density, volume, and basal area (Axelson et al. 2018; Lieffers et al. 2024; Bassil et al. 2025). Beyond the overstory, understory plant communities respond to increased light availability following tree mortality, with diversity and productivity increasing across the gradient of MPB-induced mortality (Pec et al. 2015), and associated changes in nutrient cycling. As standing snags collapse over time, large surface fuel loads accumulate, increasing surface fire intensity (Schoennagel et al. 2012). Salvage logging is commonly implemented to recover economic value from beetle-killed timber and reduce these fuel loads, but also removes standing dead trees and coarse woody debris compared to naturally recovering stands, with potential consequences for wildlife habitat, soil nutrient dynamics, and understory community composition (Lewis 2009).

With the epidemic now in decline, millions of hectares of MPB-affected lodgepole pine forest remain across Alberta, many with standing dead trees that have been accumulating for nearly two decades. As these snags collapse and surface fuel loads increase, the window for active management is narrowing. This study addresses whether salvage logging produces different long-term outcomes than natural recovery, to inform decisions about wildfire risk reduction, lodgepole pine regeneration, and vegetation management in affected areas.

Objectives

To evaluate long-term overstory recovery dynamics following mountain pine beetle mortality and salvage logging, to inform whether active management is needed to maintain lodgepole pine dominance.
To evaluate whether mountain pine beetle-induced mortality and salvage logging increase the risk of high-intensity wildfire compared to undisturbed lodgepole pine stands, by quantifying long-term changes in downed woody material accumulation across disturbance types.
To characterize understory plant community successional trajectories across disturbance types over a 17-year period, to determine whether beetle-affected and salvage logged stands converge or diverge in community composition.
To guide decisions about whether active interventions such as lodgepole pine planting or vegetation management are needed, by determining whether soil nutrient availability is compromised across post-disturbance recovery trajectories.

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