Effects of lodgepole pine death due to mountain pine beetle and forestry management on understory plant communities

Background/Question/Methods

Subalpine forests throughout the western United States are experiencing an unprecedented irruption of mountain pine beetle (Dendroctonus ponderosae) resulting in widespread death of lodgepole pine (Pinus contorta). Managers aim to enhance lodgepole regeneration through dead tree removal and soil disturbance, but little is known of how this will affect composition and invasibility of understory plant communities. The understory will experience decreased competition and increased disturbance, possibly leading to an increase in available nutrients, moisture, and light. This may in turn affect community composition, richness, and diversity. This study aims to examine the abiotic and biotic effects tree death and management activities have on lodgepole forest understory. Species adapted to disturbed environments, including invasive species, may increase in abundance, as will community functional traits associated with those species. To test this, four management treatments of varying disturbance intensity were imposed in Colorado lodgepole pine forests with extensive beetle damage. Plant cover was measured over two years and community functional traits were collected and measured according to standard functional trait procedures. Gravimetric soil moisture and soil nitrogen content were also measured. Ordination was run to quantify community changes and community functional trait and abiotic differences between treatments were analyzed using repeated measures.

Results/Conclusions

We found that abiotic characteristics were affected by management, with treatments of higher disturbance exhibiting higher moisture, plant available nitrogen, and cover of bare soil. Community composition varied by treatment, with higher richness, and increased richness over time, in more disturbed treatments. Species with functional traits advantageous to disturbed environments (high leaf area and specific leaf area), which included several invasive species, comprised higher cover in more disturbed treatment plots. In addition, invasive species richness was higher in more disturbed treatments. Finally, leaf area and percent cover bare soil were positively correlated, suggesting that intensity of disturbance may drive changes in functional trait abundance in the understory community. Results indicate that management disturbance alters abiotic and biotic conditions of subalpine understory in forests affected by mountain pine beetle. While management treatments may enhance lodgepole recruitment, they alter plant communities, increasing the abundance of species adapted to disturbed environments. In response to lodgepole death due to mountain pine beetle and subsequent forest management, we may see widespread shifts in understory communities. This may include an increased risk of establishment and spread of invasive species, which could threaten native plant communities in the subalpine understory.