The response of a Rocky Mountain forest system to a shifting disturbance regime

Climate change is likely to drive widespread forest declines and transitions as temperatures shift beyond historic ranges of variability. Warming temperatures and shifting precipitation patterns may lead to increasing disturbances from wildfire, insect outbreaks, drought, and extreme weather events, which may greatly accelerate rates of ecosystem change. However, the role of disturbance in shaping forest response to climate change is not well understood. Better understanding the impacts of changing disturbance patterns on forest decline and recovery will allow us to better predict how forest ecosystems may adapt to a warming world. Severe wildfires and bark beetle outbreaks are currently affecting large areas of forest throughout western North America, and increasing disturbance size and severity will have uncertain impacts on forest persistence. The goal of my dissertation was to investigate the factors shaping disturbance response in a region of the San Juan Mountains, Colorado, which has undergone impacts from a high-severity spruce beetle outbreak and wildfire in the last 15 years. I conducted three separate studies in the burn area of the West Fork Complex wildfire, which burned in 2013, and in surrounding beetle-affected spruce-fir forests. The goals of each study were to 1) assess whether the severity of spruce beetle outbreaks occurring before wildfire resulted in compounded disturbance interactions affecting vegetation recovery, 2) determine how the severity of each disturbance type influenced fine-scale below-canopy temperature patterns across the landscape, and 3) assess how conifer seedling regeneration densities were influenced by effects of disturbance severity on seed dispersal, temperature, and vegetation structure. I found that disturbances influenced seedling regeneration and ecosystem resilience through several mechanisms. First, pre-fire beetle outbreak severity was negatively correlated with post-fire vegetation cover, indicating that the combined disturbances were inhibiting regeneration beyond what may have been expected with fire alone. Second, disturbances had significant effects on below-canopy temperatures, with burned areas ~0.5 °C warmer than unburned forest areas and differences in overnight minimum temperatures resulting from loss of live canopy in unburned, beetle-killed forests. Third, the large fire size and high severity resulted in very little spruce seed dispersal or conifer regeneration in most of the burned area, while spruce regeneration in unburned forest was negatively correlated with increasing overstory mortality from the spruce beetle. My results indicate that disturbance is playing an important role in determining the future trajectory of the forest in my study area. The West Fork Complex fire has caused a severe ecosystem transformation, has increased landscape exposure to warming temperatures, and is preventing forest re-establishment as a result of a lack of seed sources. The spruce beetle outbreak has not resulted in such a severe transformation, but is possibly leading to reduced forest resilience by reducing spruce seedling re-establishment and by altering fuel structures to make forests more prone to high soil burn severity if fire follows within ~10 years. Warming of below-canopy microclimates is not exacerbated by spruce beetle outbreak, and is rather partially offset by cooling of overnight temperatures. These findings provide insights into how forest responses to climate change may be shaped by disturbance processes, which are occurring with increasing severity and frequency worldwide.

Name thesis:degree_name

Doctor of Philosophy (Ph.D.)

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Ecology

Grantor dc:publisher

Colorado State University. Libraries

Year dc:date.issued

2019

Authors dc:creator

• Carlson, Amanda R., author
• Sibold, Jason S., advisor
• Assal, Timothy J., committee member
• Hobbs, N. Thompson, committee member
• Rocca, Monique E., committee member

• disturbance
• spruce beetle
• wildfire
• forest
• climate change
• subalpine

Statement dc:rights

• Copyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.

Language dc:language.iso

eng, English

Identifier URI

https://doi.org/10.25675/3.018669

OAI identifier oai:identifier

oai:mountainscholar.org:10217/199841