Post-fire Vegetation Response at Chickaree Lake

Lake cores taken from Chickaree Lake provide insight into fire history and vegetation response to fire.

Photo courtesy of Barrie Chileen

The Questions: How has vegetation responded to wildfires over the past 2,500 years at Chickaree Lake? Is there evidence that regional climate changes influenced the long-term vegetation dynamics over this time period?

Wildfire activity is a topic of interest to many people living in Colorado and the Western United States. Wildfires are a naturally occurring process, an important component to pine forest ecosystems, and a natural ‘change agent’. But, they can be devastating to nearby human communities. Are wildfires occurring more frequently? Do forests recover differently now than they did thousands of years ago? How has changes in climate affected vegetation composition following wildfire?

Charcoal fragment and pine pollen, as seen under a microscope.

Photo courtesy of Barrie Chileen

The Project

Fire history and vegetation composition were examined using a sediment core taken from Chickaree Lake, in Rocky Mountain National Park. Chickaree Lake is within a lodgepole pine dominated forest ecosystem on the West side of the park. Charcoal fragments were used to identify fire events; while fossilized pollen grains were used to identify vegetation species present at different intervals in core depth. Previously conducted radiocarbon dating of the sediment core provided an age-depth chronology so that charcoal and pollen samples can be compared over time.

The Results

Thirty-one types of pollen were identified within 52 samples. Pine tree (Pinus) pollen was the most dominant pollen type found throughout the record. Other dominant pollen types were Picea (spruce), Artemisia (sage, sagebrush, etc.) and Chenopodiaceae (goosefoot). Less dominant but significant in their contribution to long-term trends were Arceuthobium (mistletoe), Rosaceae (rose, cinquefoil, etc.), and Poaceae (grasses).

While Pinus remained the dominant pollen type throughout the 2,500 years investigated, two distinct pollen zones were evident in the record – Zone 2: 1,155-2,500 years ago, and Zone 1: 1,155 years ago to present. Zone 2 had higher pollen representation from grasses, fir, alder, ragweed and greasewood, whereas Zone 1 had higher percentages of pollen from mistletoes, spruce, sagebrush and members of the rose and sunflower families. Pinus was also found in a higher overall percentage of the pollen make up within Zone 2. This shift between zones is consistent with a regional change in climate, which has been interpreted by related studies to be a regional increase in winter precipitation.

Evidence of four high severity fires and another eleven lesser fire events were found within the sediment core record, with the most recent high severity fire happening 161 years ago (~1782). Tree pollen was found in significantly lower amounts following fire events. Artemisia pollen was found in higher abundance following fire, likely due to the opening of the forest canopy. A decrease in fire activity was observed in Zone 1, with six of the fifteen fires (including two of the four high intensity fires) happening during that time period. Lower abundance of Pinus pollen following Zone 1 fires suggests that more recent fires could have been more intense, which suggests potential recovery stress in modern fire contexts.

Stratigraphic diagram of selected pollen taxa found within the 52 samples. The relative amounts (%) of each pollen type is shown throughout the chronology of the sediment core. High severity fire events are represented by red lines. Lower severity fires are represented by the blue lines.

Diagram courtesy of Barrie Chileen

Barrie Chileen spent hours at the microscope deciphering the mysteries of lake mud. She identified 31 types of pollen within 52 lake core samples.

Photo courtesy of Barrie Chileen

The Researcher

Barrie Chileen is a recent graduate from Kansas State University. Using a sediment core from Chickaree Lake, Barrie examined pollen and charcoal to reconstruct past wildfire occurrence and vegetation communities over the past 2,500 years. Barrie earned her Master’s degree in geography in May 2019 and plans to continue her investigations into vegetation and fire ecology.

Reference: Chileen, Barrie V. Vegetation response to wildfire and climate forcing in a Rocky Mountain lodgepole pine forest over the past 2,500 years. Master’s thesis, Kansas State University, 2019.