Post-fire Understory Plant Communities: the Calwood Fire

In Colorado, 2020 was notable for being the most destructive wildfire season in the state's recent history. That year alone, more than 660,000 acres burned with multiple new records set for Colorado’s largest wildfires. The Calwood Fire, a mixed-severity fire that burned north of the City of Boulder, burned approximately 10,000 acres, primarily made of ponderosa pine and Douglas fir forests. 

While low-elevation ponderosa pine and Douglas fir forests are fire-adapted, they are adapted to frequent, low to moderate severity fire. In contrast to low severity fire that maintains open forests of mature trees and abundant understory communities (shrubs, wildflowers and grasses), high severity fire results in high tree mortality, post-fire erosion and flooding, and delayed or inhibited forest recovery. While the effects of high severity fire on trees are evident, long-term impacts on understory communities are not as apparent. Depending on the ecosystem or time since fire, moderate and/or high severity fire has been found to both increase or decrease the diversity of understory species. 

To examine how fire severity influences understory plant communities, I have been collecting data this summer in the Calwood burn scar within Heil Valley Ranch in Boulder County for my master's thesis. In addition to capturing data on what species are growing and how abundant those different species are, I am also collecting soil cores to examine what species are present in the soil seed bank. This upcoming fall I will be germinating my soil samples in the greenhouses at the Gardens’ York Street location. This experiment will contribute to our understanding of what species have dormant seeds resting in the soil seed bank and thus what species are likely to return following subsequent fire or disturbance events. 

This field season has been humbling with many hot days, endless difficulty in identifying grasses, charcoal-stained clothes and expansive fields of downed logs. I am beyond grateful for all the amazing volunteers and friends who have joined me out in the field to collect some quality data!

This article was contributed by Botany Assistant Meredith Prentice.