As wildfires burn across the western United States, people are asking why does the West burn so frequently? Was it like this in the past? To piece together ancient landscapes, paleoecologists act like biological sleuths by digging through layers of sediment in search of clues. Traditionally, they analyzed pollen grains to infer what types of plants existed in an ecosystem and how they changed in abundance over time. Thomas Brussel, doctoral candidate in the Department of Geography at the University of Utah wanted to know more. To understand how plants actually interacted with their environment, he linked plant characteristics, called functional traits, to pollen data to show how a plant community developed structurally and in terms of adaptation to natural disturbances.
Brussel and collaborators developed a new method for understanding how plant communities’ function in relation to wildfires over the past 13,000 years. The researchers combined pollen, charcoal and functional trait data with their new method to assess a plant community’s functional trait variation to understand ecosystems in a new way. They found that as wildfires burned more frequently, the forest shifted from having more fire-sensitive traits, such as short heights, to more fire-adapted traits, such as taller heights.
“The traditional methods look at who was there, like Douglas fir or hemlock, and relate changes in vegetation to a disturbance, like fire. Our method allows you to see what the plants were actually doing, what functional role they were playing, and how disturbance might play a role in selection,” said Brussel, lead author of the study that published in the Journal of Vegetation Science in late May. “This method has the potential to reveal things that we haven’t seen before. It’s a new approach to understanding how ecosystems worked in the past.”
The method has broad applications outside of just plant communities and wildfires; scientists working with paleo-proxies could adapt it to look at other natural disturbances or to a different environment altogether.
Fire-sensitive vs. fire-adapted
A common hypothesis in ecology states that fire-prone ecosystems, like those in western United States, burn frequently because fire has selected for a particular mix of plant species with more flammable characteristics over time. Brussel’s new method revealed a new insight into how modern fire regimes may have developed today.
“People ask whether modern fire activity is a trend due to climate change. People have questioned why the West is on fire all the time. But the key to understanding present conditions is in understanding the past. There’s great research out there that shows that fire activity in western United States increased over the last 10,000 years, possibly because forests have expanded and fuel conditions promoted fire,” said Brussel, who began the research during his master’s thesis at the University of Wyoming. “This research showed that the functional interactions between vegetation and fire may have also played a large role in why the West burns.”
Brussel piloted the method at a single site. Today, Breitenbush Lake, Oregon, is a mixed-conifer forest near the crest of the Cascade Mountain range. To understand the Breitenbush Lake of the past, Brussel analyzed vegetation and fire history data going back 13,000 years. Traditionally, paleoecologists use pollen grains as a proxy for the types of vegetation existing in an area—the abundance of pollen from different species indicate what vegetation was around, in what quantities, and how that changes over time. To assess fire activity, they use bits of charcoal. Brussel used existing data of 19 pollen types and charcoal analysis for the site, then assigned each plant species 14 functional attributes that describe either ability to withstand fire. He calculated an overall functional attribute score that describe changes in the abundance of functional attributes through time.
“We selected traits that represent opposite ends of the spectrum, in terms of adaptation to fire. For example, bark thickness. In the context of this study, if a fire erupts, trees and shrubs with thin bark ignite and die more readily than vegetation with thick, chunky-monkey bark that would be more adapted to surviving fire,” said Brussel.
The analysis provided evidence against the theory that fire-dependent plant communities evolve more flammable characteristics over time. Despite Breitenbush Lake’s increasing trend in the number of fires throughout the record, functional attributes scores that were significantly related to fire activity showed that the community became more fire adapted.
Expanding into other regions and fields
The proof of concept study analyzed one site, and Brussel is now applying the method to regions comprising North America. The project will assess whether the site-level analysis fits a larger pattern, or is an anomaly, and look at how other ecosystems developed. He hopes to collaborate with scientists across different fields.
“Our method can be extended to other types of disturbances, such as drought or insect outbreaks, and potentially other proxy data outside of pollen,” said Brussel. “It has broad applicability to a range of ecological and anthropological questions, including archaeology.”
Other co-authors include Simon Brewer, assistant professor of geography at the University of Utah, Thomas Minckley, associate professor of geography at the University of Wyoming, and Colin Long, professor of geography at the University of Wisconsin Oshkosh.
Brussel, T., & Minckley, T.A., & Brewer S.C., & Long C.J. (2018). Community-level functional interactions with fire track long-term structural development. J Veg Sci. 2018;29:450-458. Doi: https://doi.org/10.1111/jvs.12654