In this blog post, Kyra Clark-Wolf discusses the research behind her recent co-authored article: Fire-regime variability and ecosystem resilience over four millennia in a Rocky Mountain subalpine watershed.
🔥 Fire: what does it do?
Fire is a longstanding process that has been part of the Earth System for millions of years. Typically, vegetation regrows following a fire, and stores of carbon and nutrients like nitrogen build back up over time. This ability of an ecosystem to recover with similar characteristics as before a fire is called resilience. However, forest resilience has limits. The combined effects of ongoing shifts in climate and fire activity could undermine the ability of forest ecosystems to recover after fires. The result might be changes in forest composition and structure (e.g., density), changes in the cycling of carbon and nutrients, or even wholesale shifts to different vegetation types.
To anticipate how forests could respond to changes in fire activity under climate change, we need to know how fires affect ecosystem processes. This understanding is difficult to get in forests that historically experienced infrequent, high-severity fires (i.e., fires that kill most trees, once every century or more on average), like subalpine forests. That’s because recovery after fires can take decades, and there are few long-term datasets over these timeframes. Therefore, we need different methods in these types of forests. This is where natural archives come in.
🔎 Our study
Contrary to what you might expect, lakes give us great opportunities to learn about fire. Lakes are excellent natural archives, because they act like big traps that accumulate materials from the surrounding landscape, and they preserve them in layers of mud over thousands of years. By taking cores of sediment from the bottoms of lakes, and measuring the amounts of charcoal, pollen from different vegetation types, and elements like carbon and nitrogen, we can reconstruct when fires happened in the past and how the ecosystems surrounding the lake changed over time.
Well, that’s just what we did. We took cores from a small, subalpine lake in the U.S. Northern Rocky Mountains, which is a region that has been experiencing increasing area burned, linked with warmer, drier climate in recent decades. We selected this site because we know how the climate changed there over the past several thousand years from previous research, showing an overall trend of increasing effective moisture. This allowed us to test whether forest recovery after fires differed between a period of overall drier climate conditions, before about 2800 years ago, and a period of overall wetter climate conditions in the past 2000 years.
🌲 What we found
Spoiler: ecosystem recovery after fires did not differ between overall wetter and drier periods in the last 4800 years. Forest vegetation and carbon and nutrient cycling consistently recovered within about 50-80 years after fires. This is a pretty striking finding, given that fire frequency and forest structure did change over this period. In the last 2000 years, fires became about 77% less frequent, and the abundance of subalpine conifer species increased, consistent with what might be expected in a cooler, wetter climate where fuels rarely dry out enough to support fire. Despite these long-term changes, the forest remained resilient to each individual fire, recovering within decades.
Even more strikingly, the patterns of how fires affect ecosystem processes varied from fire to fire. We interpreted this variation as resulting from differences in fire severity, which determined what happened in the decades after the fire. About a third of the fires in the record were high-severity fires that killed most trees in a significant proportion of the watershed, resulting in post-fire soil erosion and losses of nitrogen that left signals in the lake sediment, similar to a site from Colorado. Another third of the fires were smaller or moderate-severity fires that killed fewer trees, resulting in increased nutrient availability but no soil erosion. The final third of the fires had indistinct ecosystem effects.
✔️ Lessons learned
Our study paints a detailed picture of the effects of fires and how subalpine forests recover over decades. We found both resilience and notable diversity in ecosystem responses to fires over our 4800-year record. This highlights that fire severity is an important factor affecting carbon and nutrient cycling in forests. Looking forward, a modest increase in fire frequency could likely be supported in our study region without undermining forest resilience to fire, given that fire frequency was higher in the past. Nevertheless, not only shifts in fire frequency, but also fire severity, will determine the overall effects of changing climate and fire activity on ecosystem processes.
Kyra Clark-Wolf, University of Montana, USA.
Read Kyra’s paper online: Fire-regime variability and ecosystem resilience over four millennia in a Rocky Mountain subalpine watershed
