Fire Causes in the Roaring Fork Valley

Who starts more fires in the Roaring Fork Valley, Mother Nature or humans? A recent study published in the Proceedings of the National Academy of Sciences found that humans ignite the vast majority of wildfires in the US. The research was conducted at the University of Colorado Boulder and examines trends in wildland ignitions and size in the US between 1992 and 2012. The study has been prominently featured national media outlets (NPR and AP) for its conclusions that humans are responsible for 84 percent of wildfires in the US.

We looked at the study in terms of our Roaring Fork River watershed. The map below is from the original study and shows the trend isn’t quite as simple as a lot of the articles made it out to be. It’s clear that geography plays a big role in results (something the original study looks at extensively). The area in the green circle is Southern Rockies eco-region, where we are in the Roaring Fork Valley (RFV). From this scale (each dot represents 50 km) it is impossible to draw any definitive conclusions about the RFV, but we can see trends in the Southern Rockies aren’t as straight forward as in the Appalachians or the southern coast of California..

Figure 1

Using the same dataset as the original study we looked at fires in the RFV. We compared the number of human-ignited fires with lightning-ignited fires:

Figure 2

Over the study period, natural ignitions are the primary cause of wildfires in the RFV, but humans are also a significant source of wildfire ignitions (Figure 2). 

Next, we looked to see if this relationship has shifted over time:

Figure 3

The two trend lines in Figure 3 show that the number of human-ignited fires is increasing, while the number of natural fires is essentially flat. 

In figure 4 we removed natural fires to focus on changes over time in human ignitions. We expected to see 2002 and 2012 (two infamously dry/low snow years in the RFV) stand out as high fire years but we were surprised that this wasn’t the case. 

Figure 4

Geography was an important variable in the original study so we plotted the fires on a map of the RFV. In figure 5 there are a two obvious trends: fires are more common at lower elevations and fires occurring in upper valleys fires are more common on the valley floors. The first trend is predictable due to the warmer weather at lower elevations and fire-prone Pinion Juniper forest which is prevalent at lower elevations in the RFV. The second trend is potentially a result of higher temperatures in valley bottoms. Overall, there’s not a clear geographic trend in fire ignition source in our area. Looking at the size of fires in Figure 5 it becomes obvious that fires in the RFV have been relatively small. The largest fire in the RFV with a known ignition source was 390 acres.

Figure 5

The big question is what does all this tell us? It’s tempting to say that the RFV is an exception to some of the trends happening elsewhere in the country and we don’t need to worry about the growing risk of human-caused fires. But we caution against this thinking for a few reasons.

To begin with, the dataset used for this study is relatively short, covering 20 years. Many of the forests found in the RFV have a long fire return interval (the amount of time between fires). Spruce-fir forest is 50-300 years and aspen forest is 30-100 years. It seems likely that the RFV has experienced increased use of wildlands and increased settlement of the wildand urban interface. These conditions have led to increases in human ignitions in other parts of the country, but in the RFV we have yet to see the impacts of fire ignitions due to a lack of longer term data.

Finally, the uptick in human caused fires over the past few years is distressing no matter how you look at it. As more people continue to access public lands for overnight use, we expect this trend is likely to continue. It is also interesting that we see human ignitions increasing in wet years, when we would not expect a high number of fires. This is corroborated by the conclusions of the original study which found human ignitions increasing the geographic range of fires as well as lengthening the fire season. Does increased management (such as fire-bans), education, and outreach during high risk fire seasons play a role in these results? 

When NPR asked Dr. Balch, the lead researcher of the study, for a solution to the increase in human-ignited fires, she suggested increasing the number of prescribed fires. Prescribed fires ignited during optimal conditions can be managed and controlled to burn a limited and carefully selected area. Much of the forest in the western United States naturally evolved with fire and depend on it for their survival. Fire will continue to be an important part of our ecosystem, but the use of prescribed fires in impacted and accessible forests enables these areas to burn in more safely and productively. This management practice was put to use in 2016 in the Hunter Creek Valley when ACES collaborated with the Pitkin County, the City of Aspen, the United States Forest Service, and Wilderness Workshop to burn nearly 900 acres outside of Aspen (see top photo). However, regardless of management using prescribed fire, we need to accept that large natural fires will still occur. 

~ Adam McCurdy, Forest Programs Director


Analysis based on original study by:

Balch, J. K., Bradley, B. A., Abatzoglou, J. T., Nagy, R. C., Fusco, E. J., & Mahood, A. L. (2017). Human-started wildfires expand the fire niche across the United States. Proceedings of the National Academy of Sciences, 114(11), 2946–2951.