Exotic annual grass alters fuel amounts, continuity and moisture content

Authors: Davies, Kirk; Nafus, Aleta

Submitted to: International Journal of Wildland Fire
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2012
Publication Date: 5/14/2013
Citation: Davies, K.W., Nafus, A. 2013. Exotic annual grass alters fuel amounts, continuity and moisture content. International Journal of Wildland Fire. 22:353-358.

Interpretive Summary: Exotic annual grass invasion is thought to alter fuels to promote large, frequent fires. However, comparisons of fuel characteristics between invaded and non-invaded plant communities are lacking. This information is needed to plan and justify treatments to reduce the risk of wildfires in annual grass-invaded communities. We compared fuel characteristics in annual grass-invaded and non-invaded plant communities. Fuel amounts were greater and fuel moisture was lower in annual grass-invaded compared to non-invaded plant communities. These results suggest that annual grass invasion substantially increases the risk of large, frequent wildfires. Information in this manuscript will assist land managers in planning and justifying treatments to reduce fuels in exotic annual grass-invaded plant communities.

Technical Abstract: 1. Invasion by exotic plants are one of the most serious threats to native plant communities, biodiversity, and ecosystem functioning. Of particular concern are exotic plants that alter disturbance regimes. Exotic annual grasses are believed to increase wildfire frequency to the detriment of native vegetation by increasing fine fuels and thus, creating a grass-fire cycle. However, information on differences in fuel characteristics between invaded and non-invaded plant communities are lacking or based mainly on speculation and anecdotal evidence. Thus, evidence is lacking that quantifies the potential for a grass-fire cycle in annual grass-invaded plant communities. 2. We compared fuel characteristics in plant communities invaded and not invaded by Bromus tectorum L., an exotic annual grass, at four sites in Oregon. At each site, an annual grass-invaded and non-invaded plant community was included in the study. Fuel biomass, cover, continuity, and moisture content were measured in 2010 and 2011. 3. Annual grass-invaded communities had higher fine fuel amounts, greater fuel continuity, smaller fuel gaps, and lower fuel moisture content than non-invaded plant communities. These conditions would increase the probability that ignition sources would contact combustible fuels and that fires would propagate. Fuel moisture content was low enough to readily burn more than a month earlier in annual grass-invaded communities, thereby expanding the wildfire season. The cumulative effect of these differences in fuel characteristics between exotic annual grass-invaded and non-invaded plant communities is a much greater risk of frequent, large-scale wildfire. 4. Synthesis and applications: Exotic annual grasses alter fuel characteristics to promote a grass-fire cycle that will perpetuate their dominance of the plant community and facilitate the invasion of adjacent communities. The decrease in fuel moisture, especially earlier in the year, in annual grass-invaded communities may facilitate the grass-fire cycle as much as or more than the increase in fine fuels. Fuel characteristics in the annual grass-invaded communities would also support faster spreading fires that would increase the risk to life and property and be more challenging to suppress. We suggest that fuel management will be needed to break the annual grass-fire cycle and reduce the threat to life and property.