Burn and learn: creating and maintaining diversity with fire

[originally posted on www.foreststeward.com on Dec. 10, 2010]

Article reviewed: Long-term vegetation responses to reintroduction and repeated use of fire in mixed-conifer forests of the Sierra Nevada

By K.M. Webster and C.B. Halpern. Published in Ecosphere, Vol. 1(5): 1-17. Available for full download here (in this new and OPEN ACCESS journal).

The plot line: Sequoia and Kings Canyon National Parks have the longest history of using prescribed fire in Sierra Nevada forests. The authors of this article analyzed data that were collected in the parks over time from sites that were either burned once, burned twice, or not burned at all. They looked for differences in how the treatments influenced understory species composition during the 10 to 20 years that followed burns. The relatively long-term nature of the monitoring allowed them to detect delayed effects of the burning that otherwise may not have been detected. The long-tenured burning program in the parks also allowed them to characterize effects of single versus follow-up second-entry burns on composition. Burning led to increases in the total number of species, especially beginning 5 years after the burns. Shrub species were especially responsive to the first-entry burns, and were then maintained with the second-entry burns. Ground cover made up of most types of plants tended to increase following burns, especially 10-20 years after burns. The authors suggest that prescribed burning programs can be very successful for reducing fuel while also achieving desired species compositions. The frequency of burns, their relative proximity to each other, and the severity of burns are discussed as critical management factors for burning programs.

Relevant quote: “If fire is to play an important role in restoration… it will need to be maintained as a frequent and spatially dynamic process on the landscape.”

Relevance to landowners and stakeholders:

Most people who have visited national or state parks in the Sierra Nevada have seen the signs and brochures that tout the important role that fire has had in shaping the forest. From an ecological perspective, the importance of fire is incontestable. It did indeed shape the forest. And now the forest has been forever altered because of fire suppression. We can never truly restore the forest conditions of the past, but using prescribed fire is one way that we can achieve modern goals of fuel reduction, species composition, and forest health.

Whether or not people who see the pro-fire signs in parks walk away as advocates for prescribed burning, however, depends a lot on their non-ecological perceptions of fire. One important factor is how sensitive their health is to smoke. In my neighborhood, I can talk to people endlessly about the benefits of fire, but all of those benefits are quickly forgotten when smoke from my prescribed fire creeps into their yard and starts to negatively impact their breathing. This is the great challenge for all of those pyro-foresters out there: How do you increase burning activity when the public's tolerance for smoke keeps declining?

This research suggests that increases in biodiversity following burning and then maintenance of diversity by repeat burns is one benefit that could be used to support fire (the more obvious one is the benefit of reducing high-severity fires that burn peoples’ houses down and put LOTS of smoke in the air, but that’s not really what this article was about). Biodiversity could even be put in the context of its importance for public health, as was demonstrated in last week’s post. Burning will likely remain a tough sell to anyone, however, who has asthma and who is already living in an area with high levels of air pollution (e.g. the Central Valley).

Relevance to managers:

• Burn when you can- many of us managers have far greater constraints than those within the parks. We work in the urban interface or have other logistical, legal, or risk-aversion challenges. While it is important to have objectives and clear plans about where/when to burn, often it is determined by weather and availability of personnel. So you end up burning when you can.

• Expect the unexpected- Fire is a blunt tool. In this study, a wide range of species composition responses resulted from patchiness in fire severity during burns. Other variable factors of species responses include the climate following the burn and availability of seed within soil banks or from nearby parent populations. Don’t expect to be able to predict exactly how the species composition will responds to fire. One thing that can be expected- continuing to suppress fire without doing anything else will decrease biodiversity until a high-severity fire occurs (which will, by the way, also increase biodiversity but not necessarily in a good way).
  

• The first burn is critical- It appeared from this study that the first burn after a long period of fire suppression was the critical one in influencing species diversity and cover over the next two decades. The second burn was important in maintaining composition, but did not appear to increase or decrease composition with anywhere near the same magnitude as the first burn. (the authors seemed to suggest that the second burns “enhanced” diversity, but I did not see that happening in the data or analysis that was given).

• The mechanical + burn option- This study did not include mechanical treatments that were followed by burns, but it makes me think of the mechanical+burn treatment as a potentially effective option for increasing biodiversity. A mechanical treatment that alters fuel structure in such a way that allows a hot yet manageable fire will likely see a distinct increase in richness and ground cover which can then perhaps be maintained by subsequent fires.

Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:

This study compares three basic treatment options: burning once, burning twice, and not burning at all. It is not a comparison of burning with mechanical treatments, so it should not be interpreted as a recommendation of burning over mechanical treatments. It is more a demonstration (a very interesting and important one) of the benefits of burning versus not burning at all.

It is also worth noting that the second-entry burns did not appear to have been applied in an experimental fashion (e.g. they were not selected randomly). It makes me wonder if they were selected for second entry burn because they burned in a particular way during the first burn. It does appear from the graphs (Fig. 1A) that the second-entry burns may have been selected for a second burn because the first burn was particularly hot. The pre-treatment tree density prior to second burns looks lower than the tree density 10 years after the first entry burns. This could be just due to chance or not important, but it does make me wonder about how these areas were selected for burning or not burning.

Their repeated measures analysis seems to give a lot of leverage to the early responses since there were fewer measurements available for later responses. Normally a repeated measures analysis will only include plots that have data that span the entire time range being considered. But they seemed to use a non-traditional type of analysis that let them use all of the plots even if they didn’t have data across the entire period. This is probably completely justifiable, but they didn’t explain why they chose this type of analysis, which I bet most other researchers have never used. Typically a non-standard approach has more discussion of why it was used.

Their management recommendation that fires be done “asynchronously” with white fir seed production in order to avoid a pulse of white fir establishing after fire does not seem feasible. Most fires are done in the fall, after seeds have already been dispersed (white fir dispersal is usually in August or September). So tree seedlings establishing after a prescribed fire will come from seeds produced after the fire. White fir cones mature in one year, so we can’t tell what the cone crop will be like following the fire. A slightly more feasible (but still challenging) option might be to time higher severity fires with bumper crops of pine species. Pine cones take 2 years to mature, so it is more feasible to time the treatment with next year’s seed crop. This wouldn’t decrease white fir establishment necessarily, but it might increase the relative amount of pine establishment compared to white fir.