Fires, Humans, Exotics, and Native Plants in Southern CaliforniaBy Rolf Muertter Last fall we experienced one of the worst wildfires in the history of southern California, resulting in the loss of thousands of homes, and several lives. But they have also had a major impact on our native plant communities. Again, we have to ask ourselves to what extent wildfires today are part of the natural fire-cycle, and to what extent they are exacerbated by human activity. The issue is complicated by a number of factors, such as the effects of fire suppression, urban sprawl, the introduction of non-native invasives, and air-pollution. Some recent research results are shedding new light on these issues.
The myth that wildfires are always destructive has been debunked decades ago. We now know that vegetation types in Mediterranean climates are usually fire-adapted. In southern California these include coniferous forests, chaparral, and coastal sage scrub. These vegetation types not only tolerate fire, but actually require it for long-term maintenance of the ecosystem. Coulter pine (Pinus coulteri) cones, for example, require fire to open. Many chaparral shrub species, such as Chamise (Adenostoma fasciculatum), resprout after a fire. In other species, such as Whispering Bells (Emmenanthe penduliflora), germination is induced by nitrogen dioxide, which is present in smoke. However, the destructive intensity of the recent forest fires was the result of decades of fire-suppression, leading to the build-up of fuel. When a fire breaks out, and is fanned by hot and dry Santa Ana winds, huge flames are created that burn the tree crowns, resulting in the death of the trees. A good example is the Cedar fire in San Diego county, which, according to Richard Minnich, professor of geography at UC Riverside, led to the "extirpation of an ecosystem." It may take more than a century for a mature forest to come back, and the pre-fire mix of oaks and conifers may never reappear. Without suppression, fires burn with much lower intensity, and they don't burn the tree crowns.
For a long time, it was thought that fire-suppression was equally problematic in chaparral. Supposedly, chaparral fires used to be frequent and small, but fire suppression efforts have led to senescent stands of chaparral with lots of dead undergrowth, leading to fewer, but much larger and more catastrophic fires. Recent research by Dr. Jon Keeley of the USGS shows that this is not the case, because fire suppression efforts have been ineffective. Fire frequency has actually increased during the last 50 years due to human activity and urban sprawl. Fire suppression does not work because the most destructive fires burn when the Santa Ana winds blow. During Santa Ana winds, fires are just unstoppable, and fuel treatments are ineffective. There is evidence that this is not due to fire-suppression, but is part of the natural fire regime for southern California chaparrel. This is based on an analysis of historical records of wildfires in southern California. What has changed is the frequency of wildfires. Fires in coastal southern California are rarely ignited by lightning, which is the most common natural cause further inland, and Keeley estimates that the natural frequency was about one to two times a century for a given area. Now, some areas experience fires several times a decade. A consequence of this increase in frequency of wildfires is the conversion of native shrublands to non-native grasslands. This is because shrubs need a few decades to recover from the effects of these fires.
To make matters worse, burnt areas are often seeded with non-native annual grasses for erosion control. There is no evidence that this works, and it may actually lead to greater erosion. And there is evidence that seeding leads to the displacement of native species. Unfortunately, once non-native grasses become established, they increase the frequency of wildfires, which are much harder to fight than shrub-fires.
It has been suggested that seeding be done with native species. However, this is not very effective for several reasons. First, if the first rain after the fire is heavy, the seeds may just end up getting washed away. Second, if the varieties used for seeding are not local, they may lead to a contamination of the local gene pool. Using locally collected seeds for seeding after a fire would require establishing seed collections for every possible location, which is probably not feasible. The best thing to do is nothing. Just wait for the seeds that have survived the fire in the soil seed bank to germinate on their own after the winter rains have arrived.
Another factor encouraging the growth of non-native annuals is smog. According to research done by plant ecologist Dr. Edith Allen at UCR, nitrogen from pollution is deposited onto the soil, fertilizing exotic grasses. This is most noticeable in areas that receive a lot of pollution from Los Angeles, such as Riverside. Based on a ten year survey of annuals of the Box Springs Mountain near UCR, Dr. Minnich has found the replacement of native wildflowers and shrubs by exotic grasses to be catastrophic. Of course, the smog doesn't stop in Riverside. Even desert areas east of LA are fertilized by airborne pollutants. The resulting growth of exotic annual grasses has led to fires, even in Joshua Tree National Park. This is very worrisome, since the desert vegetation there is not fire-adapted.
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