COS 106-1 - Impacts of Increased Fire Activity on Biodiversity in California Shrublands

Abstract: In recent decades the Western U.S. has experienced an increase in frequency of intense wildfires. This is a threat to native plant diversity in some ecosystems, producing vegetation type conversion (VTC) from native to non-native species. Where this is a major threat is California chaparral, as many dominant species are sensitive to short fire intervals. Those with limited resilience are obligate seeding shrubs because they lack resprouting ability and are entirely dependent on a dormant seed bank, which requires sufficient time between fires to accumulate. Short fire-return intervals are frequently implicated in VTC although some studies cast doubt on this as a significant factor. Better understanding of this is needed because of an upsurge in state and federal programs to utilize prescription burning to reduce fire hazard; treatments at at short intervals are often preferred although despite the threat of losing some species.

But, how short is too short? It is commonly assumed in the literature that this threshold is less than 8 years, however, this threshold is likely to be species specific. Our recent postfire studies have focused on fires that overlapped with prior fires at various intervals distributed throughout California. We investigated over 50 fires that burned in the last 3 years and included fire-return intervals from 2 to 96 years. Randomly distributed 32m2 plots were sampled for each species of woody shrubs and trees, recording the number of dead skeletons, resprouting individuals and seedlings.

The results show that the likelihood of VTC is a function of which species dominate the site. Obligate resprouting shrubs such as species of Quercus, Prunus, Rhamnus, Rhus are highly resilient to short fire return intervals. However, across the state we have found that many obligate seeding shrubs are at risk of extirpation with fire return intervals between 10-20 years. For example, some Ceanothus species have been extirpated by return intervals between 10-13 years of age. Arctostaphylos species may require even longer intervals. Facultative seeding species, which combine seeding and resprouting, are variable in response. The most widespread of these species is Adenostoma fasciculatum, which seems to be most resilient to short fire-return intervals.

In summary, thresholds of short fire-return intervals are species-specific and at current fire frequencies of 10-20 years we are seeing reductions in native biodiversity. This is an important consideration when doing prefire fuel treatments in chaparral as young stands are often targeted for treatment because burning is less hazardous.