Wednesday, August 4, 2010 - 4:40 PM

COS 70-10: The causes and consequences of natural tree clearings that reduce fire severity in western forests

Sarah E. Dalrymple, University of California, Davis and Hugh Safford, USDA Forest Service, Pacific Southwest Region.

Background/Question/Methods   The feedback between fire and vegetation in an ecosystem often results in regionally characteristic fire regimes and plant adaptations to fire. For example, Jeffrey pine trees, which inhabit a fire-prone ecosystem, have thick bark and self-pruned branches that resist heat damage and prevent the spread of fire to the tree crown. Recent observations indicate that a more indirect method of fire-resistance also exists in Jeffrey pines. Following fires, ring-shaped clearings, free of litter, are maintained around tree trunks, even when needles accumulate elsewhere on the forest floor. Evidence from a wildfire near South Lake Tahoe suggests that these clearings reduce both damage and mortality to trees. Therefore, determining the forces responsible for maintaining clearings is crucial to understanding a previously unknown natural process by which fire severity is mediated. This study uses field surveys and experiments to determine 1) how common and geographically widespread clearings are and 2) what mechanisms underlie the maintenance of clearings around trees.

Results/Conclusions   Surveys in Jeffrey pine forest indicate that clearings are four times more common in recently burned forest than in unburned forest. In addition, the proportion of trees with clearings declines with time since the last fire, indicating that recurrent fire is necessary to maintain clearings. Thus the exclusion of fire from forests eliminates these features and their ecological effects. Formica sibylla ant nests are found in a majority of clearings, and results from field experiments suggest that ants play a role in removing needles deposited in clearings. However, this behavior appears to be independent of ant nest site and may instead be related to creating optimal ant foraging space. Although the exact motivation behind ant clearing behavior remains to be determined, it appears that ant ecosystem engineering dramatically alters fire severity and tree fitness. This highlights an unexpected benefit of reintroducing fire to an ecosystem where it was once an integral process.