Tuesday, 18 November 2003: 9:30 AM
Anthropogenic or Lightning: Ignition Source of Pre-Euro-American Fire Regimes in the Southern Sierra Nevada?
Dramatic change in southern Sierra Nevada fire regimes, unprecedented in the last two millennia, began in the 1860s. Considerable debate exists as to the specific source and importance of past ignitions that governed this regime. The debate centers on the relative influence of Native American versus weather generated lightning ignitions. However, understanding the root cause is confounded because reduction of indigenous populations was followed immediately by intense grazing that affected fine fuels. Comparison of pre-Euro-American fire history, reconstructed from fire scarred trees, with fire records may provide some insights into resolving the debate. In the East Fork watershed of Sequoia National Park a network of fire history sites is being developed that provides an unusual level of detail about the past spatial and temporal patterns of fire. These patterns are being compared to contemporary patterns of lightning ignitions recorded over the same landscape during the last 82 years. Patterns are interpreted in light of past vegetation, fuel, and topographic influences. They suggest that lightning caused ignitions could account for much of the pre-Euro-American fire occurrence within the drainage but not all of it, particularly at some locations within the watershed. The results indicate that the importance of past ignition sources, Native American or lightning, probably varied spatially across the landscape but still remains poorly understood.