Wednesday, 19 November 2003: 2:30 PM
Climatic influences on fire regimes in the Lake Tahoe Basin
The role of natural disturbances such as fires, insect attacks, and windstorms in maintaining and changing species distribution and abundance patterns is widely recognized. In the Sierra Nevada Mountains, fire has affected most areas with the greatest frequency and patterns of succession, community structure, and composition and diversity have all been linked to recurring fires. Identifying processes that influence or modulate fire regimes is therefore central to understanding long-term vegetation dynamics. This study uses fire scars in Jeffrey pine (Pinus jeffreyi) to reconstruct the spatial and temporal pattern of surface fires in 8 contiguous watersheds for a 200 year period (1650-1850) and tests the hypothesis that fire occurrence is influenced by climate variation. Variation in winter precipitation was reconstructed using independent tree ring chronologies from near the Lake Tahoe Basin. Tree ring based reconstructions of the Southern Oscillation Index (SOI) and Pacific Decadal Oscillation Index (PDOI) were used to assess the influence of equatorial and north Pacific teleconnections on fire occurrence. For the entire period of record, high winter precipitation was characteristic of years without fires. Fire years, in contrast, were associated with low winter precipitation. The intensity of the winter drought also influenced fire extent with the most widespread fires occurring during the driest years. Years with the most widespread fires were also preceded by wet winters three years before an event. Widespread fires were also associated with changes in the phase of the PDOI with the most widespread fires occurring during the change from warm (positive) to cold (negative) conditions. We detected no relationship between the SOI and the timing and extent of fires. The relationship between climate and fire occurrence, however, was not stable over the period of record. From 1700-1775 there is no relationship between drought, PDOI, and fire occurrence. In the period from 1775-1850, the pattern of wet years prior to and dry conditions during widespread fire years emerges. This is also the period where the influence of PDOI phase changes is most pronounced. In the Lake Tahoe Basin, fire events are strongly influenced by climate variation. This suggests that ecosystem structure and composition may be more strongly controlled by non-equilibrium processes associated with large scale climate variation rather than local processes that control vegetation growth and development.