Fire history of coniferous riparian forests in the Lake Tahoe Basin

Background/Question/Methods

Fire is an important ecological process in many western U.S. coniferous forests, yet high fuel loads and rural home construction lead to the suppression of most wildfires. Using mechanical thinning and prescribed burning, managers often try to reduce fuels in strategic areas with the highest fuel loads. Riparian forests, however, are often designated as areas where limited management action can take place within a fixed-width zone. These highly productive zones have developed heavy fuel loads capable of supporting stand-replacing crown fires that erode stream channels, eliminate important wildlife habitat and degrade ecosystem function. Objectives of this study are to determine whether adjacent coniferous riparian and upland forests burned historically with different frequencies and seasonalities, whether they had different stand structures and fuel loadings, and whether the relationship varied by forest type, riparian zone widths, and precipitation regimes. We measured dendrochronological fire records, current stand structures, and fuel loadings in adjacent riparian and upland sites in three forest types (mixed-conifer, white fir, Jeffrey pine), two riparian zone widths (broad and narrow), and two precipitation regimes (dry east side and wet west side). Using the fire-scar records and the FVS growth and yield model, we reconstructed historic stand structures and fuel loadings.

Results/Conclusions

There was significant variation between fire regimes in different forest types, riparian zone widths, and precipitation regimes. Preliminary results indicate that adjacent riparian and upland forests did not differ significantly with respect to historic fire frequency and seasonality. Riparian forests burned primarily during mid to late summer (80% of scars in the dormant season) with a mean point fire interval of 31.6 years (range 3 to 77 years). Upland forests also burned primarily during mid to late summer (90% of scars in the dormant season) with a mean point fire interval of 36.2 years (range 30 to101 years). Trends in the data suggest riparian fire regimes may differ from adjacent upland sites only where stream channels are sharply incised or bordered by extensive mesic understory vegetation. Outside of these conditions, the historic similarity between adjacent riparian and upland forest fire regimes indicates that they likely had similar stand structures and fuel loadings. Historic stand structures and fuel loadings may serve as approximate guidelines for fuel reduction treatments in riparian forests. Current forest practices which generally do not treat riparian areas leave these zones at risk of burning with uncharacteristic severity due to their high stem density and fuel loading.