Fire history of coniferous riparian forests in the Sierra Nevada

Abstract

Fire is an important ecological process in many western U.S. coniferous forests, yet high fuel loads, rural home construction and other factors have encouraged the suppression of most wildfires. Using mechanical thinning and prescribed burning, land managers often try to reduce fuels in strategic areas with the highest fuel loads. Riparian forests, however, are often designated as areas where only limited management action can take place within a fixed-width zone. These highly productive forests have developed heavy fuel loads capable of supporting stand-replacing crown fires that can alter wildlife habitat and ecosystem function, and contribute to stream channel erosion. Objectives of this study were to determine whether adjacent coniferous riparian and upland forests burned historically with different frequencies and seasonalities, and whether these relationships varied by forest, site, and stream characteristics. We measured dendrochronological fire records in adjacent riparian and upland areas across a variety of forest, site and stream conditions at 36 sites in three sampling areas in the northern Sierra Nevada. Riparian fire return intervals (FRI) ranged from 8.4 to 42.3 years under a liberal filter (mean 16.6), and 10.0 to 86.5 years under a conservative filter (mean 30.0). Upland FRI ranged from 6.1 to 58.0 years under a liberal filter (mean 16.9), and 10.0 to 56.3 years under a conservative filter (mean 27.8). Riparian and upland fire return intervals were significantly different in only one quarter of the sites we sampled. Riparian and upland areas did not burn with different seasonalities, and fire events occurred primarily during the late summer-early fall dormant season in both riparian and upland areas (88% and 79% of scars, respectively). FRI was shorter in forests with a higher proportion (>22.7–37.6%) of fire-tolerant pine ( Pinus spp.), sites east of the Sierra crest, lower elevation sites (<1944 m), and riparian zones bordering narrower, more incised streams (width/depth ratio <6.2). Upland areas exhibited a greater degree of fire–climate synchrony than riparian areas. Our study suggests that Sierra Nevada coniferous riparian forests bordering many montane streams might be managed for fuel loads and fire return intervals similar to adjacent upland forests.