Top - down and boTTom - up conTrols on fire regimes along an elevaTional gradienT on The easT slope of The sierra nevada , california , usa

Fire is an ecologically significant process in the fire-prone ponderosa pine and mixed conifer forests of the northern Sierra Nevada. Fire regimes are influenced by processes that operate over a range of scales that can be grouped broadly as bottom-up (e.g., topography, forest type) or top-down (e.g., climate variation, human land use) controls. To identify the bottom-up versus top-down controls on fire regimes, we quantified spatial and temporal variation in fire occurrence and extent using fire-scar dendrochronology. Inter-annual climate variability and human land use patterns strongly influenced fire regimes. Years of widespread burning and fire-free years were associated with dry and wet years, respectively. Variation in fire activity was also associated with variation in the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Widespread burning occurred during La Niña years (cool ENSO) and during positive conditions of the PDO (warm PDO). Fire occurrence declined with Euro-American settlement in the nineteenth century and only two fires were recorded in the study area after 1905, the date fire suppression was implemented. Fire regimes were also influenced by bottom-up controls. Fire return intervals (FRI) were shorter in pine-dominated low-elevation forests than in high-elevation fir-dominated mixed conifer forests, although FRI did not vary by slope aspect.