Climatic and human influences on fire regimes in mixed conifer forests in Yosemite National Park, USA

The goal of this study was to identify the influence of interannual and interdecadal climate variation and changes in land use on fire regimes in fire prone mixed conifer forests in the central Sierra Nevada in Yosemite National Park, California. We quantified fire frequency, fire return interval, fire extent, and season of fire for a 400 year period using fire-scar dendrochronology. The influence of regional climate variability and land use on fire occurrence and extent was assessed by relating the fire record to regional climate reconstructions and to documentary records of settlement and land use. The timing and extent for fires was related to interannual and interdecadal variation in drought and temperature linked to variation in the Pacific North America Pattern (PNA), the Pacific Decadal Oscillation (PDO) and El Niño–Southern Oscillation (NINO) had little effect on fire. The occurrence of large fires was also influenced by interactions among climate patterns and they occurred more often than expected in PNA+ NIÑO3-years. At interdecadal time scales area burned was positively correlated with temperature and the PNA. Fire occurrence and extent declined with mid-19th century Euro-American settlement and land use change and fire was nearly eliminated after 1900 when a fire exclusion policy was implemented. A two-fold increase in rate of burning in the late 18th and early 19th century corresponds with spread of non-native disease to Native American populations during the Spanish Colonial Period but the PNA was also mainly positive during this period. Fire regimes were sensitive to shifting modes of climate and land use which can lead to variable pathways of forest development and hence forest structure. Forest structure at the time of Euro-American settlement reflects this sensitivity and managers should consider presettlement conditions as only a guide for restoration planning in forests highly altered by fire exclusion under a changing climate. Moreover, considering winter PNA could provide managers with an early indication of conditions during the fire season that are conducive to widespread fire.

► Annual and decadal fire extent in Yosemite was studied over 400 years.
► Fire regimes were sensitive to shifting modes of climate and land use.
► Fires in dry warm years were strongly related to interacting climate patterns.
► Native American decline, settlement, and fire exclusion affected fire regimes.
► Considering winter PNA could provide an early fire season forecast.