Tropical and north Pacific teleconnections influence fire regimes in pine-dominated forests of north-eastern California, USA

Aim To assess the importance of drought and teleconnections from the tropical and north Pacific Ocean on historical fire regimes and vegetation dynamics in north-eastern California. Location The 700 km 2 study area was on the leeward slope of the southern Cascade Mountains in north-eastern California. Open forests of ponderosa pine ( Pinus ponderosa var. ponderosa Laws.) and Jeffrey pine ( P. jeffreyi Grev. & Balf) surround a network of grass and shrub-dominated meadows that range in elevation from 1650 to 1750 m. Methods Fire regime characteristics ( return interval, season and extent) were determined from crossdated fire scars and were compared with tree-ring based reconstructions of precipitation and temperature and teleconnections for the period 1700 - 1849. The effect of drought on fire regimes was determined using a tree-ring based proxy of climate from five published chronologies. The number of forest-meadow units that burned was compared with published reconstructions of the El Nino/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Results Landscape scale fires burned every 7 - 49 years in meadow-edge forests and were influenced by variation in drought, the PDO and ENSO. These widespread fires burned during years that were dryer and warmer than normal that followed wetter and cooler years. Less widespread fires were not associated with this wet, then dry climate pattern. Widespread fires occurred during El Nino years, but fire extent was mediated by the phase of the PDO. Fires were most widespread when the PDO was in a warm or normal phase. Fire return intervals, season and extent varied at decadal to multi-decadal time scales. In particular, an anomalously cool, wet period during the early 1800s resulted in widespread fires that occurred earlier in the year than fires before or after. Main conclusions Fire regimes in north-eastern California were strongly influenced by regional and hemispheric-scale climate variation. Fire regimes responded to variation that occurred in both the north and tropical Pacific. Near normal modes of the PDO may influence fire regimes more than extreme conditions. The prevalence of widespread teleconnection-driven fires in the historic record suggests that variation in the Pacific Ocean was a key regulator of fire regimes through its influence on local fuel production and successional dynamics in north-eastern California.