Historical fire regime and forest variability on two eastern Great Basin fire-sheds (USA)

Abstract

Proper management of naturally forested landscapes requires knowledge of key disturbance processes and their effects on species composition and structure. Spatially-intensive fire and forest histories provide valuable information about how fire and vegetation may vary and interact on heterogeneous landscapes. I constructed 800-year fire and tree recruitment chronologies for two eastern Great Basin fire-sheds using fire-scar and tree establishment evidence from 48 gridded plots (500m spacing) and from fire-scarred trees between plots. Fire-sheds are located in the Snake Range of eastern Nevada (BMC) and Wah Wah Range of western Utah (LAW) and span a range in elevation and vegetation zones typical for the region. Estimates of point mean fire interval varied more than 10-fold at both BMC (7.8–125.6years) and LAW (13.3–138.4years). At BMC, a distinct within-fire-shed contrast in fire frequency was difficult to explain without invoking the possibility of spatially-variable human-caused ignitions. A majority of fires were small (<10ha) but large fires (⩾100ha) accounted for 78% (BMC) to 89% (LAW) of cumulative area burned. Tree recruitment for mid-elevation, mixed-conifer stands was somewhat episodic and asynchronous among plots. Recruitment pulses were synchronous with multi-decade fire quiescent periods, and often followed large fires. I concluded that fire frequency was under strong topographic control and that fire severity was mixed and variable through time and space resulting in a dynamic mosaic of variable-aged, fire-initiated vegetation intermixed with long-lived, fire-resilient trees and open shrub–steppe communities (BMC). A major change in fire regime and forest composition began in the 1800s cusing shifts in composition and structure at the stand scale and homogenization at the landscape scale. I recommend that management strategies prioritize the use of fire and surrogate treatments on mid-elevation forests that have deviated most from historic conditions and on associated shrub–steppe communities where conifer encroachment has occurred. Planned disturbances should be of mixed severity and sized to recreate vegetative mosaics at historic spatial scales.