U.S. Joint Fire Science Program


Date of this Version


Document Type



Final Report: JFSP 06-2-1-10


US government work.


After decades of fire exclusion many stands now have historically unprecedented (high) levels of fuels and overstocking. As a result these stands have developed a variety of health issues. In response to these problems land managers are using prescribed fire to restore fuel loads and stand stocking to pre-fire exclusion levels. The impacts of season of prescribed burn as well as burn interval are not well understood. Managers need the ability to better predict these impacts on such things as tree mortality from fire, associated insect attack and disease development, tree growth, and plant communities. The “Season of Burn” study complex was established in 1997 in six stands of mixed-age ponderosa pine (Pinus ponderosa Dougl. ex Laws.) with scattered western junipers at the south end of the Blue Mountains near Burns, Oregon. Three treatments, a fall burn, spring burn, and unburned control, were randomly assigned to 12-ha experimental units within each stand. Within each experimental unit, six 0.2 ha-plots were established to evaluate responses to the burns. Prescribed burns occurred during mid-October of 1997 or mid-June of 1998. In summer 2002 the objectives of the original study were expanded to include an examination of the impact of interval of burn and cattle grazing on the trees and understory vegetation. Each of the burn units was split and units were randomly assigned to either a 5-year or 15-year burn interval. Within each of the controls and the 5 year interval treatment units 0.2 ha cattle exclosures were randomly located to study the effects of the interaction of cattle grazing and season of burn on the understory vegetation. The 5 year interval units were prescribed burned for a second time in fall 2002 and spring 2003. The JFSP project reported here continued the expanded Season of Burn study. The five year interval of burn treatment units were successfully burned for the third time in fall 2007 and spring 2008 and previous protocols for monitoring were continued. Our results indicate that Ponderosa Pine growth rates are not negatively impacted and the incidence of insect attack or disease does not increase as a response to repeated prescribed burns. With each burn iteration further reducing the fuel load accumulated during the period of fire exclusion, repeated prescribed burns may be needed to significantly reduce fuel loads after prolonged fire exclusion. We found that removing cattle after prescribed fire reburns significantly enhanced the native plant community. Five years of grazing exclusion increased: (1) total vegetative cover, (1) native perennial forb cover, (3) shrub cover (4) grass stature and, (5) grass reproductive success. Remarkably, total cover increased by more than one-third. Neither spring nor fall reburning increased perennial native species cover or richness, and reburning reduced sedge cover. Fall reburning increased cover of native colonizers and exotic species cover and richness. We demonstrate that removing a novel disturbance process such as grazing significantly benefits the native plant community, but we document few benefits associated with adding the historic disturbance of fire, even in the absence of grazing.