Changes in stand structure and fuel load have complicated the safe application of prescribed fire to ponderosa pine forests. Mechanical fuel reduction has become a necessary first step to reduce ladder fuels and tree density in some burn units. Thinning specifications included leaving 20 pole-size trees per acre of good vigor at 20 feet minimum spacing, with maximum size of tree taken at 5" dbh. At Devils Tower, a 200-acre unit, North Terrace, was mechanically thinned in 2001 and burned in fall of 2005. There are five plots located within this unit and tree density, vegetative cover, and fuel loading was monitored after thinning and burning.

The figure below displays ponderosa pine pole (1 - 6" dbh) and overstory density (> 6"dbh) following the thinning and burning treatment. Pre-burn sampling occurred immediately prior to the thinning treatment, with post-thin and post-burn monitoring occuring at years one and two. Ponderosa pine pole density decreased by 98% two seasons after the burn treatment with almost all of this occurring from the thinning treatment. There was a 19% reduction in overstory density two years after the burn which met the objective of less than 30% mortality.

Below are mean cover measurements by lifeform following thinning and prescribed burning. Pre-burn values represent cover values prior to thinning and burning, 01yr01 values are those one growing season after thinning, and 02yr01/02yr02 represent cover one and two growing seasons after the prescribed burn treatment. Native grasses increased by 53% and native forbs 132% two years after the prescribed burn. Non-native grasses and forbs also show increasing trends in cover at 44% and 111% respectively.

The figure below shows the change in fuel loading from pre-thin/burn conditions to two years after the prescribed burn. There was a 40% decrease in woody fuels (fine and 1000 hr fuels combined) one year after the burn while litter/duff and total fuel loading decreased 69% and 57% respectively.