Refined Burning Prescriptions for Yosemite National Park
NPS Occasional Paper No. 2
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The experimental fires burning under varying conditions of fuel and weather reduced fire hazards and changed vegetative composition in the mixed conifer ecosystems studies. Using the results of the experimental fires, an ecologically sound burning program for the mixed conifer forests of Yosemite is proposed.

Prescribed Fire Effects

Fuel moisture content was the principal factor influencing fire characteristics and subsequent effects on fuel and vegetation. Fuel moisture appeared to integrate the effects of air temperature and relative humidity and combined with fuel type to produce considerable variation in fire behavior within the range of conditions prescribed by Schimke and Green (1970).

The inverse relationship of fuel moisture on fire characteristics and fuel consumption has been studied under laboratory conditions (Byrum et al. 1966; Beaufait 1965). This study defined this relationship for field conditions in the mixed conifer type. Similar studies by Hough (1968) in the southern pine type and Kilgore (1971) in the red fir type did not burn over a specified range of moisture conditions, but rather only measured fuel moisture content at the time of burning. Both investigators reported 50% reductions of the litter layers and considerable change in the density of the understory vegetation. This corresponds to the fuel reduction for the 10 and 13% moisture level fires of this study.

Changes in the understory tree composition were caused by the 10% fires. Density was reduced by 50% with most of the loss being incense-cedar trees. The loss of bear clover stems, while not important from a composition standpoint since the plants resprout after burning, is an important part of fire hazard reduction. The new sprouts are not as flammable as the older, larger, mature plants because they contain less dead material and have higher moisture contents.

The effect of direction of burning was not significant for fuel reduction. Hough (1968) and Beaufait (1965) found that backfires consumed more fuel in the litter layer than headfires. They attributed this difference to the fact that the backfires spread slowly and burned deeper into the litter layer while the headfires spread rapidly, often burning only the upper litter layer. In this study both head- and backfires burned primarily in the upper or fresh needle layer since the lower layers were too wet to burn. The amount of fuel consumption was therefore independent of the rate of consumption since both methods of burning consumed equal amounts of fuel.

Rate of spread and intensity were affected by the burning direction as explained previously. Scorch height, which is dependent on the rate of energy release, was also affected. These results are in agreement with those of other investigators (Hough 1968; Beaufait 1965; Byram et al. 1966).

The insignificance of low wind speeds concurs with the conclusion reached by Hough (1968) and Byrum et al. (1966) that the rate of spread did not change for wind speeds between 0 and 8 mph. At higher speeds, wind does become important and assumes a nonlinear effect (Beaufait 1965). These speeds, however, are beyond the range considered for this study.

Management Applications

The results from the experimental fires can be used by the resources manager. Refined burning prescriptions will enable the manager to plan and implement ecologically sound burning programs which will meet specific management objectives.

REFINED PRESCRIPTIONS. It was found that fire behavior was very sensitive to changes in fuel moisture, and that the behavior varied between fuel types. Fuel moisture provides the key to prescription refinement. Not only is it easy to measure in the field but also it is a reliable predictor of fire characteristics and subsequent effects.

The prescriptions developed here refine fuel moisture ranges. Although these refinements would be applicable to most of the central and southern Sierra, they are specifically designed for the lower portion of the mixed conifer zone in Yosemite National Park. Fuel moisture prescriptions are given for the bear clover, pine needles, and incense-cedar understory fuel types, and, in addition, for the unique condition in Yosemite Valley.

The bear clover fuel type burned throughout the range of moisture levels with variable results. At the 19% level, the fires did not burn satisfactorily. Fires burning at less than 9% would have been too hot for control. The range for safe, effective burns in the bear clover type should be from 9 to 17% fuel moisture. Within this range, fuel reduction will vary, but practically all of the fresh needle layer and the bear clover stems will be consumed. At the drier moisture levels, the weathered needle layer will be reduced and scattered understory trees killed. In many instances a fuel moisture of 17% will be reached within days after snow has melted in the spring.

The pine needle fuel type would not burn at the 19% moisture level. The same range from 9 to 17% is recommended for the needle type. Consumption at the 16% level was limited to the fresh needle layer, while the weathered layer was reduced at the 13% level. Some of the decomposed layer was reduced by the 10% fires. Reduction in the lowest layer is more dependent on the seasonal drying trend than on the fuel stick readings. At the lower two moisture levels, scattered understory trees were killed.

Fires burned satisfactorily in the incense-cedar fuel types only at the 10% level. Although the 7% level would probably still be safe, it was not tested since that level was never reached during the period of experimentation. The recommended range for the incense-cedar type is from 9 to 11% fuel moisture. Within this range incense-cedar trees up to 5 cm in diameter can be killed. Fuel reduction was primarily in the fresh needle layer although some weathered needles were consumed.

In Yosemite Valley, two refinements are suggested. For the shaded and cooler southern side, the fuel moisture range should be from 9 to 14%. Burning could be accomplished from 9 to 17% on the northern side of the valley. The northern side is similar to the needle type at Wawona, while the southern side is intermediate between the needle and incense-cedar types.

The following prescriptions are recommended for burning within the lower portion of the mixed conifer zone in Yosemite National Park. All but the fuel moisture ranges are from Schimke and Green (1970).

Wind Speed:0 - 10 mph
Air Temperature:20 - 84°F
Relative Humidity:20 - 64%
Fuel Stick M.C.:
   Bear Clover9 - 17%
   Needle9 - 17%
   North Side9 - 17%
   South Side9 - 14%
   Incense-cedar9 - 11%
Timber Burning Index:22 - 6
Ignition Index:25 - 52

2California Wildland Danger Rating System (USFS 1962).

BURNING TECHNIQUES. Even within the refined ranges the manager can exert considerable influence on fire behavior. By waiting to burn when a specific fuel moisture level is reached, he can control the amount of fuel consumption. In conjunction with time of burning, the method of burning controls the rate of spread and intensity of the fire. The manager can control scorch height and understory mortality through control of intensity.

Method of burning is also important at the moist end of the fuel moisture range, where a headfire is often effective while a backfire is not. Conversely, at the drier moisture levels, a headfire is often exceedingly destructive while a backfire is safe.

Specific objectives will dictate which combination of fuel moisture and fire direction to use. In general, however, the following prescriptions will give desirable results. Backfires and headfires burning within the 9-14% range in the bear clover type are satisfactory. Only headfires are effective between 15 and 17% fuel moisture. The headfires spread rapidly and should be used only with extra caution. Figure 9 shows a headfire in bear clover burning at the 13% fuel moisture level. This fire produced sufficient heat to kill the incense-cedar in the right center of the picture.

FIG. 9. This fire was burning uphill with a headfire in the bear clover fuel type at the 13% fuel moisture level. Sufficient heat was produced to kill the small incense-cedar tree.

In the needle fuel type, headfires again were the only ones which were effective within the range from 15 to 17% moisture. The backfires did consume fuels, however, but were very slow.

Between 9 and 14%, the headfires were excessively hot, while the backfires were satisfactory. The intensity of the 10% and 13% headfires in the needle type were beyond acceptable levels. Extreme caution must be exercised when these fires are used. Figure 10 shows the 10% fire which produced scorch up to 10 m and killed several 3-m tall trees. On the other hand, the backfires at these levels were easy to control and consumed satisfactory amounts of fuel without doing excessive damage such as scorching the crowns of large trees.

FIG. 10. Trees up to 3 m tall were killed by this headfire in the pine needle type burning at the 10% fuel moisture level. Fires of this intensity are not recommended for application by management.

Headfires are recommended in the incense-cedar fuel type since satisfactory kills could be accomplished only with higher intensity fires. Backfires were equally effective, however, in reducing fuels within the 9-11% moisture range. Figure 11 shows a headfire in this type.

FIG. 11. In the incense-cedar fuel type, only this headfire burning at the 10% fuel moisture level was effective in killing understory trees.

Either headfires or backfires can be used on the south side of the valley. On the north side, the prescription for the needle fuel type should be used. A backfire on the south side of the valley is shown in Figure 12.

FIG. 12. This fire was burning against the wind with a backfire in the valley fuel type at the 13% fuel moisture level.

In general, it must be remembered that spring fires burning at moisture levels above 10% are effective primarily in reducing flashy fuels such as bear clover and fresh needles. Some reduction of understory density can be accomplished in the incense-cedar type. The manager can determine the extent of the reduction and mortality by controlling the time and method of burning.

In addition, in many sensitive areas, such as roadsides and picnic areas, scorch height can be easily controlled. By using a backfire in such areas, the manager can avoid excessive scorch and resultant public criticism.

BURNING PROGRAMS. The manager can use the refined prescriptions and techniques to plan an integrated burning program. As a first step, an inventory of areas to be burned must be made. Fuel types and natural or man-made fuel breaks should be recorded.

The objectives of the burning program must also be examined. The manager must decide how much fuel reduction and understory density reduction is desirable. By using fuel type and fuel moisture level controls, he has considerable flexibility in planning his program. Several possible programs are discussed below.

If, for instance, the objective is to burn off the flashy fuels in the bear clover type, the bear clover patches could be burned with a headfire at 19% fuel moisture when the other types would not burn. Such a fire could burn up slope without the danger of escape. As soon as the snow leaves an area, fuel sticks are placed in representative bear clover patches. Burning commences when the 19% level is reached. Two- or three-man teams could cover a wide area by going from patch to patch with fuel torches. No fire lines would be needed since the patches would be surrounded by other fuel types which would not burn or fuels still wet from the receding snow. The burned areas can serve as fuel breaks for later spring fires burning at lower moisture levels, or as a means of breaking up continuous ground fuels to ease wildfire control.

Another program would be to burn all the flashy fuels in the bear clover and needle types but not in the incense-cedar type. Backfires set at the tops of ridges or other natural fire breaks, when the fuel moisture is between 12 and 17% would accomplish such an objective. These fires would burn down hill to a natural or man-made break. In this case, the incense-cedar patches could be burned at 10% or lower later in the season without the danger of fire escape since they would be surrounded by previously burned areas. Fuel sticks placed in each fuel type would enable the manager to predict the effects of fire in each type.

Since fuel moisture varies with local topography, south-facing slopes could be burned when north-facing slopes are too wet. When the north slopes dry sufficiently, they would be ignited using the previous burn as a fuel break.

In Yosemite Valley, grass-filled meadows are interspersed with the forests. Although not specifically studied in this project, it is known that the previous year's herbaceous growth will burn early in the spring before the forest fuel types will ignite. The meadows could be burned at this time, providing fuel breaks for later forest burning.

The reduction of heavy fuel is a primary concern in a hazard reduction program. This can be done with fall burning, although extreme caution is necessary since the fuels are very dry. Alternatively, the heavy fuel left by a spring fire could be burned in the fall with a great reduction in risk. Since the flashy fuels have been removed, the danger of escape is reduced. Individual logs or accumulations of heavy fuels can be burned by teams going from one to the other.

In each of the above programs the sensitivity of fire behavior to fuel moisture is used by the manager. He can design his program to meet his needs by using a good survey and the fuel sticks.

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Last Updated: 01-Mar-2005