Yosemite Valley―The 4th fuel type studied was in Yosemite Valley. The valley floor is nearly level and surrounded by steep canyon walls. The study area is on the south side of the valley near Sentinel Creek. Elevation was not reported but is approximately 1,400 m. Steep, high canyon walls result in moister sites on the south than the north end of the valley [6]. Like the needle fuel type at Wawona, forest floor cover was mostly ponderosa pine needles with sparse live vegetation, although there was more groundlayer herbaceous vegetation than on the Wawona needle type. This fuel type was called valley fuel [4,6].

Yosemite Valley―Ponderosa pine and canyon live oak (Quercus chrysolepis) dominated the overstory. Pole-sized incense-cedar was successionally replacing canyon live oak. Incense-cedar seedlings dominated the understory. There were occasional coast Douglas-fir (Pseudotsuga menziesii var. menziesii), white fir (Abies concolor), and ponderosa pine seedlings in the understory. Fuel loads were heavy due to tree mortality from insect and fomes root rot (Fomes annusus) infestations [6].

Stand structure was similar at the 2 sites. Over half the basal area was ponderosa pine, mostly large ponderosa pines (>50 cm in diameter). Scattered large and numerous smaller incense-cedars comprised about one-third of the overstory. The rest of the basal area was mixed Douglas-fir, white fir, and California black oak. The Yosemite Valley site had more downed trees than Wawona. Basal area was not significantly different (all significance levels are 0.05 for this study) between sites. Overall basal area for the 2 sites was [6]:

Understory basal area and density varied among fuel types, with significantly more vegetative cover on the Sierra mountain misery fuel type compared to other fuel types [6]:

Study sites are classified in the following plant community and historically experienced the fire regime described below:

On the 2 study sites, ignitions were started in spring 1970 at 4 fuel moisture contents (19%, 16%, 13%, and 10%) and with 2 methods of burning (backfires and head fires). Areas with "excessive" rock outcrops or heavy fuels were not included in analyses [6].

The general fire prescription called for windspeeds ≤10 mph, air temperatures from 20 to 84 ºF, and relative humidities from 29% to 65%. Backing fires were ignited either at the perimeter edge away from the wind or the upper end of the plots. Head fires were ignited from either the windward or bottom side of the plots [4,6]. Burn areas were 5 to 10 ha [6].

Prefire mean fuel loads were [4]:

Fire behavior: Energy released by the fire for the 4 understory and forest floor fuel types was: Sierra mountain misery, 575.5 kcal/m²; needle, 98.5 kcal/m²; incense-cedar, 402.1 kcal/m²; and valley, 94.1 kcal/m². Total energy release did not differ between backfires and head fires. Windspeed, fuel type, and fuel moisture content variables significantly affected energy release, and there was a significant 2-way interaction between fuel type and fuel moisture for energy release [6].

Energy release was significantly higher in Sierra mountain misery and incense-cedar fuel types compared to needle and valley fuel types. Fireline intensity was significantly less at fuel moisture contents of 19% and 16% than at 13% and 10% [6]:

Fuel reduction: These low-intensity spring fires consumed most understory fine fuel and fresh needles but burned only slightly into weathered needle and organic soil layers. Since the bulk of the fine fuel load on these sites was in the organic soil layer, total fine fuel reduction from fires was not significant. Fuel type and percent fuel moisture significantly affected fuel reduction, while fire type did not. When fires carried, both backfires and head fires consumed similar amounts of fine fuel at any particular moisture content. As is typical for Yosemite Valley, heavy fuels did not dry until midsummer in the low elevations of Yosemite National Park, so none of the fires reduced heavy fuel loads. Sierra mountain misery and incense-cedar fuel types had greater fine-fuel reductions than needle and valley fuel types. Sierra mountain misery plots burned at higher fuel moisture contents than other fuel types largely because needles draped on Sierra mountain misery plants provided a well-aerated fuel bed. The fuel bed was more compact for the incense-cedar type, so the fires did not carry until fuel moistures were ≤10%. Fuel reduction was significantly greater on needle than valley fuel types, probably because added moisture content of herbaceous vegetation on valley plots caused less fresh needle reduction. Fuel reductions are shown below [6].

FIRE EFFECTS ON PLANT COMMUNITY:
As hypothesized, fires with different fuel moisture contents had different effects on understory ponderosa pine and incense-cedar. Total understory basal area of ponderosa pine was not affected by the fires, while total understory basal area for incense-cedar was significantly affected by fuel type, fuel moisture, and the interactive effects of fuel type and fuel moisture. On incense-cedar fuel plots, only plots at the 10% fuel moisture content were sufficiently dry to kill incense-cedars 1 to 3 m in height. Ponderosa pine showed insignificant mortality in all fuel types [6]:

Fuel type, fuel moisture contents, and their interactive effects significantly affected density levels for Sierra mountain misery, incense-cedar, ponderosa pine, and all understory species combined in the 0- to 0.3-m height class. Significance for all species combined was mainly due to the large number of Sierra mountain misery stems on Sierra mountain misery fuel plots. Ponderosa pine in the 0- to 0.3-m (first-year seedling) size class was well represented in the understory only on valley plots. There, the fires killed the ponderosa pine seedlings [6].

Fuel moisture content affected densities of Sierra mountain misery and ponderosa pine in the 0- to 0.3-m size class but not did affect 0- to 0.3-m incense-cedars. For Sierra mountain misery, plots burned at 19% and 16% fuel moisture contents showed significantly greater density losses than plots burned at 13% and 10% fuel moisture contents. For ponderosa pine, plots burned at 10% fuel moisture content showed significantly greater density loss compared to plots burned at 13% fuel moisture content. In the 0.3- to 1.0-m size class, density reductions were insignificant for all 3 species. Most trees in the 1- to 3-m height class were on incense-cedar fuel types. Again, density reductions were due to fuel type, fuel moisture contents, and their interactive effects. Density reductions for the 0- to 0.3-m and 1- to 3-m height classes are shown in the table below [6].

Fuel moisture content was the single most important factor affecting fire characteristics, fuel consumption, and fire effects on vegetation. Fuel type also influenced fire behavior. Fires on the 10% fuel moisture plots caused greatest changes in postfire understory composition. Reduction of Sierra mountain misery is particularly important for fire hazard reduction. Although Sierra mountain misery sprouts after fire, new sprouts are less flammable than old stems because sprouts have higher moisture contents and less dead biomass [6]. Based on these findings and other prescribed fires on similar mixed-conifer Yosemite sites from 1972 through 1976, van Wagtendonk [4,5,6] presented the following prescription for the lower mixed-conifer zone of Yosemite National Park. This prescription may be applicable in lower mixed-conifer zones elsewhere in California.

Van Wagtendonk [5] also provides prescriptions for upper mixed conifer (1,700-2,400 m) and giant sequoia (Sequoiadendron giganteum) groves.

REFERENCES:

1. Hann, Wendel; Havlina, Doug; Shlisky, Ayn; [and others]. 2005. Interagency fire regime condition class guidebook. Version 1.2, [Online]. In: Interagency fire regime condition class website. U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior; The Nature Conservancy; Systems for Environmental Management (Producer). Variously paginated [+ appendices]. Available: http://www.frcc.gov/docs/1.2.2.2/Complete_Guidebook_V1.2.pdf [2007, May 23]. [66734]

2. LANDFIRE Rapid Assessment. 2005. Reference condition modeling manual (Version 2.1), [Online]. In: LANDFIRE. Cooperative Agreement 04-CA-11132543-189. Boulder, CO: The Nature Conservancy; U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior (Producers). 72 p. Available: http://www.landfire.gov/downloadfile.php?file=RA_Modeling_Manual_v2_1.pdf [2007, May 24]. [66741]

3. LANDFIRE Rapid Assessment. 2007. Rapid assessment reference condition models, [Online]. In: LANDFIRE. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab; U.S. Geological Survey; The Nature Conservancy (Producers). Available: http://www.landfire.gov/models_EW.php [2008, April 18] [66533]

4. van Wagtendonk, Jan W. 1974. Refined burning prescriptions for Yosemite National Park. National Park Service Occasional Paper Number 2. Washington, DC: U.S. Department of the Interior, National Park Service. 21 p. [50524]

5. van Wagtendonk, Jan W. 1977. Fire management in the Yosemite mixed-conifer ecosystem. In: Mooney, Harold A.; Conrad, C. Eugene, technical coordinators. Proceedings of the symposium on the environmental consequences of fire and fuel management in Mediterranean ecosystems; 1977 August 1-5; Palo Alto, CA. Gen. Tech. Rep. WO-3. Washington, DC: U.S. Department of Agriculture, Forest Service: 459-463. [4895]

6. van Wagtendonk, Jan Willem. 1972. Fire and fuel relationships in mixed conifer ecosystems of Yosemite National Park. Berkeley, CA: University of California. 163 p. Dissertation. [40173]