Using Fire as a Management Tool in Southwestern Ponderosa Pine1

Michael G. Harrington and Stephen S. Sackett2

CHANGING CHARACTERISTICS OF infrequently, rarely accumulated over stagnation has been reported on many sites SOUTHWESTERN PONDEROSA PINE extensive areas. When single or small (Cooper 1960, Schubert 1974), and FORESTS groups of trees fell, they were generally persists where natural or artificial thinning consumed by subsequent fires, creating a has not taken place In addition to stand The ponderosa pine (Pinus ponderosa mineral soil seedbed and reducing grass changes, 75 to 100 years of general fire Laws.) forests of the Southwest have gone competition in microsites, favoring absence has also led to uncharacteristically through extensive structural and ponderosa pine seedling establishment large accumulations of surface and ground compositional changes in the last century. (Cooper 1960). These circumstances fuels (Kallander 1969). Numerous references document the open, created an uneven-aged stand structure Sackett (1979) reported average park-like appearance of historic ponderosa composed of small, relatively even-aged loadings of naturally created fuels at 22 pine stands (Biswell et a1.1973, Brown groups. tons per acre (range 8-48 tons per acre) for and Davis 1973, Cooper 1960), where Change began in the southwestern 62 southwestern ponderosa pine stands. herbaceous vegetation was vigorous and ponderosa pine forests during extensive Harrington (1982) verified the heavy fuel abundant. Fires were a regular feature of livestock grazing in the late 19th century loadings, with an average of 34 tons per these forests, burning the light surface (Faulk 1970). As grazing intensified acre in southeastern Arizona. Another fuels at intervals usually averaging less herbaceous vegetation could not respond, formerly uncommon feature is the than 10 years and as often as every 2 years and its coverage declined drastically. This abundance of large, woody fuels, (Dieterich 1980, Weaver 1951). The decline led to two subsequent changes: averaging about 8 tons per acre. Much of frequency of these fires resulted from the reduced fire spread because of the these down, woody fuels have continuity of grass and pine needle fuels, decrease in fine fuels, and an eventual accumulated in sapling thickets, creating the high incidence of lightning, and the increase in ponderosa pine regeneration an even more severe hazard. A final warm, dry weather common to the because of reduced competition and fire characteristic of current southwestern Southwest. Light surface fuels built up mortality, and more mineral seedbeds ponderosa pine stands is the sparse sufficiently with the rapid resprouting of (Cooper 1960). Beginning in the early understory vegetation, created from thick grasses and the annual pine needle cast. 1900's, forestry practices, including fire forest floor layers and dense pine Large, woody fuels, which fall control, further reduced the spread of canopies, that resulted from fire inevitable fires, leading to unprecedented suppression (Arnold 1950). 1Panel paper presented at the conference, fuel accumulations and stagnation of The changes that have taken place Effects of Fire in Management of Southwestern seedling and sapling thickets. primarily within the last century have Natural Resources (Tucson, AZ. November These human-induced changes have created several undesirable conditions in 14-17.1988). resulted in ponderosa pine forests that the ponderosa pine forests of the 2Research foresters, USDA Forest Service. have little similarity to the presettlement Southwest. The extreme fuel hazard is Harrlngton is with the Intermountain Fire forests. In the uncut or lightly harvested probably most apparent. The combination Sciences Laboratory, Intermountain Forest and stands, old-growth trees still stand. The of heavy forest floor fuel loadings and Range Experiment Station in Missoula MT; open structure is gone, however, as dense dense sapling thickets coupled with the Sackett is at the Forest Fire Laboratory, Pacific Southwest Forest and Range Experiment sapling thickets and small pole groups normally dry climate and frequent Station in Riverside, CA. have developed in the understory. Stand lightning and human-caused ignition

122 potential result in a severe wildfire threat SOUTHWESTERN PONDEROSA PINE reduced by 55% and dead woody fuels (Biswell et al. 1973, Harrington 1982). FUELS were reduced by 64% to 80%. Emphasis Additionally, trees of all sizes have was placed on the consumption of generally poor vigor and reduced growth The deteriorating and hazardous forest thousands of snags and windfalls, which rates (Cooper 1960, Weaver 1951). This conditions described above have greatly lessened the fire hazard. The effect condition is likely due to the reduced contributed to numerous severe wildfires of this burning operation was evident the availability of soil moisture caused by in Arizona and New Mexico. Several following year by a dramatic reduction in intense competition and by moisture examples include the 1951 Escudilla the number of wildfires and in the acreage retention in the thick forest floor (Clary Mountain Fire (19,000 acres) and Gila burned (Weaver 1952). and Ffolliott 1969). The thick forest floor Black Ridge Fire (40,000 acres), the 1956 In a well documented study in 1950, six also indicates that soil nutrients, especially Duddly Lake Fire (21,000 acres), the 1971 small plots were burned under two sets of nitrogen, may be limiting because they are Carrizo Fire (57,000 acres), and the 1977 fuel moisture and weather conditions to bound in unavailable forms (Covington La Mesa Fire (15,000 acres) (Biswell et al. determine fuel consumption and tree and Sackett 1984). 1973, Cooper 1960, USDA 1977). The responses (Gaines et al. 1958). A low In the past, pine regeneration generally need to alleviate the hazards was intensity burn was set at the end of developed in openings in the stand after recognized years ago, but widespread use September, and a high intensity burn was fire had produced a receptive seedbed. of fire under specified conditions to create set in early October with higher air tem- Now, long after tree mortality and the designated results has been adopted peratures and lower fuel moistures, creation of openings, a poor quality, slowly. Reasons for this reluctance include humidities, and winds. The September organic seedbed remains, allowing little a shortage of guidelines for prescription burn resulted in a 57% reduction in opportunity for establishment of pine burning and insufficient information on surface, ground, and aerial fuels. The seedlings. Also, forage production for fire effects. October burn consumed more fuel, but wildlife and livestock is commonly also created new fuel by killing or minimal, because of severe competition Fire Effects on Fuels damaging numerous trees. When the with trees and the physical effect of the newly deadened fuels were added to the deep forest floor (Biswell 1972, Clary et The use of prescribed fire has increased residual fuels, only a 15% reduction was al. 1968). in recent years. Some of the earliest, realized. Ponderosa pine is also found with extensive prescribed burning took place on Another large scale, fuel-reduction burn several other conifers in the higher the Fort Apache Indian Reservation in on the Fort Apache Reservation was elevation mixed-conifer forests. Fire Arizona with about 3,000 acres burned in conducted in November 1956 under cool, occurred less frequently in these more the late 1940's (Kallander 1969). From clear days with a moderately high drought mesic forests. A 22-year average interval 1950 to 1970, over 300,000 acres were index and low rate-of-spread index was found in northeastern Arizona burned, primarily for hazardous fuel (Lindenmuth 1960). Fires were ignited at (Dieterich 1983), but its impact was still reduction. The effectiveness of this specific locations, but allowed to burn un- important for site preparation, species burning program in reducing size and checked for 33 days within project selection, disease and insect distribution, severity of subsequent wildfires has been boundaries. Fire effects were un- and nutrient cycling (Jones 1974). Little documented (Biswell et al. 1973, Knorr documented in fall 1957 (Lindenmuth documentation of fire effects and burning 1963). This burning operation used the 1962). Fuel reduction was deemed prescriptions is available for southwestern cool, dry conditions encountered in late unsatisfactory because fuel consumption mixed-conifer forests, therefore, this fall to moderate fire behavior for initial on 75% of the area was only minimal. In subject will not be addressed here. fuel reduction burns. Burning was not to addition, a large portion. of the area that This paper addresses the use of begin until after November 1, but dry litter had good fuel reduction also had extensive prescribed fire in southwestern ponderosa and duff layers were preferred (Kallander tree mortality or injury. pine forests. From research and 1969). These fires were strategically In central Arizona, a small scale observations, recommendations are made ignited, then allowed to burn, unattended, research burn was conducted on two to apply fire to reduce fuel hazards, thin over vast acreages. In a series of three distinct sites. The first had 75% more fuel dense sapling thickets, and provide burns in 1950, this procedure was used to by depth and weight, and 85% greater conditions for natural regeneration in burn portions of 65,000 acres (Weaver overstory basal area than the second natural forest stands. 1952). Forest floor fuel loadings were

123 (Davis et al. 1968). Under similar fuel the results of an October 1982 initial fuel reached at 2 inches below the soil surface. moisture and weather conditions, more reduction burn at the Fort Valley At flinch depths, temperatures frequently fuel was consumed on the site with the Experimental Forest (Covington and exceeded 100° F for at least 6 hours, with most fuel. However, 2 years after the Sackett in press). Under low humidities some reaching 140° F. On a burn at the burns, the net fuel change including (15% to 24%) and moderate air Long Valley Experimental Forest under consumption by fire and litter temperatures (52° to 67° F), nine plots wetter conditions, 69% of the 45.5 tons per accumulation demonstrated that the site with differing stand structure were burned. acre of forest floor was consumed. with less initial fuel had a 37% reduction Surface fuel moisture ranged from 7% to Temperatures failed to reach 100° F at the compared with a 23% reduction on the 10%, and humus moisture ranged from 8-inch soil depth, but ranged from 140° to other site. This indicates more damage was 12% to 20%. In stagnated sapling thickets, 210° F at the 2- to 4-inch depth. done on the site with more fuels, resulting about 34% of their 12 ton per acre loading Live ladder fuels, which also add to the in greater fuel accumulation. was consumed, 52% of the 16 tons per hazard, need particular attention. These In a more recent study in uncut acre in the pole stands was consumed, and fuels can take the fire from ground level ponderosa pine in north-central Arizona, 89% of the 55 tons per acre was burned in into the overstory crowns. The vertical the importance of fuel moisture on the mature, yellow pine stands. A high, fuel continuity can be broken by a consumption was seen (Sackett 1980). positive correlation was found between reduction in ladder fuels. Some of the Two similar sites at Fort Valley and Long percent forest floor consumed and preburn important live fuels are medium-sized Valley Experimental Forests, maintained loading (r2 = 0.89). shrubs which are easily top-killed by fire by the Rocky Mountain Forest and Range Although there are valuable fuel but generally resprout. Examples include Experiment Station, were burned in fall hazard, nutrient, and regeneration benefits Gambel oak (Quercus gambelii), silverleaf under contrasting moisture and weather derived from the consumption of heavy oak (Q. hypoleucoides), and wavy leaf oak conditions. Surface litter had similar forest floors, there are also liabilities. (Q. invaginata). Gambel oak can be moisture on both sites, but the humus Consumption of large quantities of fuel temporarily reduced by prescribed burning layers (lower duff) differed 10% to 15%. generates large amounts of heat energy. (Harrington 1985). Low crowns of Since about 80% of the forest floor weight Studies at the Fort Valley and Long Valley ponderosa saplings also increase crown is in the ground fuels, the ground fuel Experimental Forests and Sequoia Kings fire threat. Scorching the lower foliage is moisture is most influential in total fuel Canyon National Park show very high effective in relieving this hazard, but the consumption. Over 60% of the ground mineral soil temperatures curing burning. burning procedure is difficult and requires fuels and about 70% of the large woody Lethal temperatures have been measured experience (Harrington 1981, Kallander fuels were consumed in the drier burn on many sites deeper than 12 inches into 1969). compared with only about 40% of both the mineral soil. More than 35% of the Understory burning in southwestern fuel groups in the wetter burn. old-growth ponderosa pines, which ponderosa pine can greatly, but only The importance of humus moisture was survived numerous presettlement fires, temporarily, reduce the fuel hazard demonstrated further by a prediction have died at Fort Valley as a result of the (Harrington 1981, Sackett 1980). equation for forest floor reduction first burns in 100 years. Consumption of the litter layer lessens developed from a series of summer burns Fires burning under these old-growth ignitability and rate-of-spread potential. in southeastern Arizona (Harrington pines are unspectacular, consuming only As more of the duff, ladder fuels, and large 1987). In the equation, the litter in the flaming front. Most of the logs are consumed, a reduction in potential forest floor is consumed by smoldering fire intensity, total energy release, and FR = 37.4 - 0.8 HM + 21.2 FD (1) combustion, which may take 72 hours to resistance to control are realized. Truesdell complete. Burning for this length of time (1969) reported a decrease in wildfire size humus moisture (HM) and preburn forest can result in either temperatures exceeding up to 7 years after prescribed burning on floor depth (FD) were highly correlated 140° F, which cause instant cambium or the Fort Apache Indian Reservation and a with percent forest floor loading (FR) root death, or lower temperatures for 3-year reduction in suppression costs after reduction (r2 = 0.91). Tree density also had longer durations, which also cause tissue burning on the Hualapai Indian a strong effect, with less fuel reduction death. Reservation. As mentioned, the fuel occurring in denser stands. Fuel loadings under these old pines at hazard reduction is only temporary as 0.6 Equation [1] clearly indicates that the Fort Valley ranged from 41 to 86 tons per to 1.8 tons per acre of needle litter can be percentage of forest floor (FR) consumed acre, and consumption was always greater cast annually depending on tree density increases as the preburn amount (FD) than 85%, regardless of humus moisture and site productivity (Dais et al. 1968, increases. This result was confirmed by up to 90%. In 13 of 14 cases, 140° F was Sackett 1980). Fuel often accumulates

124 rapidly to hazardous levels after initial fuel hazardously, or not at all. On sites will determine fire behavior and fuel con- reduction burns. As needles from scorched requiring reduction of natural fuels, sumption. Using the same prescription trees fall, the amount of surface litter can maximum daytime air temperatures should ranges during the summer rainy season actually become greater than preburn be between 50° and 75° F. Below 50° F, should permit successful fuel reduction levels. This litter increase is the result of moderately dry fuels (9% to 12% burns. More attention to air temperature the inevitable tree injury caused by con- moisture) burn poorly and above 80° F limits and erratic winds is needed, sumption of the unnaturally great fuel extensive overstory crown scorching is however. quantities in dense stands. Therefore, likely. Minimum relative humidities reburns are essential to remove these should not drop below 20% or exceed Follow-up fire-created fuels and generally maintain 40%. Fuels subjected to a series of low low fuel hazard, even when initial burns humidity days become hazardously dry. Maintenance burning is necessary to are effective (Harrington 1981, Sackett Also, very low humidities are frequently keep the reccurring fuel hazard to a 1980). accompanied by temperatures above 80° minimum (Davis et al. 1968, Gaines et al. F. If minimum humidity exceeds 40%, 1958, Harrington 1981, and Sackett 1980). Reducing Fuels with Prescribed light surface fuels are generally too moist Since most of the light, fire-created fuels Fire to burn well. Windspeed at flame height accumulate within 3 years of burning, we should be between 3 and 8 miles per hour. recommend a repeat burn within that A lot has been learned within the last Slope effects can compensate for lack of period. Generally, repeat burns in light, 50 years about the use of fire in the wind. A fire burning with little or no wind needle fuels are easily managed. The win- Southwest. Many fire experts have and no effective slope either will not dow of burning season and ambient developed their skills primarily through spread well or will cause extensive crown conditions is broader than for initial burns, personal experience, learning from failures heating, if fuels are dry. Windspeeds with warmer, drier, windier situations as well as successes. This type of greater than 10 miles per hour can result in being advantageous to the conduct of the knowledge is difficult to pass on to less erratic fire behavior. Surface pine needles burn (Harrington 1985). Air temperatures experienced individuals. However, there is ideally should contain 5 to 12% moisture. should range between 55° and 85°F, now enough documentation of research Below 5%, ignition and rates-of-spread are humidities from 15% to 40%, windspeeds and operational burns to provide general too rapid, and above 12%, burning is from 5 to 12 miles per hour, and litter guidance for fire prescription and effects. patchy and incomplete with slow rates-of- moisture from 5% to 10%. After the sec- Unique combinations of stand, fuels, spread. ond or third burn, annual litter accu- vegetation, and terrain may preclude the Not all combinations within the range mulation should return to a level relative use of the following prescriptions and of temperatures, humidities, windspeeds, to natural attrition. From this point, effects information. Therefore, we and fuel moisture described above are safe burning need only be conducted at recommend a thorough assessment of site and effective. For example, if burning intervals of about 7 to 10 years to maintain characteristics. A generalized set of fire conditions are approaching the upper a low hazard. prescription parameters was derived from temperature and windspeed limits and the If a reduction in sprouting shrubs is a the prescribed burns discussed earlier. lower humidity and fuel moisture limits, a major management goal for fuel and very intense, rapidly spreading fire will competition reduction, then a distinct Season result. However, experienced burners can program of repeat burning is needed. For use the upper limits of one parameter to Gambel oak management, we suggest an In forested sites where fire has been make up for a deficiency in another. For initial fuel reduction burn in fall followed absent for decades, the initial fuel example, a combination which provides by 2 or 3 mid-August burns, 2 years apart reduction burns should be conducted in the good burning conditions is low humidity (Harrington 1985). fall or early spring when temperatures and (15% to 20%) and low temperatures (40° humidities are moderate. Fall burning can to 50° F). These situations do occur in late begin as early as mid-September and can fall throughout the Southwest. Predicting Fuel Consumption continue in some years into December. Because damp, cool fall weather often results in poor burning conditions, summer The ability to predict fuel consumption Weather Parameters burning during the monsoon season has from prescribed burning would be valuable to forest managers. If too little The following prescription parameters been studied as a successful alternative (Harrington 1981,1987). The amount of fuel is removed, the hazard might not be are the primary variables that determine relieved; if too much fuel is consumed, whether afire will burn successfully, drying that follows fuel-saturating rains

125 then tree mortality might be excessive, and animals also benefit from thinning of 1983). Even less information is available site quality might be compromised. dense stands. Forage production is on the use of fire to accomplish thinning. Equation [1] has not been extensively increased to a higher level by reducing the Fire is not a very selective thinning tool, tested, but should work reasonably well in basal area in typical ponderosa stands producing a rather unpredictable, patchy stands with characteristics within the (Clary and Ffolliott 1966, Jameson 1968). residual stand. However, most studies following ranges: preburn fuel loading = 25 Forest visitors find the dense thickets of dealing with fire as a thinning tool have to 40 tons per acre, large woody fuel reproduction uninviting. Access to the lacked a long-range process to accomplish loading = 3 to 15 tons per acre, and stand forest is often inhibited and esthetic values the objective. A number of fires are density = 800 to 2200 trees per acre are reduced. These dense stands do not required to reduce fuels, change the (Harrington 1987). The estimation of fuel indicate a healthy ecosystem. Treatment understory, and overcome the changes reduction was fairly accurate for fires measures that are silviculturally acceptable caused by fire exclusion. reported by Sackett (1980). However, it and economically sound are needed to Thinning by fire was a natural process in was highly over estimated for fires reported improve the situation. ponderosa pine before settlement. The by Davis et al. (1968) and Harrington These "dog hair" thickets may be degree of thinning is dependent on the (1985), because fuel loadings and stand modified for various reasons in a variety of quantity of fuel on the ground (Cooper densities were one-half or less of those ways. Sometimes an entire thicket should 1961). The more dense the thicket, the used in the equation development. be eradicated when infected by disease or more fuel, and the more intense the fire; insects, or when release potential is thus resembling a self-regulating feedback Reducing Logging Slash minimal. Fuel breaks are sometimes mechanism governed essentially by stand thinned to a low basal area for fire density. Reduction of fuels from silvicultural suppression. Stand density in travel A number of investigators have dealt activities is also important for lessening the influence zones is also reduced to improve with the use of fire as a thinning tool-an chance of severe wildfire in residual stands, visibility. Thinning as a silvicultural emulation of the "natural" pine processes. especially in ecosystems where the wildfire treatment is generally quite limited by cost, Weaver (1947) reported on a comparative potential is so great. However, little but some reduction of tree density could study of stand conditions in an area burned documentation exists concerning effective benefit over-dense stands on more than 4 in September 1914 and an adjacent area not combinations of cutting methods and fire. million acres of southwestern ponderosa burned, on the Colville Indian Reservation Buck (1971) proposed burn prescription pine land (Schubert 1974). in Washington. Thirty years after the fire, parameters and techniques that have Many of these stands need thinning to the 40-year-old, fire-thinned stand had worked well in reducing logging slash provide for more productive forests. substantially fewer stems per acre, greater while causing acceptable tree damage. Mechanical or hand thinning are probably height, and larger diameters than the During harvesting, generally a large the most common methods used, but they adjacent unburned stand. The conclusion amount of logging slash is added to the do not appear to be economically feasible was that fire was an effective thinning tool. existing natural fuel component discussed over extensive areas. In addition, the Tests on the Fort Apache Indian earlier. This added slash creates an extreme hazard created by extensive thinning Reservation using prescribed fire as hazard and leaves the fuel manager or operations would encourage insect a thinning tool showed that thinning was silviculturalist with a complex condition to outbreaks and make fire protection even spotty from fall fires, but the prescribed attempt to relieve. An informal proposal more difficult. Obviously any one fire did a "reasonably effective and has been suggested in which a preharvest treatment is not the answer. The problem conservative job of thinning" (Weaver burn would be conducted under conditions must be spread over a number of methods, 1952). Gaines et al. (1958) wrote a described above to reduce natural fuels, depending on circumstances and situations supplement to Weaver's report providing followed by a postharvest burn to reduce present. data that more or less supported the activity-generated fuels and aid site previous observations. Their conclusions preparation. Effects of Fire on Stand Density were not as optimistic because of the injury to the commercial overstory, and STAND DENSITY IN SOUTHWESTERN There has been limited research on the noted the need for additional information. PONDEROSA PINE FORESTS use of fire to eliminate the fuels produced Wooldridge and Weaver (1965) reporting by thinning. One documented case of on a prescribed fire on the Colville Indian Besides reducing the wildfire hazard, successfully thinning slash used high, Reservation designed to thin dense sapling thinning of such stands releases the green fuel moisture to minimize ponderosa stands, concluded that prescribed fire was residual trees, allowing faster growth pine mortality in Oregon (Smith et al. a rough and largely unpredictable thinning (Schubert 1971). Domestic and wild

126 tool. The fire drastically reduced the (Henderson 1967, p. 57). He used backing a wet summer preceding the fall burn. An number of stems, had no significant effect fires on "high intensity days," but could average of only 180 stems per acre was on diameter growth, and caused a slight not adjust to any other technique because killed by the fire in the net reduction in height growth. of the severity of conditions. At low fire reproduction/sapling size classes. Virtually Lindenmuth (1960) studied the effects of danger, weather and fuel conditions were none of the small poles were killed two prescribed fires in east-central not adequate to sustain an effective fire outright. Arizona near McNary and Maverick. The spread. Medium conditions allowed for No known studies and reports deal 1956 fires burned continuously for 33 days adjustment of burning technique to specifically with the problem of under a variety of topographic, fuel, and regulate intensity. In general, high developing and using definitive burning weather conditions. The fire released from intensity fires eliminated more stems than techniques for thinning. Most references competition 24.3% of the potential crop did low intensity fires, but no prescriptions deal with a single fire as an answer to the trees that needed releasing (a novel way of could be developed from the few tests and problem. From experiences at the Fort presenting thinning data). The fires also the inherent variability involved. Valley and Long Valley Experimental destroyed 10% of the potential crop trees In test fires on the Apache National Forests, southwestern Colorado, and and damaged an additional 7.4%. Forest with logging slash, after southern Arizona, quality of fuel rather Lindenmuth concluded that these shelterwood cutting, Buck (1971) than quantity, as Cooper (1961) suggested, particular fires demonstrated an imperfect observed overstory mortality. Although appears to be more essential for producing tool, and rightly so, since no specific the prescribed fires were not designed as a high intensity fires in dense stands. thinning objective was intended, and thinning tool, they did accomplish some Work in surface fuel characteristics and because of the many varied conditions effective thinning from below. experience with many prescribed fires under which the fire burned. Eighty-three percent of the losses were in indicate that only the newly cast needles In a study of prescribed fire in suppressed and intermediate trees. Most (L layer) and upper portion of the California ponderosa pine, Gordon (1967) experiments in thinning with fire have had fermentation layer (F) actually burn as minimized the benefits derived from fire. their emphasis in the ponderosa pine type flaming combustion in heavy, old forest Using a limited data base (three fires), primarily because of the large acreages floor accumulations. The lower F layer is small areas, and severe burning conditions, that have not burned for so long, resulting matted and bound tightly together by he concluded that dense seedling and in stand stagnation. mycelium hyphae. As a result, the lower sapling groups would be completely killed In some recent prescribed fires in portion of the F layer acts more like a solid by broadcast (prescribed) burning, and that Arizona designed to reduce fuel hazards, piece of fuel rather than as individual fire is not a feasible tool for hazard thinning was also an important benefit. In particles as in the L layer, and does not reduction or thinning in eastside pine three distinct fires in the Santa Catalina burn well. areas. Volumes of data were collected, but Mountains, Harrington (1981) reported In an undisturbed, well-developed no regard was given to burning technique. tree density reductions in the small or forest floor, newly cast needles become Ffolliott et al. (1977) reported an suppressed classes of 24%, 56%, and 43% rapidly colonized and bound by mycelium effective thinning response from an in stands with preburn densities of about and therefore less burnable. When fire experimental prescribed fire near 2000 trees per acre. Percent tree reduction spreads over the forest floor, most of the Flagstaff, Arizona. However, they was positively correlated with amount of fungi are destroyed. Needles that fall after concluded that basal area was not reduced fuel reduction indicating that, with more a fire do not become readily infected and a enough for optimal growth of the residual research, degree of thinning could possibly much deeper layer of pure litter stand. As stated before, one fire seldom be predicted. accumulates. When fire is applied a corrects problems associated with 100 Two other initial fuel reduction fires second time, all material cast since the years of fire exclusion. netted similar results. At the Fort Valley initial fire is consumed (for up to at least 4 A study specifically designed to Experimental Forest, initial prescribed years accumulation). Fire intensity, evaluate the effects of prescribed fire on fires designed to reduce natural fuels rate-of-spread, and flame length are much thinning in ponderosa pine was conducted (Sackett 1980) reduced the number of higher in response to the greatly increased in western Montana (Henderson 1967). stagnated reproduction and sapling stems available fuel. Hence, repeat burning in Small plots were set up on three different from an average of 1553 to 912 per acre. higher quality and quantity fuel does a areas and burned under low, medium, and Small poles, many of which are also better job of thinning stagnated stands. high fire intensity days as defined by stagnated in thickets, were reduced from Crown scorch and consumption kills fire-danger rating levels. Thinning success 192 to 156 stems per acre. In a companion trees and thins stands more effectively was dependent on "close supervision of study at the Long Valley Experimental than bole girdling. Many of the stagnated the fire intensity through manipulation" Forest, fewer intense fires occurred due to sapling stands arose from the famous

127 1918 seed crop and subsequent Season 1974). Prescribed burning is valuable regeneration. Although the trees have for increasing the probability of obtaining grown little in diameter, tree height and Burning during different times of the natural regeneration, especially on the bark thickness have progressed normally year can be used to take advantage of silty, volcanic soils of northern Arizona. through the past 70 years. The unusually various phenological and physiological Schubert (1974) listed the optimum thick bark prevents heat of low intensity conditions of the trees to modify their conditions for obtaining adequate natural fires from penetrating enough to kill trees. susceptibility to fire damage. Spring and regeneration: Subsequent burns in deep litter result in summer may be superior to the traditional high intensity fires which cause extensive fall season for thinning with fire 1. A large supply of good seed. crown damage yet do not damage the bole. (Harrington 1987). We still recommend initial burning in fall. Repeat or rotational 2. A well-prepared seedbed. Thinning Stands with Prescribed burns can be made at other times of the Fire year. 3. Little or no competition from other vegetation. Manipulating the Fire Ambient Conditions 4. A low population of seedeating The most critical element in the use of Taking advantage of ambient conditions insects. fire as a thinning tool is the burner's ability on any given burn day is another way of to manipulate the fire or the fire manipulating the fire environment. Death 5. Sufficient moisture for early seed environment or both to achieve of pine needles occurs when temperatures germination and seedling growth. slow-dissipating, high temperature air in are sustained above 125° F (Hare 1961). the crowns. Manipulation of each fire can When air temperatures are already high, 6. Protection from browsing animals and be achieved in a number of ways. needle and bud temperatures do not have insect pests. Adjusting the direction of fire spread to be raised much by the fire to kill plant relative to wind direction is the most tissue. Likewise with low humidities and Certain of the conditions are common technique. Heading or uphill fires drier fuels, less energy is required to unmanageable (precipitation, seed crops), move at a speed commensurate with evaporate moisture and therefore is more others are partially manageable (seed windspeed creating longer flame lengths, available to heat the crowns. eaters, insects), and a few can be managed greater speed, and higher intensities. Thinning ponderosa pine stands with to improve regeneration success (quality Backing fires, moving against the wind (or prescribed fire is an art that takes skillful seedbeds, competing vegetation). Soil down hill), progress very slowly with short manipulation of the fire environment and moisture seems to be the most critical flame lengths and low intensities. Back the fire itself. Conditions are so diverse, factor in seedling establishment. fires seldom thin stands. spatially and temporally, that the burner Therefore, any activity that results in an Using ignition techniques that interact must skillfully prescribe the proper increase in available moisture or an with one another is probably the most treatment for each thinning situation. increase in soil volume tapped for effective way to thin stands. For example, moisture by roots would be beneficial. a head fire and a back fire coming together NATURAL REGENERATION IN Mineral soil with a light litter covering is create a vertical heat rise that is slow to SOUTHWESTERN PONDEROSA PINE generally thought to be the optimum FORESTS dissipate and concentrates the heat in the seedbed (Pearson 1950, Schubert 1974), crowns. The same effect can be because it allows best seed and seedling accomplished by lighting a spot fire in the Ponderosa pine is considered a difficult contact with available moisture. Much center of a thicket followed by a ring fire species to regenerate in the Southwest precipitation can be absorbed by a deep around the thicket. This technique primarily because regular periods of forest floor and then lost through generally eliminates the center, but leaves moisture stress are caused by droughts and evaporation without reaching the root zone the outer ring of trees. Merging flank fires competition from grasses early in the (Clary and Ffolliott 1969). have the same effect as a head and back growing season (Larson and Schubert fire coming together. Junction zones 1969a, Pearson 1950). Numerous papers Fire Effects on Natural created by spot fires joining will have a point out the difficulties encountered with Regeneration similar effect, yet spread the high heat planting, seeding, and natural regeneration concentrations around and not in a (Heidmann et al. 1982, Larson and Removal of forest floor material is continuous path as with the other Schubert 1969b, Rietveld and Heidmann beneficial. Pearson (1923) noted long situations mentioned (Sackett 1968).

128 ago that spots where slash piles burned the burned plots were carpeted with new rapid seedling emergence. More produced large numbers of rapidly seedlings. Plots that had recently burned extensive monitoring of soil temperatures growing pine seedlings. Reduction of were surveyed extensively, along with the at Fort Valley4 showed soils to be warmer grass competition was the suggested unburned controls. The burned seedbeds on burned sites, but rarely high enough to benefit. Reports by Weaver (1952) and averaged over 90,000 seedlings per acre. be damaging. Warmer soils could also Ffolliott et al. (1977) showed much The unburned plots had 26,000 seedlings result in larger-rooted seedlings (Larson greater pine seedling establishment on per acre. In fall 1984, two of the three 1967), which should have better survival burned than unburned seedbeds. previously burned plots were reburned as during the normal dry periods. Heidmann et al. (1982) studied sites of part of the burning rotation study. One Favorable seedling development on best natural ponderosa regeneration in a plot had 4 years of litter accumulation and seedbeds with improved moisture harvested watershed in central Arizona. the other had 8 years accumulation. Four conditions, and warmer soils is also Of the sites adequately stocked, 70% had years after burning, the following seedling enhanced by greatly improved nitrogen been burned before a moderate cone crop distribution was found: all seedlings were availability. Covington and Sackett (in was produced. Harrington and Kelsey killed on the plots burned with 8 years of press) showed a large increase in available (1979) illustrated the deleterious effect of litter, 7,800 seedlings per acre remained nitrogen from initial prescribed fires and a deep organic layer and competing on the plots burned with 4 years of litter, rotational burning. The increased nitrogen vegetation on ponderosa establishment in 15,000 seedlings per acre remained on the on burned seedbeds allows seedlings to Montana. An additional finding was the plots burned before seed fall, and only attain deep roots and large crowns, which much greater size of pine seedling crowns 1,200 seedlings per acre remained on the facilitate survival through fall drought and and roots in burned plots, presumably controls. the first winter. Since seedlings are from an increase in available nitrogen. Not only does prescribed burning generally much stouter on a burned site, As part of the fire research at Fort provide for favorable seedbeds for perhaps a greater resistance to frost Valley Experimental Forest, burned and germination, it also enhances the growing heaving, common on basalt sites in unburned seedbeds were surveyed after environment for survival. Soil moisture northern Arizona, is developed. the 1976 seed crop was produced (Sackett on burned sites at Fort Valley 1984). Burned plots had 2600 seedlings Experimental Forest was greater than on Preparing Seedbeds With per acre compared with 833 seedlings per unburned sites because moisture can reach Prescribed Fire acre on unburned controls. After 2 years, soil unimpeded by the forest floor no seedlings remained on control plots, material (Haase 1986, Ryan and The effectiveness of prescribed fire in whereas burned plots still supported over Covington 1986). Work with soil consuming the forest floor for hazard 500 seedlings per acre. In a companion thermocouple psychrometers at Fort reduction was discussed earlier in this study, seeds falling on an undisturbed Valley3 confirmed Haase's findings. On paper. The same process produces forest floor seldom reached mineral soil burned sites, soil moisture was slightly to seedbeds of various qualities. Fires that (Haase 1981). Sackett (1984) showed a significantly more available in the 6 to consume the forest floor, leaving little high correlation (r2 = 0.85) between 12-inch soil depths than on unburned organic matter, create microsites that quadrat bare area (square feet) exposed by sites. Since tap roots of seedlings surpass unburned areas in moisture and fire and quadrats stocked: 83% of the new exhumed on burned sites at Fort Valley nutrient status. The burn prescription pine seedlings germinated on microsites are generally 12 inches long after the first parameters listed earlier for fuel reduction where the forest floor was partially or full growing season, more moisture burning also apply to burning for seedbed totally consumed by fire. Another apparently reaches the major rooting production. Once the weather and surface confirmation of this benefit came from a zones where the forest floor has been fuel moisture conditions have been met, prescribed burning study in southwestern consumed. the humus moisture will determine the Colorado (Harrington 1985), where 20 Soil temperatures are also higher on amount of forest floor consumed, and times more pine seedlings per acre were burned seedbeds. At Fort Valley, Milne therefore the quality of the seedbed. located on burned units than on units with (1978) found burned soil averaged 9° F unburned forest floors and Gambel oak. warmer than unburned soils for the time

A more recent seedling survey at Fort period for active pine seed germination. 3 Data on file with Michael G. Harrington at Valley Experimental Forest revealed a Since germination of southwestern the Intermountain Fire Sciences Laboratory, more pronounced regeneration success. In ponderosa pine seed is temperature- Missoula, MT. 1983, seeds were cast at a rate possibly dependent (Larson 1961), burned sites 4Data on file with Stephen S. Sackett of the rivaling that of 1918. By summer 1984, should favor earlier and more Forest Fire Laboratory, Riverside, California.

129 Pattern of Consumption Follow-up CONCLUSIONS

Even though soil with minimal organic When a crop of seedlings establishes, it Very few forest ecosystems compare covering is optimal for seedling is important to defer the suggested with southwestern ponderosa pine in the establishment, the entire site need not be rotational burning program for a number importance of presettlement fire for burned to that degree. In fact, a burn of of years to keep from injuring the young maintenance of forest health and stability. that severity would be hazardous to trees. If seedlings are needed on a Fire history from this region confirms this. conduct and would likely cause extensive particular site (understocking), not much Prescribed fire,in mimicking the natural damage to much of the vegetation and fuel will accumulate after the initial burn. role of fire, can be an ideal tool for accom- soils. Experience shows that a ponderosa We have found seedlings 6 feet high at age plishing many forest management pine forest floor burns in uneven patterns, 10 that are growing where oldgrowth pines objectives. with microsites of mineral soil alternating have died. They have survived successive The successful reduction of natural fire with unburned islands. This pattern is fires because there was no fuel close by to from ponderosa pine stands within the last probably due to variations in fuel scorch them. Once the trees can withstand century has created hazardous, unhealthy moisture, forest floor bulk density, and some fire, the interval between burns forest conditions. With careful fire use consumption of woody fuels. So, a burn should be relatively short. Less fuel under the general guidance of the consuming 50% to 75% of the forest floor produces a less intense, uneven burn, prescriptions and cautions presented would have a variety of partially to almost which makes it easier for seedlings to earlier, the vigor and stability of these completely burned microsites. Partial survive the fire environment. forests should return. With time, we may forest floor consumption is preferred, even be able to improve upon the because high quality seedbeds result and Silvicultural Treatments multiple-use production of presettlement, other site characteristics generally are not natural conditions. The ideas and damaged. The use of prescribed fire in prescriptions presented here are very conjunction with different silvicultural general. Prescribed burning anywhere is Fuel Moisture Contents treatments in the Southwest has not been site-specific. Land managers must learn researched. However, dense slash fuels how prescribed fire relates to their It is difficult to propose specific forest should be reduced, just as should forest resources, develop a prescription pertinent floor moisture contents at which an floor fuels, to facilitate pine seedling to each situation, and monitor results. Each optimum seedbed will result from burning. establishment. One lesson learned years fire is a new experience; therefore, the This is because of the variable pattern of ago was that large clearcuts followed by learning process never ends. consumption. Generally, in dense, or intense burning failed to promote new The state-of-the-art in using fire for fuel otherwise fully stocked groups within generations of ponderosa pine seedlings management, overstory thinning, and stands, prescribed burns will create few (Schubert 1974). Small group selection, natural regeneration is only in the mineral seedbeds, probably because of the shelterwood, or seed tree cuts followed by beginning stages of development. Other high forest floor bulk density. However, broadcast or pile burning favors seedling aspects of prescribed fire have not been on sites where mature trees have been or establishment and reduces the fuel hazard. addressed in this paper or studied. In will be removed, fires burn to mineral soil Fire has not been effective in reducing addition to the tangible parameters, the within a large range of moisture contents. grass or shrub competition (Schubert intuitive nature or art form of fire Places that do not need seedling 1974), but research in different seasons of application is the key to attaining success. regeneration probably will not have much fire application suggest a possible Anyone who has the desire to use fire to mineral soil exposed, and the places where alternative to the current use of mechanical help manage land resources must learn pine regeneration is desired will have scarification (Harrington 1985). Lessons both the art and the science aspects. mineral soil exposed by fire. Given the can be learned from the Pacific Northwest, Desired results can be achieved only when weather conditions and surface fuel where fire is used with various ponderosa practitioners master the techniques of moistures mentioned earlier as general pine silvicultural treatments for logging burning and learn the principles of fire burning guides, applying fire with humus and thinning slash disposal, brush ecology. moisture contents between 25%a and 65% reduction, mistletoe control, as well as site will likely result in adequate mineral soil preparation (Barrett 1979). exposure.

130 LITERATURE CITED Range Experiment Station. 4 p. Range Experiment Station. 4 p. Cooper, Charles F. 1960. Changes in Gaines, Edward M.; Kallander, Arnold, J. F.1950. Changes in ponderosa vegetation, structure, and growth of Harry R.; Waganer, Joe A. 1958. pine-bunchgrass ranges in Arizona southwest pine forests since white Controlled burning in southwestern resulting from pine regeneration and settlement. Ecological Monographs ponderosa pine: results from grazing. Journal of Forestry 30(2):129-164. the Blue Mountain plots, Fort 48:118-126. Cooper, Charles F. 1961. The ecology of Apache Indian Reservation. Journal of Barrett, James W. 1979. Silviculture of fire. Scientific American 304(4): Forestry 56: 323-327. ponderosa pine in the Pacific 150-160. Gordon, Donald T.1967. Prescribed Northwest: The status of our Covington, W. W.; Sackett, S. S. 198 The burning in the interior ponderosa pine knowledge. Gen. Tech. Rep. PNW-97. effect of a prescribed burn in types of northeastern California...a Portland, OR: U.S. Department of southwestern ponderosa pine on preliminary study. Res. Paper PSW-45. Agriculture, Forest Service, Pacific organic matter and nutrients in woody Berkeley, CA: US. Department of Northwest Forest and Range debris and forest floor. Forest Science Agriculture, Forest Service, Pacific Experiment Station. 106 p. 30(1): 183-192. Southwest Forest and Range Experiment Biswell, H. H. 1972. Fire ecology in Covington, W. W.; Sackett, S. S. 198 Station. 22 p. ponderosa pine-grassland. In: Effect of periodic burning on soil Haase, S. M.1981. Effects of prescribed Proceedings of the Tall Timbers Fire nitrogen concentrations in ponderosa burning on ponderosa pine seed Ecology Conference, No. 12. 1972 June pine. Soil Science Society of germination in the Southwest. Flagstaff, 8-9; Lubbock, TX. Tall Timbers America journal 50: 452-457. AZ: Northern Arizona Univ.; 67 p. MS. 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Effect of temperature Effects of a prescribed burn in burning prescriptions for ponderosa on initial development of ponderosa ponderosa pine on inorganic nitrogen pine fuel reductions in southeastern pine seedlings from three sources. concentrations of mineral soil. Res. Arizona. Res. Note RM-402. Fort Forest Science 13: 286-294. Note RM-464. Fort Collins, CO: U.S. Collins, CO: U.S. Department of Larson, M. M.; Schubert, Gilbert H. Department of Agriculture, Forest Agriculture, Forest Service, Rocky 1969a. Effect of osmotic water stress on Service, Rocky Mountain Forest and Mountain Forest and Range germination and development of Range Experiment Station. 5 p. Experiment Station. 7 p. ponderosa pine seedlings. Forest Sackett, Stephen S.1968. A field trial for Heidmann, L. J.; Johnson, Jr., Thomas N.; Science 15: 30-36. regulating prescribed fire intensities. Cole, Quinten W.; Cullum, George. Larson, M. M.; Schubert, Gilbert H. Fire Control Notes 29(3): 5-6. 1982. 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132 Smith, D. F.; Mrouka, Robert; Maupin, John. 1983. Underburning to reduce fire hazard and control Ips beetles in green thinning slash. Fire Management Notes 44: 5-6. Truesdell, P. S.1969. Postulates of the prescribed burning program of the Bureau of Indian Affairs. In: Proceedings of Tall Timbers Fire Ecology. 1969 April, Tallahassee, FL; Tall Timbers Research Station. 235-240. U.S. Department of Agriculture. 1977. Natural forest fire report. Washington, DC.: U.S. Department of Agriculture, Forest Service; 54 p. Weaver, Harold. 1947. Fire-nature's thinning agent in ponderosa pine stands. Journal of Forestry 45: 437444. Weaver, Harold. 1951. Observed effects of prescribed burning on perennial grasses in ponderosa pine. Journal of Forestry 49: 267-271. Weaver, Harold. 1952. A preliminary report on prescribed burning in virgin ponderosa pine. Journal of Forestry 50: 662-667. Wooldridge, David D.; Harold Weaver. 1965. Some effects of thinning a ponderosa pine thicket with a prescribed fire, II. Journal of Forestry 63: 92-95.

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