USDA -- United States 

a Department of Agriculture 

Forest Service 

Rocky Mountain 

Research Station 

Research Note 

RMRS-RN-10 

September 2001 

Fire Frequency Effects on Fuel 

Loadings in pine-Oak Forests of 

the Madrean Province 

Francisco J. Escobedo 

Peter F. Ffolliott 

Gerald J. Gottfried 

Florentino Garza 

Abstract-Loadings of downed woody fuels in pine-oak 

forests of the Madrean Province are heavier on sites in 

southeastern Arizona with low fire frequencies and lower on 

sites in northeastern Sonora, Mexico, with high fire frequencies. Low fire frequencies in southeastern Arizona are attributed largely to past land uses and the fire suppression 

policies of land management agencies in the United States. 

Ecologists and land managers interested in reintroducing 

fire into these forests to reduce fuel loadings and meet other 

land management objectives could use information about 

fuel buildups in their planning efforts. Quantifying these fuel 

loadings could also be useful in improving fire behavior 

models for the forests. 

Keywords: Madrean Province, pine-oak forests, fire frequency, fuel loadings, downed woody fuels, coarse 

woody debris 

1999). Incomplete knowledge of wild-land fuel conditions, however, often constrains successful reintroductions of fire. Quantifying fuel buildups under different 

fire frequencies and severities could provide useful 

information on fire regimes that might be prescribed in 

the reintroduction efforts. We discuss the effects of 

contrasting fire frequencies on fuel loadings in the 

pine-oak forests of the Madrean Province in the Southwestern United States and Northwestern Mexico 

(Brown 1994). 

"Fire frequency" is defined (in this paper) as the 

number of fires occurring on the sites studied since 

1900, when fire suppression polices of land management agencies in the Southwestern United States 

dictated that, in general, fires should be suppressed as 

quickly as possible after their ignition (Pyne and others 

1996; Williams 2000). "Fuel loadings" are the total 

oven-dry weights of fuels per unit of surface area 

(DeBano and others 1998; Pyne and others 1996; 

Whelan 1995). Fuel loadings, therefore, are measures 

Introduction of the potential energy thatmight be released by a fire. 

The "downed woody fraction" of wild-land fuel loadings 

Ecologists and land managers are reintroducing fire are the focus of this paper. Accumulations of duff and 

into ecosystems of the Madrean Province in the South- humus on soil surfaces and herbaceous fuels were not 

western United States to reduce excessive buildups of measured in this study. 

fuels that have a high probability of igniting and 

becoming unnaturally severe wildfires. Prescribed fire 

can also meet other land management objectives such Study Area 

as improving ecosystem functioning, watershed conditions, wildlife habitats, and stocking of natural regenMadrean Province 

eration (Ffolliott and others 1996; Gottfried and others The Madrean Province is situated at the convergence 

of the Sonoran, Chihuahuan, Madrean, and Rocky 

Mountain biogeographic regions (Brown 1994; DeBano 

and others 1995) and the Sierra Madre Occidentalis 

Francisco J. Escobedo is a Graduate Student and Peter F. Ffolliott and Rocky Mountain ranges. Parts of southeastern 

is a Professor, School of Renewable Natural Resources, University of Arizona and New Mexico, and northArizona, Tucson. Gerald J. Gottfried is a Research Forester, U.S. western Chihuahua and northeastern Senora in Mexico 

Department ofAgriculture, Forest Service, Rocky Mountain Research 

Station, Phoenix, AZ. Florentino Garza is a Biologist, Reserva are included in the Province. Elevations range from 

Forestal Nacional Sierra Los Aios. Secretaria de Medio Arnbiente. 29300 to more than 89500 feet. The mountains ". 

Recursos Naturales y Pesca, Hermosillo, Sonora, Mexico. are characterized by oak and pinyon-juniper w oodlands 

USDA Forest Service Res. Note RMRS-RN-10.2001 1 
on the lower elevations and pine-oak and other montane forests on the higher elevations (Brown 1994; 

Gottfried and others 1995; McLaughlin 1995). The 

mountain ranges are separated by broad plains and 

valley floors covered with a variety of desert-shrub and 

desert-grassland plant communities. 

Study Sites 

Fuel loadings were measured on five study sites in 

the pine-oak forests of the Madrean Province. Two 

sites, the McCluqe a~q IJpper Sawmill, were in the 

Huachuca Mountains bf$oytheastprn Acizona, and 

one site, the El Tigre, Gas in the Chiricahua Mountains 

of southeastern Arizona. Two sites, the Plantio and 

Pinos Grandos sites, were in the Sierra de Los Ajos of 

northeastern Sonora, Mexico. The climate, vegetation, 

and physiography of the five study sites have been 

described elsewhere (Kaib and others 1999; Swetnam 

and Baisan 1996a,b). All of the sites have the same 

general climatic, vegetative, and physiographic characteristics (Escobedo Montoya 1998); therefore, it might 

be assumed that fuel loadings on the sites would be 

similar. However, their land-use histories differ. 

Livestock ranches, timber harvesting enterprises, 

and mining operations spread across southeastern 

Arizona in the late 1880s, causing profound changes in 

forest landscapes (Bahre 1991,1995; Bahre and Shelton 

1996; Sayre 1999). There was also a significant reduction of widespread recurring fires with this settlement 

period (Swetnam and Baisan 1996a,b). The scarcity of 

tree fire scars after the 1880s is attributed largely to 

the influence of heavy livestock grazing and other 

intensive land uses that fragmented the forest landscapes and, as a result, reduced the spread of surface 

fires over large areas (Baisan and Swetnam 1990). The 

fire suppression policies of land management agencies 

after the 1900s also contributed to this reduction in 

fires. 

The existing forests in northeastern Sonora evolved 

under a different land-use history. Some fire-regime 

changes coincided with land tenure and agricultural 

reforms in the 1940s and 1950s (Ful6 and Covington 

1994; Kaib 1998). In more recent decades, commercial 

loggingventures also entered the last remaining stands 

of uncut pine-oak forests to modify forest structures 

(Gingrich 1993). Because lightning ignitions have been 

and continue to be infrequently suppressed, fire continues to be a forest-structuring process (Kaib and others 

1999). Fire regimes in northeastern Sonora have continued unchanged for more than a century. 

All of the sites studied are classified as a Pinus 

engelmaniilMuhlenbergia longiligula habitat type association (Muldavin and others 1996). Trees, the primary contributors to the downed woody fraction of fuel 

loadings on the sites, included (in varying intermixtures) Apache pine (Pinus engelmanii), ponderosa pine 

(P. ponderosa), southwestern white pine (P. strobiformis), alligator juniper (Juniperus deppeana), border 

pinyon (P. discolor), Gambel oak (Quercus gambelii), 

Arizona white oak (Q. arizonica), silverleaf oak (Q. 

hypoleucoides), and netleaf oak (Q. rugosa). The occasional shrubs were New Mexican locust (Robinia 

neomexicana), Chihuahua ash (Fraxinus papillose), 

and mountain-mahogany (Cercocarpus rnontanus). 

While not all of the tree and shrub species were present 

on all of the sites, there was no pattern in the composition of woody species and fire frequencies. No attempt 

was made to separate the fuels measured on a site by 

species or species groups in the analysis of this study. 

The densities of all tree species 5 inches d.b.h. and 

larger on the Upper Sawmill and McClure sites in 

southeastern Arizona and the Plantio site in northeastern Sonora were statistically similar and averaged 

about 85 square feet of basal area per acre. Densities of 

these trees on the El Tigre site in southeastern Arizona 

and the Pinos Grandos site in northeastern Sonora 

were also similar and averaged 70 square feet of basal 

area per acre. There was no relationship between 

overstory densities and fire frequencies of these sites. 

Fire frequencies on the sites differed. The three sites 

in southeastern Arizona were characterized by changed 

fire regimes in which the (historical) frequent and low 

severity regimes ceased in the early 1900s, largely 

because of the occurring forest fragmentation and fire 

suppression policies in the region. These sites have had 

relatively fewer fires of generally higher severity since 

1900. In contrast, the two sites in northeastern Sonora 

have had more continuous and higher frequencies of 

fire since 1900. Local people's interest and involvement 

in fire suppression activities had been insignificant 

throughout Mexico until the middle 1980s, when the 

federal government initiated active programs to suppress uncontrolled forest fires (Bojorquez-Tapia 1990). 

The fire regimes on the northeastern Sonora sites 

mostly reflect the earlier lack of the public's concern 

about fire. Fire frequencies since 1900 for all study 

sites (table 1) had been established earlier from fire 

chronologies of fire-scarred trees on or near the sites 

(Kaib and others 1999; Swetnam and Baisan 1996a,b). 

Methods 

Fuel loadings on each site were measured on a 2.5-acre 

grid of systematically located plots established at equally 

spaced intervals on the site. Similar grids have been 

used in other studies to measure fuel loadings in pine 

and other pine-oak ecosystems (Arno and others 1997; 

Ful6 and Covington 1994,1995; Harrington 1985). The 

grids consisted of 30 plots on the Chiricahua and 

Huachuca Mountain sites and 25 plots on the Sierra de 

Los Ajos sites. Errors of 20 percent or less, adequate 

levels of precision for most fuel inventories (Brown 

1974), were obtained with these plot numbers. Plot 

USDA Forest Service Res. Note RMRS-RN-10. 2001 
Table 1-Means and standard errors of loadings of downed woody fuels and the fire frequencies for study sites. 

- ---- 

Coarse Total fuel Fire 

Smaller fuels Larger fuels woody debris loadings frequency 

Sound wood Sound wood Sound wood Sound wood Decaying wood (all fuels (all downed (fires since 

Site 0.1-0.25 inch >0.25-1 inch >1-3 inches >3 inches >a-inches) >3 inches) woody fuels) ' 1900) 

................................. tonsperacre .................................... number 

McClure 0.25f 0.0091 0.40 f 0.018 1.0 + 0.091 1.8 k 0.38 6.5 f 0.22 8.3 f 0.44 9.9 f 0.46 5 

Uppersawmill 0.16f0.0073 0.56f0.019 3.0k0.093 8.750.40 0.9f0.19 9.6f0.43 13.3f0.47 2 

El Tigre 0.73k0.0092 0.9850.036 4.1f0.090 12.6k0.39 3.2f0.20 15.8f0.46 21.6k0.48 0 

Plantio 0.15f0.012 0.16f0.021 0.9f0.12 3.3f0.48 0.6f0.27 3.9k0.52 5.1k0.48 9 

Pinos Grandos 0.14f 0.011 0.22+ 0.020 1.3f 0.26 3.1 f 0.46 0.3 + 0.26 3.4 f 0.54 5.0 k 0.42 13 

centers were the starting point for a randomly oriented 

planar-intersect transect that was established to measure fuels. The planar-intersect technique has the same 

theoretical basis as the line-intersect technique (Van 

Wagner 1968). It involves counting intersections offuels 

with vertical sampling planes that resemble guillotines 

dropped through the accumulated fuels. 

Diameter classes of fuels that were intersected by the 

planar transects were tallied by the classification of 

Brown (1974), that is, sound woody fuels 0.1 to 0.25 

inch, >0.25 to 1 inch, >1 to 3 inches, >3 inches, and 

decaying (rotten) woody debris >3 inches. These diameter classes correspond to the 1-hour, 10-hour, 100- 

hour, and 1,000-hour time-lag classes of the National 

Fire Danger Rating System (Martin and others 1979; 

Pyne and others 1996), respectively. Fuels >3 inches in 

diameter, both sound and decaying, are also called 

"coarse woody debris." Coarse woody debris includes 

limbs, stems, and roots of trees and shrubs in various 

stages of decay (DeBano and others 1998; Graham and 

others 1994; Pyne and others 1996). Tallies of the 

respective diameter classes with their standard specific gravity values were used in determining fuel 

loadings in tons per acre. 

The null hypothesis that fire frequencies since 1900 

did not affect the loadings of downed woody fuels on the 

study sites was tested by comparative statistics at the 

0.05 level of significance. Analyses of variances 

(ANOVAs) were tested for homoscedacity by Levene's 

test for unequal variances (Ramsey and Shafer 1997). 

Normality assumptions were assessed by residual and 

normal probability plots. Data sets obtained in the 

study were skewed, which is typical of fuel loading 

studies (Harmon and others 1986). The data were 

transformed to natural logs to meet the ANOVA assumptions (Zar 1974). One-way classification ANOVAs 

were used to determine the effects of fire frequencies on 

fuel loading. The Tukey-Kramer multiple range test 

was used to determine differences in fire frequencies to 

account for unequal sample sizes between the sites in 

the southeastern Arizona and the sites in northeastern 

Sonora (Ramsey and Shafer 1997). Only the transformed data were used in the statistical tests. 

Results and Discussion 

Fuel loadings and fire frequencies since 1900 for the 

five study sites are summarized in table 1. In general, 

there were heavier fuel loadings on the sites in southeastern Arizona than on the sites in northeastern 

Sonora. These differences are attributed largely to 

fewer fires since 1900 on the southeastern Arizona 

sites than on the sites in northeastern Sonora. Consequently, fuels tend to accumulate for longer periods of 

time on the southeastern Arizona sites than on the 

northeastern Sonoran sites. Loadings of downed woody 

fuels have been arbitrarily separated into those of 

"smaller fuels" up to 1 inch in diameter, "larger fuels" 

greater than 1 inch in diameter, and "coarse woody 

debris" for discussion purposes. 

Smaller Fuels 

El Tigre, the study site with no fires since 1900, had 

the heaviest loading of downed woody fuels in the 0.1- 

to 0.25-inch diameter class (table 1). This site had 

about five times the amount of fuels in this diameter 

class than the Plantio and Pinos Grandes sites, the 

sites with the highest fire frequencies. El Tigre also 

had heavier loadings of fuels in this diameter class 

than the Upper Sawmill and McClure sites. There were 

no statistical differences in the loadings of fuels in this 

diameter class among the Upper Sawmill (two fires 

since 1900), McClure (five fires), Plantio (nine fires), 

and Pinos Grandes (13 fires) sites. Loadings of fuels in 

the >0.25- to 1-inch diameter class were similar to the 

pattern observed for the 0.1- to 0.25-inch diameter 

class. That is, the loading of downed woody fuels in this 

diameter class was significantly heavier on the El 

Tigre site than the other sites, and there were no 

statistical differences in loadings of fuels in this diameter class among the other sites. 

Downed woody fuels up to 1 inch in diameter are 

more readily ignited and consumed by fire than are 

larger fuels and, consequently, are less likely to accumulate and persist in large amounts on sites with 

repeated or recent fire (Anderson 1982; DeBano and 

USDA Forest Service Res. Note RMRS-RN-10. 2001 
others 1998; Pyne and others 1996). This trend in fuel 

loadings is borne out by the results of this study. While 

the fire chronology for the Upper Sawmill site indicated that only two fires had occurred since 1900, the 

most recent fire burned in 1983, and therefore, a 

relatively low loading of smaller fuels was measured. 

Larger Fuels 

Loadings of downed woody fuels >1 to 3 inches in 

diameter were heavier on the two sites with lowest fire 

frequencies, El Tigre (0) and Upper Sawmill (2), than 

on the other study sites (table 1). There were no 

statistical differences in the lower loadings in this 

diameter class among the McClure, Plantio, and Pinos 

Grandes sites. The El Tigre and Upper Sawmill sites 

also had three to four times heavier loadings of sound 

woody fuels >3 inches in diameter than the sites with 

higher fire frequencies. Loadings of fuels in this diameter class on the McClure, Plantio, and Pinos Grandos 

sites were not statistically different. Loadings of decaying woody debris >3 inches in diameter were heaviest 

on the McClure and El Tigre sites. One explanation for 

the heavier loading of decaying fuel on the McClure site 

is that soil on this site had the deepest A horizon of all 

the study sites (Escobedo Montoya 1998), suggesting 

that this site also had the greatest surface stability 

which, in turn, could contribute to a higher rate of 

decomposition than the other sites (Edmonds 1991). 

There were no statistical differences in the lower loadings ofthis fuel class among the Upper Sawmill, Plantio, 

and Pinos Grandes sites, likely because all of the sites 

have experienced some fire occurrence since the late 

1970s. 

Larger fuels tend to ignite and burn more slowly than 

smaller fbels with fire of low to medium severity and, 

therefore, might not be totally consumed in such a fire 

(DeBano and others 1998; Pyne and others 1996). Some 

larger fuels, therefore, can remain following burning of 

the site. With fire of high severity, however, larger fuels 

can burn vigorously, and the rate of fuel reduction is 

greater than in the case with smaller fuels. The general 

pattern of loadings illustrated for both smaller and 

larger fuels in this study is that lesser loadings occurred 

on the sites with a more frequent occurrence of fire. 

Coarse Woody Debris 

El Tigre has the heaviest loading of coarse woody 

debris, followed by the loadings on the Upper Sawmill 

and McClure sites, which were statistically similar, 

and the Plantio and Pinos Grandos sites, which were 

also statistically similar (tablel). The range of loadings 

of coarse woody debris observed in this study is comparable to the loadings reported in other pine-oak forests 

of the Madrean Province. Sackett (1979) found an 

average loading of coarse woody debris of 7.8 tons per 

acre in ponderosa pine-Gambel oak stands on the San 

Carlos Reservationin east-central Arizona. Harrington 

(1981) indicated that the range of loadings of coarse 

woody debris was 4 to 10 tons per acre in pine-oak 

stands that had not been burned for 70 years on the 

Santa Catalina Mountains in southeastern Arizona. 

Alanis-Morales (1996) reported a range of 5 to 20 tons 

per acre Pinus durangesis stands near Madera, Chihuahua, Mexico. 

Total Loadings of Downed Woody Fuels 

The heaviest total loadings of downed woody fuels 

occurred on the El Tigre site, followed by the Upper 

Sawmill and McClure sites, which were statistically 

similar (table 1). The lower total loadings found on the 

Plantio and Pinos Grandes sites were statistically 

similar. All of the sites had experienced some timber 

harvestingin the past and, therefore, unknown amounts 

of logging slash likely contributed to the total fuel 

loadings of downed woody fuels measured in this study. 

The total loadings of downed woody debris found in this 

study were similar to values reported for ponderosa 

pine forests in the Southwestern United States in 

previous studies. Sackett (1979) found an average 

loading of downed woody material fuels greater than 

1 inch of 9.2 tons per acre. Harrington (1982) reported 

loadings of 5 to 11 tons per acre. Graham and others 

(1994) reported total fuel of 5 to 13 tons per acre. 

Management Implications 

Information on the effects of fire frequencies on wildland fuel conditions could be helpful to ecologists and 

land managers considering the reintroduction of fire as 

a natural disturbance process in the pine-oak forests of 

the Madrean Province. While the information presented in this paper is both site-specific and specific to 

the type of fuels measured, the trends illustrated show 

that sites with higher fire frequencies accumulate less 

downed woody fuels than sites with lower fire frequencies. Depending on the fuel loading that might be 

acceptable on a site, an initial schedule of introduced 

fire occurrences could be prescribed, tested, and evaluated in reference to maintaining fuel loadings at the 

acceptable level while attaining other land management objectives. This initial schedule might then be 

modified through time and, as necessary, to sustain the 

desired fuel loading conditions and land management 

objectives. 

The fuel loading values presented in table 1 could 

also be incorporated into fuel models to describe the 

fuel component in fire behavior models. Fire behavior 

models simulate how a fire reacts to the available fuels, 

local weather conditions, and topography (DeBano and 

USDA Forest Service Res. Note RMRS-RN-10. 2001 
others 1998; Pyne and others 1996). Differences in fire 

behavior are largely related to fuel loadings and the 

diameter-class distributions and moisture contents of 

the fuels. However, further investigations of the applicability of available fuel models and fire behavior 

models, such as BEHAVE (Burgan 1987; Burgan and 

Rothermel1984), to the pine-oak forests of the Madrean 

Province are necessary before their widespread usage 

in these ecosystems. 

Reintroducing fire to reduce fuel loadings and meet 

other land management objectives must be balanced 

against the importance of sustaining the cycling of 

nutrients in the ecosystem to be burned. Excessive 

removal of coarse woody debris by repeated fires could 

disrupt the mycorrhizal activity that is associated with 

this debris and, in doing so, affect the functioning of soil 

that is necessary to maintain a "high level" of plant 

productivity (Graham and others 1994; Harmon and 

others 1986; Jurgensen and others 1997). Depending 

on the habitat type association, Graham and others 

(1994) concluded that loadings of coarse woody debris 

from 5 to almost 25 tons per acre were "optimal" to 

maintain properly functioning soils in the forest types 

of the Rocky Mountains. Loadings of coarse woody 

debris necessary to sustain the "health" of forest soils 

in the pine-oak forests of the Madrean Province have 

not been determined. 

Conclusions 

Fire is important to the functioning of ecosystems 

because of its impacts on nutrient cycling, vegetative 

structure and composition, and the spatial distributions of plants and animals. Natural fire regimes have 

been altered in the Southwestern United States by 

past land-use practices and by the fire suppression 

policies of land management agencies. One consequence of this alteration has been the changes in 

loadings of downed woody fuels compared to those in 

similar forest stands in northwestern Mexico where 

more natural fire regimes continue. 

Study sites in southeastern Arizona where fires have 

occurred less frequently since 1900 have heavier fuel 

loadings than sites in northeastern Sonora where fires 

are more frequent. The combination of heavier fuel 

loadings on the former sites provides the potential for 

high-intensity, stand-replacing fires. Information presented in this paper on relationships between fire 

frequencies and loadings of downed woody fuels in 

pine-oak forests should be helpful to ecologists and 

land managers in reintroducing periodic fire into this 

ecosystem of the region. Although these results are site 

specific, the information should be applicable to the 

larger region covered by the pine-oak forests of the 

Madrean Province. 

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