USDA Forest Service Research Note RMRS-RN-36. 2008 1

United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

Research Note

RMRS-RN-36

September 2008

Abella, Scott R.; Fulé, Peter Z. 2008. Changes in Gambel oak 

densities in southwestern ponderosa pine forests since EuroAmerican settlement. Res. Note RMRS-RN-36. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station. 6 p.

Scott R. Abella is Assistant Research Professor, Public Lands 

Institute and School of Life Sciences, University of Nevada Las Vegas, 

Las Vegas, NV. 

Peter Z. Fulé is Associate Director and Associate Professor, Ecological Restoration Institute and School of Forestry, Northern Arizona 

University, Flagstaff, AZ.

Introduction

It is well documented that small-diameter ponderosa 

pine (Pinus ponderosa) trees have increased in density in 

southwestern forests since the late 1800s. It may be less 

well known, however, that dramatic changes in Gambel 

oak (Quercus gambelii) densities have also occurred during this period in these forests. Gambel oak is one of only 

a few deciduous trees in ponderosa pine forests and has 

numerous ecological and human values (Reynolds and 

others 1970). This understory or mid-story tree provides 

mast, forage, and habitat for wildlife and is prized by 

humans for fuelwood and aesthetics (Harper and others 

1985). However, Gambel oak is often passively managed in ponderosa pine forests. This may partly result 

from poorly known treatment effects on Gambel oak or 

a perception that oak abundance has decreased.

Changes in Gambel Oak Densities in 

Southwestern Ponderosa Pine Forests 

Since Euro-American Settlement 

Scott R. Abella and Peter Z. Fulé

In this note, we review published literature to ascertain: 

(1) changes in Gambel oak densities in ponderosa pine 

forests since Euro-American settlement in the late 1800s, 

(2) the timing and possible mechanisms responsible for 

these changes, and (3) whether data exist to substantiate a hypothesis that densities of large-diameter oaks 

have decreased. We begin by briefly reviewing the basic 

characteristics of Gambel oak’s biology, specifically for 

the frequent-fire pine-oak forests (such as in Arizona and 

New Mexico) that are the focus of this synthesis.

Biological Characteristics

In ponderosa pine forests, Gambel oak stems can exceed 

36 inches (91 cm) in diameter at breast height and 400 

years in age (Reynolds and others 1970; Swetnam and 

Brown 1992). This clonal oak recruits new stems by vigorous sprouting, particularly after existing stems are top 

killed (Brown 1958; Harper and others 1985). Regeneration by seedlings to produce new genets, however, does 

occur. Neilson and Wullstein (1986) found that seedling 

densities ranged from 49 to 534/acre (120 to 1320/ha) on 

15 plots in Arizona and New Mexico pine-oak forests. 

Fire can kill more than 50 percent of small stems less than 

2 inches (5 cm) in diameter (Harrington 1985), but mortality probably decreases with increasing diameter (Fulé 

and others 2005). This observation concurs with Gambel 

oak’s persistence in pine-oak forests that often burned at 

least once every 10 years before Euro-American settlement 

(Fulé and others 1997; Madany and West 1983).

Abstract––Densities of small-diameter ponderosa pine (Pinus ponderosa) trees have increased in southwestern ponderosa pine forests during a period of fire exclusion since Euro-American settlement in the 

late 1800s. However, less well known are potential changes in Gambel oak (Quercus gambelii) densities 

during this period in these forests. We reviewed published literature to summarize changes in oak density in ponderosa pine forests over the past 140 years and evaluated the possibility that large-diameter 

oaks have decreased in density. All nine studies examining oak density changes found that densities of 

small-diameter oaks have escalated. Increases ranged from 4- to more than 63-fold. These increases are 

comparable on many sites to those of ponderosa pine. Studies in northern Arizona, which analyzed cut 

stumps and past and present diameter distributions, did not find strong evidence that large-diameter oaks 

on average have declined in density. However, since oak cutting varied across the landscape, this important 

question needs additional study. Actively or passively managing Gambel oak requires decisions about 

desired future conditions and how to attain them. A possible contention for passive management—that 

the overall abundance of oak has decreased—is not supported by research published to date.
2 USDA Forest Service Research Note RMRS-RN-36. 2008

Density Changes

Our literature review uncovered nine studies reconstructing Gambel oak densities in the late 1800s in 

southwestern pine-oak forests. We were able to obtain 

numerical data from eight of the nine studies (table 1). 

The other study (Ruess 1995) provided graphical data 

with results consistent with those of the other studies. 

All studies reported increases in Gambel oak densities 

since settlement. Magnitudes of the increases ranged from 

16 to 450 trees/acre (40 to 1112 trees/ha). These increases 

were often comparable to irruptions in ponderosa pine 

densities (table 1). While these studies were performed in 

northern Arizona, except for one in southern Utah, these 

increases are probably representative of upsurges in other 

pine-oak forests due to the nearly ubiquitous exclusion of 

historically frequent surface fires (Grissino-Mayer and 

others 2004). These fires probably partly kept woody 

regeneration in check (Fulé and others 1997).

Timing and Mechanisms for 

Increased Densities

Data from Fulé and others (1997) provide insight into 

the timing and probable mechanisms for the oak density 

increases. At Camp Navajo in northern Arizona, Fulé and 

others (1997) found that 40 percent of the 32 oaks/acre 

(80/ha) at the site in 1883 were less than 2 inches (5 cm) 

in diameter in 1883. Prior to 1883, frequencies of surface 

fires averaged 4 years, but 1883 represented the start of 

fire exclusion associated with settlement. The authors 

hypothesized that many of the small oaks present in 1883 

would have been killed had frequent fires continued. This 

hypothesis concurs with prescribed burning experiments 

in southwestern Colorado (Harrington 1985) and northern Arizona (Abella and Fulé 2008) that documented 

fire-induced mortality of small stems less than 4 inches 

(10 cm) in diameter. Further augmenting oak densities in 

the study by Fulé and others (1997), a pulse of new stems 

also established from about 1885 to 1914 shortly after fire 

exclusion began. Several factors could have contributed to 

recruitment of these new stems. Pine logging and livestock 

grazing may have reduced tree and herbaceous competition 

with the oaks. Cutting of large oaks possibly stimulated 

dense sprouting. Finally, fire exclusion likely facilitated 

survival of the fire-susceptible small stems. In summary, 

the elevated oak densities of 191/acre (417/ha) at the site 

in 1994 probably arose due to survival of small-diameter 

oaks on-site in 1883, followed by the establishment of new 

stems during a period of fire exclusion.

Waltz and others (2003) reported a similar sequence 

of events in pine-oak forests in Grand Canyon-Parashant 

National Monument in northern Arizona. Their study 

documented a large irruption in oak density around 

1870 coinciding with the cessation of frequent fires. 

Additional oaks recruited after that time. In southern Utah, Madany and West (1983) hypothesized that 

livestock grazing made grassy interspaces between oak 

clumps more susceptible to invasion by oak stems whose 

survival was facilitated by fire exclusion. Historical photos also suggest that any early post-settlement increases 

were further augmented by later stem recruitment during 

the 1900s (fig. 1). Stem maps shown in Ruess (1995) 

suggest that oak increases may have resulted from both 

vegetative expansion of existing clones and increased 

seedling establishment.

Table 1––Summary of studies measuring changes in oak and pine densities in southwestern ponderosa pine-Gambel oak forests.

Gambel oak Ponderosa pine

Location Prea Posta Pre Post Pre yearb Reference

- - - - - - - - - - - - - - - -Trees/acre- - - - - - - - - - - - - - - -

Beaver Creek Watershed, N. AZ 1 63 17 769 1867 Covington and Moore 1994

Walnut Canyon, N. AZ 6 44 22 102 1876 Menzel and Covington 1997

Camp Navajo, N. AZ 34 191 26 291 1883 Fulé and others 1997

Kaibab National Forest, N. AZ 6 to 28c 64 to 177 18 to 43 167 to 1,353 1887 Fulé and others 2002a

Grand Canyon National Park, N. AZ 1 to 29 32 to 264 26 to 63 78 to 261 1879,1887 Fulé and others 2002b

Zion National Park, UT-pine/oak 0 2 to 104 1 to 23 16 to 102 1883 Madany and West 1983

Zion National Park, UT-oak woodland 31 to 115 459 to 565 0 to 1 0 to 48 1883 Madany and West 1983

Mt. Trumbull, N. AZ 17 to 30 75 to 127 13 to 17 173 to 276 1870 Roccaforte 2005

Mt. Trumbull, N. AZ 1 to 35 17 to 244 6 to 26 110 to 684 1870 Waltz and others 2003

a

Pre = presettlement; post = postsettlement.

b

Year for which presettlement densities were reconstructed, normally the last year in which surface fire occurred. Postsettlment measurements were made a few 

years before a study’s publication date. c

Range of means averaged on a site basis for studies reporting results for multiple sites.
USDA Forest Service Research Note RMRS-RN-36. 2008 3

Large-Diameter Oaks

Research published to date suggests that densities of 

small-diameter oaks have sharply increased in the past 140 

years in ponderosa pine-Gambel oak forests. Less clear, 

however, is whether large-diameter oaks have dwindled 

because of fuelwood harvest or other factors (Mast 

2003). Diameter distributions at Camp Navajo, Arizona, 

in 1883 and in 1994/1995, suggest that densities of oaks 

greater than 10 inches (25 cm) in diameter have actually 

increased slightly since 1883 (fig. 2). Brischler (2002) 

analyzed densities of oak cut stumps in seven stands on 

the Coconino National Forest in northern Arizona and 

found that stump densities averaged 13/acre (33/ha). 

Figure 1––Ninety-three year photo series of a Gambel oak clump (left center) that increased in density by approximately 

75 percent from 1913 to 2006. The photos were taken near Coulter Cabin in the Coconino National Forest, 12 miles (19 km) 

south of Flagstaff, Arizona. The 1913 photo was taken following a group selection harvest of ponderosa pine. The 1913 and 

1928 photos were taken by Hermann Krauch (U.S. Forest Service photo 16491A), the 1959 photo was taken by M.M. Larson 

(U.S. Forest Service photo LA-86), and the 2006 photo was taken by A.J. Sánchez Meador. Photos provided by A.J. Sánchez 

Meador (U.S. Forest Service). 

1913

1959

1928

2006

More than 70 percent of stumps, however, were less 

than 8 inches (20 cm) in diameter (inside bark at stump 

height), consistent with the high proportion of live stems 

and snags in these size classes (fig. 3). Brischler (2002) 

hypothesized that harvesting was greater for small- to 

medium-diameter (4 to 8 inch diameter) oaks than for large 

oaks because smaller oaks were more available, easier to 

cut and remove, less likely to be hollow, or less likely to 

be noticed as missing (in the case of illegal cutting). On 

the Kaibab National Forest near the South Rim of the 

Grand Canyon, Fulé and others (2002b) found that oak 

stumps less than 16 inches (40 cm) in diameter averaged 

6/acre (15/ha) compared to 0.7/acre (1.7/ha) for stumps 

greater than 16 inches in diameter.
4 USDA Forest Service Research Note RMRS-RN-36. 2008

Figure 3––Diameter distributions of standing Gambel oak stems and cut stumps measured in 2000 and 

averaged for seven pine-oak stands, Coconino National Forest, Arizona. Data from Brischler (2002).

Figure 2––Gambel oak diameter distributions in 1883 and in 1994/1995 at Camp Navajo, northern 

Arizona. Data from Fulé and others (1997) and P.Z. Fulé (unpublished data). 
USDA Forest Service Research Note RMRS-RN-36. 2008 5

In forest reconstruction studies, it is possible that stumps 

from long-ago oak harvesting (for example, at the time 

of settlement) have decomposed and are missing from 

tree density estimates (Brischler 2002, Fulé and others 

1997). However, Fulé and others (1997) noted that old 

oak stumps could be identified on their plots. Their 

observation is corroborated by Long’s (1941) study of 

different-aged fences composed of oak posts in Arizona 

and New Mexico. He found that while 90 percent of the 

fence posts were from oaks less than 7 inches (18 cm) 

in diameter, posts persisted for more than 60 years (the 

oldest fences examined). A study on ponderosa pine by 

Moore and others (2004) provides additional support 

for the accuracy of forest reconstruction methods. By 

comparing reconstructed tree densities with quantitative forest inventory reports from the early 1900s, these 

authors found that reconstruction methods were accurate 

to within 10 percent for ponderosa pine tree density.

Past oak cutting varied across the landscape, making 

generalizations difficult about the possible losses of 

large oaks due to cutting from place to place (Brischler 

2002). However, evidence has not been published to date 

indicating that densities of large oaks on average have 

diminished, certainly not to the extent that densities of 

large ponderosa pine have diminished due to harvesting 

(Covington and Moore 1994; Mast 2003). Nevertheless, 

additional research is needed to enrich our understanding of the demographics of large oaks in the past and the 

present. Furthermore, consideration should be given to 

conserving existing large oaks because of their ecological value for plant and animal habitat (Harper and others 

1985, Reynolds and others 1970).

Management Implications

On many sites in the past 140 years, small-diameter 

stems of Gambel oak have increased in density as dramatically as those of ponderosa pine. Although additional 

research is needed to better understand the long-term 

dynamics of oak density and sprouting under different 

fire frequencies, there is evidence that the exclusion of 

fire since settlement has allowed many of these small 

oak stems to become established. Establishment of these 

stems probably would have been limited had a frequentfire regime continued. Prescribed fire or mechanical 

thinning of small oak stems can temporarily reduce oak 

density while stimulating sprouting to provide a variety 

of shrubby and tree forms of oak. However, any type of 

disturbance that stimulates sprouting, but is followed by 

a disturbance-free period, is likely to further increase the 

density of small stems.

Although published evidence is limited for decreases 

in large Gambel oak since settlement, maintaining existing large oaks could be a priority because of their high 

ecological value. Tactics to protect large oaks during 

prescribed fire might include avoiding lighting near the 

bases of large oaks and keeping fuels (for example, pine 

slash) away from oaks (Abella and Fulé 2008). To further 

maintain health and crown vigor of large oaks, thinning 

competing trees (such as ponderosa pine) appears effective (Fulé and others 2005). Due to oak’s clonal habit, it 

is unclear if the small stems that have proliferated since 

settlement compete for resources with large, old oak 

stems. This is an important research need. Managing 

Gambel oak, including the option of passive management, requires decisions about desired future conditions 

and how to attain them. A possible reason for passive 

management, that overall oak abundance has decreased, 

is not supported by published research.

Acknowledgments

We thank Amy Waltz for providing raw data from her 

2003 paper; Andrew Sánchez Meador for providing the 

93-year photo series in figure 1; Wally Covington for 

suggesting and supporting literature synthesis work; and 

Peter Ffolliott, John Paul Roccaforte, Charlie Denton, 

Judy Springer, Dennis Lund, and Kristi Coughlon for 

reviewing the manuscript. Funding was provided by 

the U.S. Forest Service and the Ecological Restoration 

Institute.

References

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southwestern ponderosa pine-oak forests. Res. Note RMRS-RN-34. 

Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Research Station. 6 p.

Brischler, Erik J. 2002. Size structure and spatial arrangement of 

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University. 128 p. Thesis.

Brown, Harry E. 1958. Gambel oak in west-central Colorado. Ecology. 39:317-327. 

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