USDA Forest Service Research Note RMRS-RN-37. 2008 1

United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

Research Note

RMRS-RN-37

September 2008

Abella, Scott R. 2008. Gambel oak growth forms: management 

opportunities for increasing ecosystem diversity. Res. Note 

RMRS-RN-37. Fort Collins, CO: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research Station. 6 p.

Scott R. Abella is Research Specialist, Sr., Ecological Restoration 

Institute, Northern Arizona University, Flagstaff, AZ. Present address: 

Assistant Research Professor, Public Lands Institute and School of Life 

Sciences, University of Nevada Las Vegas, Las Vegas, NV. 

Gambel Oak Growth Forms: 

Management Opportunities for 

Increasing Ecosystem Diversity

Scott R. Abella 

Introduction

Gambel oak (Quercus gambelii) is one of only a 

few deciduous trees in southwestern ponderosa pine 

(Pinus ponderosa) forests. Oak is important for wildlife 

(Reynolds and others 1970), plant habitat (Brown 1958), 

soil fertility (Klemmedson 1987), and human values 

including aesthetics and fuelwood (Harper and others 

1985). Recent reports reviewing Gambel oak literature 

have highlighted several principles of oak ecology in 

ponderosa pine forests. First, densities of small-diameter 

oaks have sharply increased in the past 140 years during a period of fire exclusion following Euro-American 

settlement (Abella and Fulé 2008b). For example, Fulé 

and others (1997) found that Gambel oak densities on 

a northern Arizona landscape increased from 32/acre 

(80/ha) in 1883, to 191/acre (471/ha) in 1994. Second, 

fires burned presettlement pine-oak forests on average at 

least once every 10 years, which is similar to pure ponderosa pine forests (Abella and Fulé 2008a). Fire is part 

of Gambel oak’s evolutionary environment, and multiple 

growth forms of the species persisted in frequently burned 

Abstract––Gambel oak (Quercus gambelii) clones have several different growth forms in southwestern 

ponderosa pine (Pinus ponderosa) forests, and these growth forms each provide unique wildlife habitat and 

resource values. The purposes of this note are to review published growth-form classifications for Gambel 

oak, provide examples of ecological effects of different growth forms, and summarize management strategies for promoting desired growth forms. Four different growth-form classifications have been published, 

which generally recognize variants of three basic forms: shrubby thickets of small-diameter stems, pole-sized 

clumps, and large trees. These growth forms exemplify ecological and management tradeoffs. For example, 

shrubby forms provide browse and cover yet produce few acorns, while larger oaks supply more acorns but 

offer little accessible browse or cover near the ground. Large oaks can be encouraged by thinning competing 

trees and protecting existing large stems from damage by prescribed fire or unauthorized fuelwood harvest. 

Pole-sized clumps may develop from thickets through time by self-thinning. Mechanically thinning within 

clumps may accelerate growth of remaining stems, depending on resource allocation within clones. Burning 

or cutting to stimulate sprouting sustains shrub-thicket forms.

forests. Third, there is no general ecological basis for 

not actively managing pine-oak sites (Abella and Fulé 

2008b), although passive management (in other words, 

no management) may be the most practical or appropriate 

depending on specific management objectives.

Gambel oak’s biological characteristics, including its 

clonal habit and resprouting ability, result in the species 

having several different growth forms (Tiedemann and 

others 1987). These growth forms range from large, 

individual trees to multi-stemmed, shrubby thickets. 

Each form provides unique wildlife habitat and resource 

values (Kruse 1992). By managing the proportions and 

characteristics of these growth forms, great opportunities 

exist to enhance multiple resource values in pine-oak 

forests (Clary and Tiedemann 1992).

The purposes of this note are to: (1) review existing 

classifications of Gambel oak growth forms, (2) provide 

examples of the influences of different growth forms on 

ecosystem components, and (3) summarize management 

strategies for promoting specific growth forms. This note 

focuses on ponderosa pine forests where oak’s tree form 

occurs, but a growth-form classification (Brown 1958) 

potentially applicable to ponderosa pine forests is included 

from the northern part of Gambel oak’s range in Utah 

and Colorado where the species forms shrublands.

Growth Form Classifications

Four classifications of Gambel oak growth forms 

have been published (table 1). Each classification distinguishes growth forms based on different tree and clump 

characteristics. It should be recognized that some oaks 
2 USDA Forest Service Research Note RMRS-RN-37. 2008

encountered in the field may not readily fit into categories, 

as the classifications are an attempt to categorize continuous variability. These classifications are useful for site 

inventories, monitoring oak condition, understanding 

oak ecology, and developing management prescriptions 

and evaluating their effects. The four classifications are 

reviewed in the following sections.

Brown (1958) Clump Area

Brown (1958) developed the earliest Gambel oak 

classification, which he based on the ground area occupied 

by oak (table 1). He developed this classification for 

west-central Colorado where Gambel oak forms shrublands. The classification probably has limited utility in 

most ponderosa pine forests because oak develops smaller 

clumps in these forests than in shrublands. However, 

Brown’s (1958) classification could be useful in pine 

forests as a canopy-area classification if it was modified 

to include smaller areas of oak canopy cover.

Kruse (1992) Successional Status

Kruse (1992) classified Gambel oak growth forms 

based on their anticipated ages and successional status. Growth forms ranged from young, brushy forms 

to old, post-mature forms (table 1). He defined poles 

as stems > 15 ft (4.5 m) tall and > 3 inches [8 cm] in 

diameter at breast height. Kruse’s (1992) interpretation 

was that oak clumps begin in the young, brushy form, 

then self thin and develop into pole stands and mature 

trees through time. Age-diameter relationships do show 

that the largest oaks tend to be the oldest (Brotherson 

and others 1983). There also is evidence that oak 

clumps self thin through time (Clary and Tiedemann 

1992). A limitation of Kruse’s (1992) classification, 

however, is that a given oak clump may contain stems 

of widely different ages. For example, the same clump 

could contain both young, re-sprout stems and older, 

pole-sized stems, making it unclear where this mixedage clump fits in Kruse’s (1992) classification. This 

classification was designed to categorize oak’s utility 

as wildlife habitat.

Rosenstock (1998) Stem Diameter

Rosenstock (1998) differentiated growth forms based on 

stem diameters, ranging from shrub-like forms (diameters 

< 1 inch) to large, old trees (> 15 inches; table 1). While 

not explicitly noted, Rosenstock (1998) also assumes 

that larger stems are the oldest. His classification focuses 

on individual stems and is less useful for classifying 

clumps that often contain stems of varying diameters. 

However, this classification could be used to categorize 

stem diameters primarily characterizing Kruse (1992) 

and Abella and Springer’s (2008) classifications of whole 

clumps. Rosenstock (1998) constructed this classification 

to measure avian habitat. For example, large-diameter 

stems may contain more cavities than smaller stems.

Abella and Springer (2008) 

Patch Structure

Abella and Springer (2008) used stem density and 

spacing to distinguish oak growth forms in a structural 

classification making no assumptions about successional 

status (table 1). Single stems can be of any size or age in 

this classification. Thickets have high densities of three 

or more closely spaced stems, which are usually smaller 

in diameter than stems in dispersed clumps (fig. 1). Dispersed clumps are intermediate between single stems and 

thickets and contain two or more widely spaced stems. 

Each growth form can contain stems of various sizes and 

ages since the growth forms are distinguished based only 

on stem density and spacing. Therefore, this classification 

has little utility for users desiring information on stem 

size or age because stems of different size and age can be 

classified into the same growth form. Another limitation 

is that any stem-stem distance or density cutoff between 

dispersed clumps and thickets would be arbitrary. The 

classification was made to support research of oak influences on soils and understory vegetation.

Table 1—Comparison of Gambel oak growth form classifications.

Brown 1958a Kruse 1992 Rosenstock 1998 Abella and Springer 2008

Area occupied by oak Successional stage Stem diameter (inches) Stem density, spacing

1. Clump (< 100 ft across) 1. Brushy (youngest) 1. Shrub like (< 1) 1. Single stem 

2. Intermediate 2. Young pole stand 2. Small tree (1 to 8) 2. Dispersed clump (low-high, wide)

3. Thicket (stand > 5 acres) 3. Mature 3. Mature tree (8 to 15) 3. Thicket (high, close)

4. Post mature (oldest) 4. Large, old tree (> 15)

a This classification was developed in west-central Colorado for Gambel oak shrublands. The other classifications were developed for 

ponderosa pine-oak forests.
USDA Forest Service Research Note RMRS-RN-37. 2008 3

Figure 1—Examples of Gambel oak growth forms on a 

basalt site north of Mormon Lake, Coconino National Forest, 

northern Arizona (35°11’48”N, 111°33’23”W). Following 

Abella and Springer’s (2008) growth form classification, (a) is 

a single stem, (b) is a dispersed clump, and (c) is a thicket. 

Photos by S.R. Abella, 9 October 2005.

a

b

c

Ecological Effects of Oak Growth 

Forms: Examples

Bird Habitat

Lesh (1999) measured Gambel oak characteristics of 

foraging areas for seven breeding bird species in northern 

Arizona pine-oak forests. At a 0.1-acre (0.04-ha) plot 

scale, she found that Virginia’s warblers (Vermivora virginiae) preferentially foraged in areas containing two to 

three times greater oak densities than were preferred by 

the other six bird species (fig. 2). Virginia’s warblers also 

preferred oak clumps, on average, exceeding 1,500 ft2

(139 m2

) in area. Brushy growth forms containing high 

densities of small-diameter stems, such as Rosenstock’s 

(1998) shrub-like form, appear critical as foraging habitat 

for Virginia’s warblers. In another example, May and 

others (2004) reported that 40 percent of all Mexican 

spotted owl (Strix occidentalis lucida) nests in northern 

Arizona pine-oak forests were in Gambel oak trees (which 

contained cavities) greater than 17 inches (44 cm) in 

diameter. Different oak growth forms may provide different benefits (for example, foraging versus nest sites) 

to different species.

Understory Vegetation

Plant species richness and composition differed among 

oak growth forms following Abella and Springer’s (2008) 

growth form classification in a study of northern Arizona 

pine-oak forests (fig. 3). Richness was greatest below 

single stems, intermediate below dispersed clumps, and 

least below thickets. Light-demanding, warm-season 

(C4) pine dropseed (Blepharoneuron tricholepis) was 

most frequent below single stems, whereas aspen pea 

(Lathyrus laetivirens) was most frequent below thickets. 

Growth forms presumably created unique environments 

below their canopies that were favorable for different 

species (Schuhardt 1991).

Acorn Production

McCulloch and others (1965) found that 10- to 15-inch 

(25- to 38-cm) diameter Gambel oaks with 80 to 100 

percent live crown produced the most plentiful acorn 

crops in northern Arizona (fig. 4). Oaks less than 5 inches 

(13 cm) or greater than 18 inches (46 cm) in diameter 

yielded few acorns. This study illustrates several tradeoffs 

with oak ecology and management. Small, shrubby oaks 

provide browse but few acorns for wildlife; larger oaks 

provide little accessible browse but copious acorns. In 

size/density tradeoffs, three times as many 6-inch (15-cm) 

diameter trees are needed to generate as many acorns as 

one 12-inch (30-cm) tree.
4 USDA Forest Service Research Note RMRS-RN-37. 2008

Figure 2—Gambel oak characteristics of foraging areas at a 0.01-acre (0.04-ha) plot scale 

for seven breeding bird species in ponderosa pine-Gambel oak forests at Camp Navajo, 

northern Arizona. Available plots represent average conditions in the study area. Virginia’s 

warblers selected foraging areas containing greater oak densities and clump areas than 

other bird species and as compared to available plots. Data from Lesh (1999). 

Figure 3—Understory plant (a) species 

richness and (b) composition among 

three Gambel oak growth forms following 

Abella and Springer’s (2008) growth form 

classification. Error bars in (a) are 1 standard 

deviation, and percent frequencies in (b) are 

based on 43-ft2

 (4-m2) plots. Scientific names 

for species: aspen pea = Lathyrus laetivirens, 

dwarf lousewort = Pedicularis centranthera, 

and pine dropseed = Blepharoneuron 

tricholepis. Data from Abella and Springer 

(2008) averaged from 10 sites in ponderosa 

pine-Gambel oak forests within the Coconino 

National Forest, northern Arizona. 
USDA Forest Service Research Note RMRS-RN-37. 2008 5

Managing Oak Growth Forms

Management prescriptions for maintaining different 

proportions of Gambel oak’s growth forms can potentially 

enhance diversity of wildlife habitat, plant communities, 

and human resource values. Existing research suggests 

that large oaks can be promoted by thinning ponderosa 

pine or other competing trees to accelerate growth of 

smaller oaks (table 2; Onkonburi 1999). During prescribed 

burning, keeping pine slash and fuels away from oak 

boles may help protect existing large trees from damage 

(Abella and Fulé 2008a). Pole-sized clumps can possibly 

develop with time by natural self-thinning of brushy thickets (Clary and Tiedemann 1992). Mechanically thinning 

oak within clumps may hasten this process, but effects 

of thinning Gambel oak are not well known (Onkonburi 

1999). Growth rates of residual stems may depend on 

how much energy clones allocate to resprouting of the 

cut stems (Harrington 1989).

Following the logic underlying Kruse’s (1992) growthform classification, young, brushy thickets can be 

promoted by reversing succession in older, pole-sized 

clumps. Stems could be cut or burned within polesized clumps to stimulate resprouting to form brushy 

thickets (table 2). Pole-sized clumps may be particularly 

amenable to management for fuelwood by thinning 

stems to potentially encourage large-tree growth forms 

or sprouting to maintain brushy thicket forms. Cutting 

live oak stems would represent a policy shift in some 

areas (Brischler 2002). However, cutting should not be 

overlooked as a possibility for manipulating growth forms 

and increasing diameter increment. A landscape-scale 

perspective is useful for identifying areas where active 

oak management would be appropriate or inappropriate 

and has few or no conflicts with other management priorities or mandates (Prather and others 2008). Because of 

their value to wildlife (Kruse 1992, Reynolds and others 

1970), protection of existing large oaks from fire damage, unauthorized cutting, and intense competition from 

other trees (even possibly from smaller oaks themselves, 

within a clone or from other clones) should be a priority. 

Ensuring that sufficient pole-sized stems survive to grow 

into new cohorts of large trees (Brischler 2002) is also an 

important consideration, although forest reconstruction 

data suggest that pole-sized stems are more abundant in 

current than historical forests (Abella and Fulé 2008b).

Summary

	 •	 In southwestern ponderosa pine forests, Gambel

oak has several growth forms distinguished by stem 

sizes and the density and spacing of stems within 

clumps.

	 •	 Four Gambel oak growth-form classifications have

been published, each with different focuses that 

classify growth forms using different criteria.

Figure 4—Gambel oak acorn production as a function of 

stem diameter and crown vigor, measured for six years on 

the Coconino National Forest, northern Arizona. Oaks 10 to 

15 inches (25 to 38 cm) in diameter and possessing 80 to 

100 percent live crown yielded the most acorns. Data from 

McCulloch and others (1965).

Table 2—Summary of potential management prescriptions to promote three basic growth forms of Gambel oak in southwestern ponderosa pine-oak forests.

Growth form Prescriptionsa

Large tree Thin ponderosa pine; thin within oak clumpsb; protect large stems from damage 

Pole/dispersed clump Allow natural self-thinning; thin dense clumps then possibly burn; fuelwood managementc

Brushy thicket Burn and cut stems to facilitate sprouting; fuelwood management

a Prescriptions summarized primarily from Abella and Fulé (2008b), Brischler (2002), Clary and Tiedemann (1992), Harrington (1989), 

and Onkonburi (1999). The magnitudes of the effects of the suggested prescriptions may vary with site conditions and prescriptionimplementation. Additional research is needed to quantify effects of these variables. 

b A particularly important area for future research is to more clearly elucidate effects of oak thinning on the growth of remaining stems.

c Fuelwood harvests should be carefully planned or regulated to ensure that only prescribed stem diameters and densities are cut.
6 USDA Forest Service Research Note RMRS-RN-37. 2008

	 •	 Oak’s growth forms afford flexibility in managing

oak for different wildlife habitat and resource values. 

For example, large-tree growth forms often contain 

cavities for wildlife, while smaller, shrubby forms 

supply accessible browse and ground cover.

	 •	 Thinning competing trees (pine, but also possibly

oak itself) likely encourages growth of large oaks. 

Preventing pine slash and fuels from accumulating 

near oak boles might help protect existing large oaks 

during prescribed fire.

	 •	 Pole-sized growth forms can develop through time,

possibly by self-thinning in younger clumps. Mechanically thinning within clumps may accelerate 

this process, but effects of thinning on the growth 

of remaining stems are poorly understood.

	 •	 Cutting or burning to stimulate sprouting maintains

brushy thicket forms.

	 •	 Existing research suggests that actively managing

Gambel oak and pine-oak sites through prescribed 

burning, thinning of small- or medium-sized oaks, or 

thinning of competing trees (for example, ponderosa 

pine) can sustain desired proportions of oak growth 

forms. Maintaining an assortment of oak growth 

forms, within or across sites, will likely enhance 

ecological diversity and human resource values.

Acknowledgments

Funding was provided by the U.S. Forest Service and the 

Ecological Restoration Institute. I thank Wally Covington 

for suggesting and supporting literature synthesis work and 

Catherine Wightman (Arizona Game and Fish Department), 

Charlie Denton (Ecological Restoration Institute), Alex Finkral (Northern Arizona University), Jill Craig (University of 

Nevada Las Vegas), and Kristi Coughlon (Rocky Mountain 

Research Station) for providing perceptive comments on 

earlier drafts of the manuscript.

References

Abella, Scott R.; Fulé, Peter Z. 2008a. Fire effects on Gambel oak in 

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.

Abella, Scott R.; Fulé, Peter Z. 2008b. 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. 

Abella, Scott R.; Springer, Judith D. 2008. Canopy-tree influences 

along a soil parent material gradient in Pinus-ponderosa-Quercus 

gambelii forests, northern Arizona. Journal of the Torrey Botanical 

Society. 135:26-36.

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

Gambel oak in northern Arizona. Flagstaff, AZ: Northern Arizona 

University. 128 p. Thesis.

Brotherson, J.D.; Rushforth, S.R.; Evenson, W.E.; Johansen, J.R.; 

Morden, C. 1983. Population dynamics and age relationships of 

8 tree species in Navajo National Monument, Arizona. Journal of 

Range Management. 36:250-256.

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

Clary, Warren P.; Tiedemann, Arthur R. 1992. Ecology and values of 

Gambel oak woodlands. In: Ffolliott, Peter F.; Gottfried, Gerald J.; 

Bennett, Duane A.; Hernandez, C., Victor Manuel; Ortega-Rubio, 

Alfredo; Hamre, R.H., tech. coords. Ecology and management of 

oaks and associated woodlands: perspectives in the southwestern 

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Harper, Kimball T.; Wagstaff, Fred J.; Kunzler, Lynn M. 1985. Biology 

and management of the Gambel oak vegetative type: a literature 

review. Gen. Tech. Rep. INT-179. Ogden, UT: U.S. Department 

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Harrington, Michael G. 1989. Gambel oak root carbohydrate response 

to spring, summer, and fall prescribed burning. Journal of Range 

Management. 42:504-507.

Klemmedson, James O. 1987. Influence of oak in pine forests of 

central Arizona on selected nutrients of forest floor and soil. Soil 

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Kruse, William H. 1992. Quantifying wildlife habitats within Gambel oak/forest/woodland vegetation associations in Arizona. In: 

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tech. coords. Ecology and management of oaks and associated 

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northern Mexico; 1992 April 27-30; Sierra Vista, AZ. Gen. Tech. 

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Lesh, Tamara D. 1999. Habitat selection by selected breeding passerine birds in pine-oak forests of northern Arizona. Flagstaff, AZ: 

Northern Arizona University. 44 p. Thesis.

May, Christoper A.; Petersburg, Mylea L.; Gutiérrez, R.J. 2004. Mexican spotted owl nest- and roost-site habitat in northern Arizona. 

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McCulloch, C.Y.; Wallmo, O.C.; Ffolliott, P.F. 1965. Acorn yield 

of Gambel oak in northern Arizona. Res. Note RM-48. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky 

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Onkonburi, Jeanmarie. 1999. Growth response of Gambel oak to thinning and burning: implications for ecological restoration. Flagstaff, 

AZ: Northern Arizona University. 129 p. Dissertation.

Prather, John W.; Noss, Reed F.; Sisk, Thomas D. 2008. Real versus 

perceived conflicts between restoration of ponderosa pine forests 

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Economics. 10:140-150.

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Rosenstock, S.S. 1998. Influence of Gambel oak on breeding birds in 

ponderosa pine forests of northern Arizona. Condor. 100:485-492.

Schuhardt, Susan. 1991. Structure, productivity, and competition in 

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