USDA Forest Service Research Note RMRS-RN-39. 2009 1

United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

Research Note

RMRS-RN-39

February 2009

Abella, Scott R. 2009. Tree canopy types constrain plant distributions in ponderosa pine-Gambel oak forests, northern Arizona. Res. Note RMRS-RN-39. Fort Collins, CO: U.S. Department 

of Agriculture, Forest Service, Rocky Mountain Research Station. 

7 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 

Department of Environmental Studies, University of Nevada Las Vegas, 

Las Vegas, NV. 

Tree Canopy Types Constrain Plant 

Distributions in Ponderosa PineGambel Oak Forests, Northern Arizona

Scott R. Abella 

Abstract––Trees in many forests affect the soils and plants below their canopies. In current high-density 

southwestern ponderosa pine (Pinus ponderosa) forests, managers have opportunities to enhance multiple 

ecosystem values by manipulating tree density, distribution, and canopy cover through tree thinning. I 

performed a study in northern Arizona ponderosa pine-Gambel oak (Quercus gambelii) forests to measure 

the influences of tree canopy types on understory plant communities and soil properties. On ten 2.5-acre 

(1-ha) sites, I sampled five 43-ft2

 (4-m2) plots below each of the following five canopy types: openings; 

single ponderosa pine; and Gambel oak single stems, dispersed clumps, and thickets. Soil properties, species richness, plant cover, and the distribution of cool- and warm-season grasses were canopy-type specific. 

Openings contained the most species/plot, three to eight times greater plant cover than any tree canopy type, 

and warm-season grasses (for example, purple threeawn [Aristida purpurea]) that were infrequent below 

trees. In contrast, aspen pea (Lathyrus laetivirens) and Fendler’s meadow-rue (Thalictrum fendleri) were 

most frequent below Gambel oak canopies. There were no species that were most frequent below ponderosa 

pine. Results suggest that canopy openings need to be reestablished and maintained on this landscape if 

understories are to be productive, diverse, and contain species dependent on these microsites.

Introduction

Two of the main variables in forest ecosystems that 

managers can manipulate are tree density and within-site 

distributions of trees. In southwestern ponderosa pine 

(Pinus ponderosa) forests, where small-diameter pine 

densities have irrupted, there is ample opportunity to 

manipulate tree density and distribution by mechanically 

thinning trees (Covington and others 1997, Naumburg 

and DeWald 1999). Tree patterns resulting from thinning 

can affect soils, plant communities, and other ecosystem 

components (Scholes and Archer 1997). Trees affect soil 

moisture, nutrients, microclimates, and other environmental variables on forest floors below their canopies 

(Barth 1980; Moir 1966; Parker and Muller 1982). These 

effects have been termed single-tree influences, and some 

authors have proposed that forests consist of mosaics of 

single-tree influences (Boettcher and Kalisz 1990).

To manage for multiple ecosystem values, managers 

need to understand the ecological effects of passively or 

actively managing within-site tree patterns. This requires 

shifting focus away from the trees themselves to the 

ecosystem components that trees influence. In ponderosa pine forests, these ecosystem components include 

canopy openings, understory vegetation, soils, wildlife, 

hydrology, and others (Covington and others 1997). I 

performed a study on a northern Arizona landscape to 

measure the influence of canopy openings and ponderosa pine and Gambel oak (Quercus gambelii) trees on 

understory vegetation and soil properties. I isolated the 

biotic influences of trees from potentially confounding 

effects of abiotic variables by holding soil parent material 

constant within sites, supported by analyses indicating 

that soil texture did not differ among canopy types. This 

study’s results, combined with previous research, provide 

insight into how different tree densities and distributions 

created by thinning may influence soils and understory 

vegetation in pine-oak forests.

Methods

Study Area

I conducted this study on ten ponderosa pine-Gambel 

oak sites on the northern half of the Coconino National 

Forest and on the Northern Arizona University Centennial Forest surrounding the city of Flagstaff in northern 

Arizona. Sites were separated by an average of 14 miles 

(23 km), with a maximum site-to-site distance of 26miles 

(42 km; table 1). I selected these sites because they 
2 USDA Forest Service Research Note RMRS-RN-39. 2009

contained pine-oak forests, spanned a range of soil parent materials and terrestrial ecosystem survey types 

(Miller and others 1995), and were studied in previous 

research (Abella and Covington 2006). Among sites, 

0- to 6-inch (0- to 15-cm) soil texture ranged from sandy 

to clay loam, pH from 5.6 to 6.8, and loss-on-ignition 

(2 hours at 572 °F [300 °C]) in open-area soils from 1.5 to 

5.6 percent. Loss-on-ignition in this study is used as a 

surrogate for organic carbon (Abella and Zimmer 2007). 

Sites also encompassed a range of stand structures, 

from dense stands exceeding 400 pine + oak/acre 

(1,000/ha) to open stands containing fewer than 100 

pine + oak/acre (250/ha; fig. 1). Annual precipitation 

across the study area averages 17 to 22 inches (42 

to 56 cm) based on three weather stations (Western 

Regional Climate Center, Reno, NV).

Field and Laboratory Procedures

On a 2.5-acre (1-ha) grid on each site, I randomly 

selected five of each of the following canopy types for 

sampling: openings; Gambel oak single trees, dispersed 

clumps, or thickets; and ponderosa pine single trees 

(fig. 1). This canopy-type classification was based on 

previous research and is articulated with examples and 

photos in Kruse (1992) and Abella and Springer (2008). 

Canopies were able to be randomly selected for sampling 

by using existing stem maps of the grids (Abella and 

Springer 2008) and enumerating all of the individual 

canopies. Openings were free of tree canopy cover and 

typically 0.025 to 0.25 acres (0.01 to 0.1 ha) in size. 

Diameters at breast height of sampled oak single stems 

ranged from 3 to 30 inches (7 to 77 cm). Compared to 

thickets, oak dispersed clumps usually contained stems 

larger in diameter with greater stem spacing. Oak thickets 

contained closely spaced stems usually of small diameter 

(< 6 inches [15 cm]). Sampled ponderosa pine ranged in 

Figure 1––Example of stand structure in a low-density 

ponderosa pine-Gambel oak forest sampled on Campbell 

Mesa, 2 miles (3 km) east of Flagstaff, Arizona, in the 

Coconino National Forest. Plots were sampled below canopy 

openings; Gambel oak single trees, dispersed clumps, and 

thickets; and single ponderosa pine. Photo by S.R. Abella, 

June 16, 2004 (35o

11’48”N, 111o33’23”W).

Table 1–Study site characteristics in northern Arizona ponderosa pine-Gambel oak forests.

Ponderosa pine Gambel oak

Site Parent material Textureb Elevation (ft) Location Trees/ac BAc

 ft2/ac Trees/ac BA ft2/ac

Turkey Hills – 513a Red volcanic cinders Sandy loam 6819 35°14’N, 111°31’E ––d –– –– ––

Garjon Tank – 536 Limestone/chert Sandy loam 7118 35°08’N, 111°53’E 222 150 53 19

Campbell Mesa – 500 Limestone/sandstone Sandy loam 6793 35°12’N, 111°33’E 25 31 68 13

Pine Grove – 586 Basalt Silt loam 7137 34°59’N, 111°29’E –– –– –– ––

Railroad Draw – 582 Basalt Loam 6845 35°07’N, 111°55’E 276 99 75 14

Little Horse Park – 585 Basalt Clay loam 6973 34°57’N, 111°35’E –– –– –– ––

Coulter Cabin – 585 Basalt Loam 7252 35°02’N, 111°36’E –– –– –– ––

Howard Mountain – 585 Basalt Loam 7186 35°03’N, 111°39’E 288 114 50 20

Dry Lake – 570 Benmoreite Loam 7380 35°11’N, 111°45’E 94 86 30 15

Fisher Tank – 570 Benmoreite Loam 7446 35°11’N, 111°46’E 412 127 83 21

a Terrestrial ecosystem survey type on the Coconino National Forest (Miller and others 1995).

b 0- to 6-inch (0- to 15-cm) mineral soil.

c Basal area.

d Not measured.

diameter from 11 to 32 inches (29 to 81 cm) and 80 percent 

were greater than 20 inches (50 cm) in diameter.

I sampled a 43-ft2

 (4-m2) circular plot below each of 

the five randomly selected canopies of each of the five 

canopy types on each grid (5 plots/canopy × 5 canopy 

types = 25 plots/grid). I selected the 43-ft2

 plot size to 

fit within the typical drip lines of the tree canopies (Gill 

and others 2000). Tree boles were the plot centers for 

single trees, and I located plots in the centers of openings and Gambel oak clumps and thickets. On each plot, 

I categorized the areal cover of each plant species rooted 

in the plot following cover classes modified from Peet 

and others (1998): 1 = < 0.1 percent, 2 = 0.1 to 1 percent, 

3 = 1 to 2 percent, 4 = 2 to 5 percent, 5 = 5 to 10 percent, 
USDA Forest Service Research Note RMRS-RN-39. 2009 3

6 = 10 to 25 percent, 7 = 25 to 50 percent, 8 = 50 to 75 

percent, 9 = 75 to 95 percent, and 10 = 95 to 100 percent. 

I included Gambel oak seedlings and sprouts shorter than 

breast height in these measurements. Estimates of total 

plant cover were comparable to total cover estimated by 

summing cover of individual species, so I report total 

plant cover summed from individual species. I assessed 

reproducibility of cover categorizations by remeasuring 

every fifth plot. Repeated measurement errors averaged 

< 1 cover class and 0.5 species/plot. Two specimens, both 

forbs, with insufficient material for accurate identification to species were deleted from the data set. Sampling 

occurred in June 2004 and represents pre-monsoon summer sampling. Nomenclature followed NRCS (2004). I 

classified graminoids (grasses and sedges) based on their 

photosynthetic pathway as cool-season (C3) or warmseason (C4) following Waller and Lewis (1979).

On one randomly selected plot of each canopy type 

on each grid, I collected a 0- to 6-inch (0- to 15-cm) soil 

sample that weighed about 2.2 pounds (1 kg) including 

coarse fragments. I analyzed the < 0.08-inch (2-mm) fraction for texture (hydrometer method), pH (1:1 soil:water), 

and loss-on-ignition (2 hours at 572 °F [300 °C]) following Sparks (1996) and Dane and Topp (2002). I also 

measured Oi horizon (litter) weight by oven drying a 

2.7-ft2

 (0.25-m2) sample at 158 °F (70 °C) for 24 hours. 

I estimated 0- to 6-inch (0- to 15-cm) gravimetric soil 

moisture on three randomly selected plots of each canopy 

type on each grid by oven drying a 13-inch3

 (207-cm3) 

core at 221 °F (105 °C) for 24 hours.

Data Analysis

I averaged all plant community measures on a grid 

basis for statistical analyses. I compared the following 

understory variables among canopy types: the sum of 

species percent cover excluding Gambel oak cover, 

total average species richness per plot (43 ft2

 [4 m2]), 

and cool- and warm-season graminoid richness per plot. 

Texture, pH, loss-on-ignition, Oi horizon weight, and 

gravimetric moisture constituted the soil variables. With 

the exception of species richness, I compared variables 

among canopy types using one-way analysis of variance, 

with grids (sites) serving as blocks and Fisher’s least 

significant difference for mean separation. I compared 

the three richness variables among canopy types using a 

Kruskal-Wallis test followed by a pairwise Wilcoxon test 

with Bonferroni adjustment. I used a non-parametric test 

for richness because these data were originally count data. 

JMP software (SAS Institute 2002) was used to perform 

analysis of variance, and R (http://www.r-project.org/) 

was used for the Kruskal-Wallis test.

Results

Plant Community Cover and Richness

Understory plant cover, excluding Gambel oak cover, 

averaged about three to eight times greater in openings 

than below tree canopies (fig. 2). Plant cover was intermediate below oak canopies and least below pine where 

cover averaged less than 4 percent. Oak seedlings and 

sprouts shorter than breast height constituted about half of 

the plant cover below oak and ponderosa pine canopies, 

while herbaceous plants predominated in openings.

Average plant species richness/43 ft2

 (4 m2) ranged from 

12 species below openings to 5 species below ponderosa 

pine (fig. 3). Among oak canopy types, richness increased 

in the order: thickets < dispersed clumps < single stems. 

Richness of cool-season graminoids was lowest below 

pine, but did not differ significantly among other canopy 

types. In contrast, warm-season graminoids were more 

than twice as rich in openings compared to single oaks, 

the next richest canopy type.

Species Composition

While understory composition varied among sites, several species showed affinities for three or fewer canopy 

types within sites (table 2). For example, the warm-season 

grass purple threeawn (Aristida purpurea) occurred in 

43 percent of plots in openings on the six sites this species occupied. The only tree canopy type that threeawn 

Figure 2––Understory plant cover among five canopy types 

in northern Arizona ponderosa pine-Gambel oak forests. 

Canopy openings contained significantly greater plant cover 

than occurred below any tree canopy. Means without shared 

letters for the other plants category (all plants excluding 

Gambel oak) differ among canopy types (F = 6.3, P < 0.01). 

Error bars are 1 standard deviation for mean total cover.
4 USDA Forest Service Research Note RMRS-RN-39. 2009

Figure 3––(a) Average total understory plant 

richness and (b) graminoid (grass and sedge) 

richness by photosynthetic pathway (C3 = coolseason, C4 = warm-season) among five canopy 

types in ponderosa pine-Gambel oak forests, 

northern Arizona. Means without shared letters 

differ among canopy types (a: Kruskal-Wallis 

χ2 statistic = 78.1, P < 0.01; b warm-season: 

χ2 = 104, P < 0.01; b cool-season: χ2 = 25.9, 

P < 0.01). Error bars are 1 standard deviation 

for mean totals. 

Table 2—Distributions of 17 prevalent plant species (of 123 total detected species) 

among five canopy types in northern Arizona ponderosa pine-Gambel oak 

forests. Values in bold represent canopy types under which a species most 

frequently occurred.

Canopy typea

Species No. sitesb Open SO DC OT SP

- - - - - - - - - Percent frequencyc

 - - - - - - - - -

Warm-season grasses

Aristida purpurea 6 26 2 0 0 0

Bouteloua gracilis 6 42 12 6 0 0

Sporobolus interruptus 6 26 8 4 8 2

Blepharoneuron tricholepis 6 32 14 2 2 0

Muhlenbergia montana 8 42 36 22 14 12

Cool-season graminoids

Carex geophila 9 48 58 62 46 22

Poa fendleriana 10 66 70 78 60 50

Elymus elymoides 10 74 94 86 94 64

Forbs

Erigeron divergens 10 64 24 14 8 0

Lithophragma tenellum 5 10 14 6 0 0

Heliomeris multiflora 8 26 20 12 10 0

Symphyotrichum falcatum 6 32 16 10 4 2

Lupinus kingii 6 36 24 10 12 10

Eriogonum racemosum 9 42 28 24 26 24

Pedicularis centranthera 6 6 18 28 14 4

Thalictrum fendleri 6 0 2 16 16 2

Lathyrus laetivirens 5 0 4 16 22 2

a SO = single Gambel oak, DC = dispersed oak clump, OT = oak thicket, and SP = single ponderosa pine. b Number of sites (n = 10 total) on which a species occurred. c Frequencies represent presence or absence in fifty 43-ft2

 (4-m2) plots for each canopy type 

(five canopies of each type sampled at each of 10 sites). 
USDA Forest Service Research Note RMRS-RN-39. 2009 5

occurred below was single oaks. As a group, warm-season 

graminoids sharply declined with increasing tree canopy 

cover. Cool-season graminoids, in contrast, were more 

evenly distributed among canopy types. The forbs aspen 

pea (Lathyrus laetivirens) and Fendler’s meadow-rue

(Thalictrum fendleri) were associated with oak clumps 

and thickets. Dwarf lousewort (Pedicularis centranthera) 

also had an affinity for oak.

Soils

On average, percent sand, silt, or clay did not differ 

significantly among canopy types (F- statistic < 1.2, 

P > 0.35). Soil pH also did not differ significantly 

among canopy types (F = 2.55, P = 0.06) but tended to 

be 0.2 to 0.4 units lower below pine compared to other 

canopies. In contrast, Oi horizon weight sharply differed 

among canopy types. Oi horizons weighed 18 tons/acre 

(4,000 g/m2

) below pine compared to 4 to 6 tons/acre 

(900 to 1,500 g/m2

) below oak and less than one ton/acre 

(200 g/m2

) in openings (fig. 4). Loss-on-ignition was 

greatest below oak canopy types and least in openings. 

Soil moisture was also lowest in openings and greatest 

below oak thickets.

Discussion

The distribution of tree canopy types constrained 

within-site patterns of understory plant cover, richness, 

and composition. These within-site patterns probably 

arise from the biotic influences of trees on soils and 

microclimates (Boettcher and Kalisz 1990), because soil 

parent material was homogeneous within sites and soil 

texture did not differ among canopy types. This study’s 

findings, such as greater loss-on-ignition in soils below 

tree canopies, are also consistent with tree influences 

reported in many experimental studies that manipulated 

tree cover (Scholes and Archer 1997).

Canopy openings contained greater understory plant 

cover, richness, and warm-season grass abundance than 

occurred below Gambel oak and pine canopies. This 

finding was consistent across the sampled soil parent 

materials and terrestrial ecosystem types. Many previous 

studies in ponderosa pine forests have also highlighted 

the importance of canopy openings for understory communities (Arnold 1950; Covington and others 1997; Moir 

1966). The greater abundance of warm-season grasses that 

I found in openings compared to below trees concurs with 

predictions that warm-season species are most competitive 

in sunnier, drier environments (Sage and Monson 1999). 

In addition to warm-season grasses, spreading fleabane

(Erigeron divergens), white prairie aster (Symphyotrichum 

falcatum), and several other forbs were most frequent in 

openings (table 2). Affinities for openings by these species could be related to greater resource availability (for 

example, light or nutrients) in openings (Moir 1966) or 

thinner litter layers that facilitate seedling establishment 

(Evenson and others 1980).

While plant cover and richness was lower below Gambel 

oak canopies than it was in openings, three forbs (aspen 

pea, Fendler’s meadow-rue, and dwarf lousewort) were 

more frequent below oaks than in openings (table 2). 

Studies in other parts of Gambel oak’s range also found 

that certain plant species are most prevalent below oak 

canopies. For example, Brown (1958) found that Geyer’s 

Figure 4—Variation in soil properties among five canopy 

types in northern Arizona ponderosa pine-Gambel oak 

forests. Averages without shared letters differ among canopy 

types (a: F = 45.9, P < 0.01; b: F = 6.1, P < 0.01; c: F = 10.1, 

P < 0.01). Averages = circles, medians = white horizontal 

lines, first quartiles = lower boxes, third quartiles = upper 

boxes, and minimums/maximums = vertical lines.
6 USDA Forest Service Research Note RMRS-RN-39. 2009

sedge (Carex geyeri) biomass in western Colorado averaged 204 pounds/acre (229 kg/ha) below Gambel oak 

compared to only 25 pounds/acre (28 kg/ha) in openings. 

In northern Utah, Evenson and others (1980) reported 

that Geyer’s sedge and tuber starwort (Pseudostellaria 

jamesiana)were most abundant below oak. It appears that 

oak commonly constitutes key habitat for certain plant 

species, but these species vary with regional floras.

This study supports previous findings in ponderosa pine (Klemmedson 1987; Moir 1966; Naumburg 

and DeWald 1999) and other forest types (Barth 1980; 

Boettcher and Kalisz 1990; Scholes and Archer 1997) 

that trees induce fine-scale patterns in soil properties 

and understory plant communities. This study’s results 

also reinforce recommendations by previous authors (for 

example, Laughlin and others 2006) that maintaining or 

creating canopy openings is critical for sustaining understory plant diversity in ponderosa pine forests.

Management Implications

	 •	 Resultssuggest that passively or actively managing

both the density and distribution of trees affects 

within-site patterns of plant communities and soil 

properties in ponderosa pine-Gambel oak forests.

	 •	 Comparedtobelowpine, canopyopenings contained

eight times more plant cover, three times more species/43 ft2

 (4 m2), and 13 times greater frequencies 

of warm-season grasses. Creating canopy openings 

when thinning ponderosa pine forests is critical if 

these forests are to contain diverse understory plant 

communities.

	 •	 SpecificgrowthformsofGambeloak—singlestems,

dispersed clumps, or dense thickets — exhibited

different effects on soils and understory richness 

and composition. Single oaks had the highest species richness, while clumps and thickets constituted 

habitat for Fendler’s meadow-rue and aspen pea on 

about half of the study sites. Maintaining areas of 

these different growth forms increases plant community diversity and is probably also important for 

some wildlife species.

	 •	 Trees are dominant structures in pine-oak forests

that affect other ecosystem components, including 

soils and plant communities. The growing space 

occupied by trees influences the living space available to other organisms, and such influences warrant 

consideration when managing forests for multiple 

ecosystem values.

	 •	 The tradeoffs of leaving large numbers of pine after

thinning operations include fewer canopy openings, 

heavier fuel loads, more acidic soils, and depauperate 

understories.

Acknowledgments

I thank Judy Springer and Mark Daniels (Ecological Restoration Institute) for help with fieldwork; 

Wally Covington (Ecological Restoration Institute) for 

discussions and guidance on disseminating summaries of research results; Jim Fowler (Rocky Mountain 

Research Station), Charlie Denton and Dennis Lund 

(Ecological Restoration Institute), and Kristi Coughlon 

(Rocky Mountain Research Station) for reviewing the 

manuscript; and David Turner (Rocky Mountain Research 

Station) and Cheryl Vanier (University of Nevada Las 

Vegas) for providing statistical reviews and help with 

statistical analyses. Funding was provided by the U.S. 

Forest Service and the Ecological Restoration Institute.

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(TDD). USDA is an equal opportunity provider and employer.

Rocky

 Mountain

 Research Station