United States Department of Agriculture / Forest Service

Rocky Mountain Research Station

May 2011

Fire Effects on Tree Overstories

in the Oak Savannas of the

Southwestern Borderlands Region

Peter F. Ffolliott, Gerald J. Gottfried, 

Cody L. Stropki, Hui Chen, Daniel G. Neary

Research Paper RMRS-RP-86
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Ffolliott, Peter F.; Gottfried, Gerald J.; Stropki, Cody L.; Chen, Hui; Neary, 

Daniel G. 2011. Fire effects on tree overstories in the oak savannas of the 

Southwestern Borderlands Region. Res. Pap. RMRS-RP-86. Fort Collins, CO: 

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

Station. 13 p.

Abstract

Effects of cool-season and warm-season prescribed burning treatments and a 

wildfire on tree overstories in oak savannas on the Cascabel Watersheds of the 

Southwestern Borderlands Region are reported in this paper. Information on the 

initial survival, levels of crown damage, species compositions and densities, annual 

growth rates, and basal sprouting following these burning events is presented. 

Impact of the fires on spatial distributions of trees in the overstories is also 

described. These events were all of low fire severities. As a consequence, effects 

of the prescribed burning treatments and the wildfire on tree overstories of the 

watersheds were similar and, therefore, the data sets were pooled. Effects of these 

fires on the tree overstories were mostly minor and often insignificant in terms of 

management implications.

Keywords: fire effects, oak savannas, prescribed burning, southwestern United 

States, tree overstories, wildfire

Authors

Peter F. Ffolliott is a Professor with the School of Natural Resources and the 

Environment, University of Arizona, Tucson, Arizona.

Gerald J. Gottfried is a Research Forester with the U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research Station, Phoenix, Arizona.

Cody L. Stropki is a Research Associate with the School of Natural Resources and 

the Environment, University of Arizona, Tucson, Arizona.

Hui Chen is a Research Assistant with the School of Natural Resources and the 

Environment, University of Arizona, Tucson, Arizona.

Daniel G. Neary is a Research Soil Scientist and Science Team Leader with the 

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

Flagstaff, Arizona.

Acknowledgments

The research results presented in this paper were supported by the Southwestern 

Borderlands Ecosystem Management Unit of the Rocky Mountain Research Station, 

U.S. Forest Service, Phoenix, Arizona; and the Arizona Agricultural Experiment 

Station, University of Arizona, Tucson.
Fire Effects on Tree Overstories

in the Oak Savannas of the

Southwestern Borderlands Region

Contents

Introduction 1

Cascabel Watersheds 1

Prescribed Burning Treatments and the Wildfire 3

Fire Severities 4

Study Protocols 4

Sampling Basis 4

Tree Overstory Measurements 5

Analytical Procedures 5

Results and Discussion 6

Initial Survival of Trees 6

Crown Damage 6

Densities 7

Annual Growth 8

Basal Sprouting 8

Spatial Distributions of Trees 9

Conclusions 10

Management Implications 10

References 11
USDA Forest Service Research Paper RMRS-RP-86. 2011. 1

Introduction

Fire was a natural part of the ecosystems of the Southwest 

Borderlands before Euro-American settlement of the 

Region. Fires caused by lightning activity in the spring and 

early summer, before the onset of monsoon rains, limited 

establishment of trees and maintained a diverse landscape 

of grasslands, savannas, and woodlands. Tree-ring evidence 

suggests that fires occurred every 5 to 10 years in the ecosystems along the United States-Mexico border prior to 

settlement (Kaib and others 1999). However, natural fire 

frequencies, their burning characteristics, and subsequent 

impacts on ecosystem resources have been altered since the 

late 1800s, largely because of past livestock grazing practices that removed significant portions of the fire-carrying 

herbaceous vegetation and because of past (aggressive) fire 

suppression policies of management agencies (Fulé and 

Covington 1995, Edminster and others 2000). These past 

practices and policies have resulted in unnaturally high tree 

densities on many sites, making trees more susceptible to 

insects, diseases, and stand-replacing wildfire and causing a 

decline in herbaceous plants in the understories. Excessive 

accumulations of flammable surface fuels are also found in 

the Region, and mesquite (Prosopis) and other woody plants 

have invaded many otherwise productive grasslands.

As a consequence of the generally undesirable conditions 

often encountered, land management agencies with support 

from their collaborators, including private organizations and 

local stakeholders, are interested in re-introducing more 

historical fire regimes into many of the ecosystems in the 

Region (Edminster and Gottfried 1999, Gottfried and others 

2000, 2007, Gottfried and Edminster 2005). Included among 

these ecosystems are oak savannas—plant communities that 

are situated between the higher-elevation, more densely 

stocked oak woodlands and lower-elevation grassland and 

shrub communities. However, managers need more information about the impacts of burning on ecosystem resources in 

the oak savannas before they can initiate such a program. 

A first step in the process of obtaining this information is 

to evaluate the effects of prescribed burning treatments 

on ecosystems resources, specifically the important tree 

overstories.

The effect of cool-season (November through April) and 

warm-season (May through October) prescribed burning 

treatments and of a wildfire on tree overstories in the oak savannas of the Malpai Borderlands, an area of approximately 

802,750 acres within the larger Southwestern Borderlands 

Region (fig. 1), is the focus of this paper. The information 

presented is useful in developing management strategies 

necessary for a re-introduction of more natural fire regimes 

into the oak savannas in order to increase site productivity 

and landscape diversity while maintaining environmental 

integrity.

Cascabel Watersheds

Twelve small watersheds, ranging from 20 to almost 

60 acres in size, in the Peloncillo Mountains of southwestern New Mexico (Gottfried and others 2000, Gottfried 

and Edminster 2005) collectively comprise the study area 

(fig. 1). The areal aggregation of these watersheds, called 

the Cascabel Watersheds, is 451.3 acres. The watersheds 

are situated between 5380 and 5590 feet in elevation. The 

nearest long-term precipitation station indicates that annual 

precipitation averages 23.5 inches, with nearly one-half of 

the precipitation occurring in the summer monsoonal season 

between June 15 and September 30. However, a prolonged 

drought impacted the area from the middle of the 1990s 

through the time of the burning events on the watersheds. 

Precipitation during this drought period averaged 14.9 inches annually.

Emory oak (Quercus emoryi) was the dominant tree 

species in the overstories on the watersheds before the 

burns, followed by alligator juniper (Juniperus deppeana). 

Intermingling Arizona white (Q. arizonica) and Toumey 

(Q. toumeyi) oak, redberry juniper (J. coahuilensis), border pinyon (Pinus discolor), and the tree form of mesquite 

(Prosopis glandulosa var. torreyana) were minor overstory 

components (Ffolliott and Gottfried 2005, Ffolliott and others 2008b). In this paper, the term “tree overstories” refers to 

these tree species regardless of their respective size or position in the canopy.

Perennial grasses were blue (Bouteloua gracilis), sideoats 

(B. curtipendula), slender (B. repens), and hairy (B. hirsuta) 

grama; bullgrass (Muhlenbergia emersleyi); common wolfstail (Lycurus pheoides); and Texas bluestem (Schizachyrium 

cirratum). Forb species of mariposa lily (Calochortus spp.), 

verbena (Verbena spp.), and lupine (Lupinus spp.) were minor components of the understory plants. Beargrass (Nolina 

microcarpa), fairyduster (Calliandra eriophylla), common 

sotol (Dasylirion wheeleri), manzanita (Arctostaphylos 

pungens), Fendler’s ceanothus (Ceanothus fendleri), and 

Mexican cliffrose (Purshia mexicana) were among the occasional half-shrubs and shrubs. Shrub forms of mesquite were 

present on many sites. Palmer’s century plant (Agave palmeri) 

and banana yucca (Yucca baccata) were scattered succulents 

on rocky slopes. Annual plants were largely absent.

Geologic, physiologic, and hydrologic characteristics 

of the Cascabel Watersheds are described elsewhere by 

Gottfried and others (2000, 2007), Hendricks (1985), Neary 

and Gottfried (2004), Osterkamp (1999), Robertson and 

others (2002), Vincent (1998), and Youberg and Ferguson 

(2001). Bedrock geology is Tertiary rhyolite overlain by 

Oligocene-Miocene conglomerates and sandstone. Soils are 

classified as Lithic Argustolls, Lithic Haplustrolls, or Lithic 

Ustorthents. These shallow soils are generally less than 20 

inches to bedrock. Streamflows originating on the watersheds are intermittent with the larger flows generated by 

storms of high-intensity rainfall (Gottfried and others 2006).
 2 USDA Forest Service Research Paper RMRS-RP-86. 2011.

Figure 1. The Cascabel Watersheds (arrow) are located in the oak savannas of the Malpai Borderlands (shaded area), 

an area of approximately 802,750 acres within the larger Southwestern Borderlands Region (from Ffolliott and 

others 2008b).
USDA Forest Service Research Paper RMRS-RP-86. 2011. 3

Prescribed Burning 

Treatments and the Wildfire

The original objective of the research program on the 

Cascabel Watersheds was to evaluate the effects of warmseason (May through October) and cool-season (November 

through April) prescribed burning treatments on the natural 

resources of the watersheds, including the tree overstories. It 

was anticipated that these evaluations would be compared to 

control (unburned) watersheds in determining the fire effects. 

Following the required watershed calibration period, four of 

the watersheds (Watersheds C, H, K, and N) were burned 

during the cool-season in early March 2008 (fig. 2). Three 

of the four watersheds designated for warm-season burning 

(Watersheds A, E, and F) were ignited on May 20, 2008, and 

the fourth watershed (Watershed I) burn was delayed until a 

later date because of shifting weather conditions (table 1). 

However, wind gusts up to 60 mph on the morning of May 

21, 2008, blew firebrands onto the remaining watershed that 

was scheduled for warm-season burning and onto the four 

control watersheds. The resulting wildfire, designated the 

Whitmire Wildfire, crossed the boundary lines of the watersheds and spread beyond them, burning almost 4000 acres. 

Average atmospheric conditions during the prescribed fires 

and wildfire are presented in table 1. The original objective of the research on the Cascabel Watersheds had to be 

modified, therefore, to evaluate the effects of cool-season 

and warm-season prescribed burning treatments as well as 

the Whitmire Wildfire on tree overstories of the watersheds 

(fig. 2).

Table 1. Average atmospheric conditions from 0700 to 1800 hours during the prescribed burning treatments 

and wildfire on the Cascabel Watersheds. The information presented in the table was obtained from a 

weather station located in the middle of the aggregated watershed area.

Treatment Date Temperature (oF) Relative humidity (%) Wind speed (MPH)

Cool season March 4 56.2 15.6 4.7

Cool season March 11 62.5 20.2 3.4

Warm season May 20 89.7 15.8 8.3

Wildfire begins May 21 85.3 15.6 17.1

Figure 2. The original 

design at the Cascabel 

Watersheds was to treat 

four watersheds during 

the cool season, treat 

four during the warm 

season, and hold four 

as controls. The design 

was changed, as shown 

here, after the Whitmire 

Wildfire. Watershed I 

was originally designated 

to be burned during the 

warm season.

"

"

"

"

" "

" "

"

"

"

" #

#

#

#

#

#

!(

!( I

E

H

J

K

B

M N

F

A

C

G

Cascabel Watershed Boundaries and Treatment Types

Legend

Cool Season Burn

Wildfire

Warm Season Burn

# Tipping Bucket Rain Gauge

" Watershed Plume with Aqua Rod

2 !( Weather Station

1:15,000

0 0.125 0.25 0.5 0.75 1

Miles
 4 USDA Forest Service Research Paper RMRS-RP-86. 2011.

Fire Severities

A system that relates fire severity to the soil resource 

response to burning (Hungerford 1996) was the basis for 

classifying severities of the cool-season and warm-season 

prescribed burning treatments and the wildfire at the 421 

sample plots on the watersheds. This system of classifying 

fire severities relates post-fire appearance of litter, duff, and 

woody material and soil conditions to discrete classes of 

severity that range from low to high. Details of the system 

are found in DeBano and others (1998), Neary and others 

(2005), and Wells and others (1979). Classifications of fire 

severity at the sample plots (fig. 3) were extrapolated to a 

watershed basis to determine the percentages of each of the 

Cascabel Watersheds that were unburned or burned at low, 

moderate, or high fire severities.

Extrapolations to a watershed basis indicated that 85 percent of the four watersheds experiencing the cool-season 

prescribed burn had been exposed to a low-severity fire; 

a moderate-fire severity had occurred on 5 percent of the 

watersheds; and the remaining 10 percent of the watersheds 

were unburned (Stropki and others 2009). Spatial distributions of fire severities on the watersheds that experienced 

the warm-season prescribed burn and wildfire were similar to the distributions of fire severities of the cool-season 

burn (fig. 3). It was concluded, therefore, that the Cascabel 

Watersheds (collectively) had been exposed to low fire severities by the three events. Occurrence of these low fire 

severities was attributed largely to the discontinuous and 

generally limited accumulations of flammable fuels before the burns (Ffolliott and others 2006, 2008a) and to the 

relatively high wind speeds during the burning events (M. 

Harrington, 2010, personal correspondence).

A high fire severity was not observed on the sample 

plots. However, there were scattered sites on the watersheds 

where high fire severities occurred in places where pockets 

of heavy accumulations of litter, duff, and other organic debris had built up before the burning events (Neary and others 

2008).

Study Protocols

Sampling Basis

On each of the Cascabel Watersheds, between 35 and 45 

sample plots were located along transects perpendicular to 

the main stream system and situated from ridge to ridge to 

provide the sampling basis to obtain data on ecosystem resources. Intervals between the plots varied with the size and 

configuration of the watershed sampled. In total, 421 sample 

plots were established on the watersheds. This sampling design was used to collect data necessary to determine effects 

of the prescribed burning treatments and wildfire on the tree 

overstories. It has also been the sampling basis for collecting data sets in other studies of the ecosystem resources on 

the watersheds (Ffolliott and Gottfried 2005, Ffolliott and 

others 2005, 2008b, Gottfried and others 2007, Stropki and 

others 2009).

It was necessary to delay the measurements of the effects of the prescribed burning treatments and wildfire on 

the tree overstories until the spring of 2009 to allow these 

effects to be fully expressed. A one- to two-year delay in 

determining the effects of fire on oak trees, the dominant 

trees in the overstories of the Cascabel Watersheds, had been 

recommended earlier by Plumb (1980), who investigated the 

response of California oak species to fire. Mortality of oak 

Figure 3. Fire severities of the 

cool-season and warm-season 

prescribed burning treatments 

and the Whitmire Wildfire on 

the Cascabel Watersheds based 

on percent of plots sampled 

(from Stropki and others 2009). 

Fire severities shown are based 

on a classification system 

relating fire severity to the soil 

resource response to burning 

(Hungerford 1996). These 

values were extrapolated to a 

watershed basis.
USDA Forest Service Research Paper RMRS-RP-86. 2011. 5

trees in this California study was observed up to two to three 

years following a fire.

Oak species on the Cascabel Watersheds become 

“drought-deciduous” in prolonged periods of drought. 

Because the Southwestern Borderlands Region experienced 

drought conditions before, during, and following the three 

burning events on the watersheds, it was necessary that measurements of fire effects be delayed until post-fire foliage 

was observed on the trees in order to distinguish effects of 

the burning events from effects of the prevailing drought 

conditions. Furthermore, post-fire basal sprouting of the tree 

species on the watersheds can be initiated up to one year 

after a fire (Caprio and Zwolinski 1992), and understanding the extent of this reproductive mechanism was necessary 

in determining the future sustainability of post-fire tree 

overstories.

Tree Overstory Measurements

Measurements to evaluate effects of the burning 

events on tree overstories were obtained on trees tallied 

on 1/4-acre plots centered over the established sample 

plot locations. Diameters at the root collar (drc) of single-stemmed trees surviving the burning events were 

measured, while equivalent diameters at the root collar 

(edrc) were measured on multiple-stemmed trees following the procedures outlined by Chojnacky (1988). These 

measurements had also been obtained in earlier studies of 

tree overstories on the watersheds (Ffolliott and Gottfried 

2005, Ffolliott and others 2008b). A relationship between 

drc or edrc and total height of the trees that was developed in the earlier studies was deemed appropriate for 

constructing a local volume table (Avery and Burkhart 

2001) to estimate volumes of the tree overstories following the burns. Tallied trees were grouped into size classes 

corresponding to those specified by O’Brien (2002) to 

describe the tree resources of the woodland types of 

Arizona. Saplings were trees 1.0 to 4.9 inches drc (edrc), 

medium trees were 5.0 to 8.9 inches drc (edrc), and large 

trees were 9.0 inches drc (edrc) and larger. The tally protocol for saplings was different in 2002 (Ffolliott and 

others 2008b), but data have been adjusted so that both 

inventories are currently similar.

Crown damage to the tallied trees was classified as follows:

• No crown damage

• Less than 1/3 of the crown killed or scorched

• Between 1/3 and 2/3 of the crown killed or scorched

• More than 2/3 of the crown killed or scorched

It was unknown if the damage to tree crowns resulted 

from convection heat of a surface fire or from a crown 

fire that spread from one tree to another tree independent 

of a surface fire. Crown damage can also be caused by fire 

impingement to the tree bole. Because of the open tree overstories on the Cascabel Watersheds and because the three 

burning events on the watersheds were largely surface fires, 

convection heating from surface fire was assumed to be the 

primary cause of crown damage to the trees.

Annual volume growth of trees in the overstories (all 

species combined) was estimated by applying the variabledensity yield table (Fowler and Ffolliott 1995) procedure 

that was followed in the earlier studies of tree overstories on 

the watersheds (Ffolliott and Gottfried 2005, Ffolliott and 

others 2008b). This yield table presents estimates of cubicfoot volume per acre as a function of stand age, site quality 

values (Callison 1988), and tree overstory density in square 

feet of basal area per acre. Pre- and post-fire estimates of 

volumes obtained by this procedure were compared to determine the effects of the prescribed burning treatments and 

wildfire on one-year volume growth.

Basal sprouting by trees was recorded in terms of species, 

size class, and the level of crown damage to determine its 

status following the burning events. Many of the tree species 

inhabiting the oak ecosystems in the Southwestern Borderland 

Region are capable of reproducing vegetatively, with basal 

sprouting as the most common mechanism (McPherson 

1992, 1997, Borelli and others 1994, Ffolliott 2002). It was 

not possible to adequately evaluate the effects of the burns 

on basal sprouting, however, because occurrences of basal 

sprouting had not been tallied before the burning events occurred. Such a comparison with unburned trees could have 

been made if the control watersheds had not burned.

Spatial distributions of the tree overstories following the 

burning events were compared to pre-fire spatial distributions (Ffolliott and Gottfried 2005, Ffolliott and others 

2008b) to determine the effects of the burning events on 

overstory stocking conditions. The respective pre- and postfire coefficients of variation for the different parameters 

served as the basis for making the comparisons.

Analytical Procedures

Tallied trees grouped by size classes (O’Brien 2002) were 

evaluated to determine the effects of the burning events on 

initial survival, crown damage, densities, and basal sprouting of trees at a 0.10 level of significance. Because the three 

size classes (saplings, medium, and large trees) were nested 

within the overall tests of all of the size classes in the tree 

overstories, individual tests of the three size classes were 

evaluated separately by a Bonferroni adjustment to maintain 

the overall 0.10 level of significance. Pre-burning measurements of tree overstories on the watersheds were analyzed 

by the same protocols (Ffolliott and Gottfried 2005, Ffolliott 

and others 2008b). Respective confidence intervals for preand post-fire annual growth estimates were compared at a 

0.10 level of significance. Coefficients of variation for the 

pre- and post-fire spatial distributions of trees were also 

compared at a 0.10 level of significance. The number of tree 

species (species richness) and species evenness (how equally abundant species are) before and after the three burning 

events (Magurran 2004) were indicative of effects of the 

burns on ecological diversity of the tree overstories.
 6 USDA Forest Service Research Paper RMRS-RP-86. 2011.

Results and Discussion

The data sets collected on the watersheds that experienced the prescribed burning treatments and wildfire were 

statistically similar. This finding was not surprising, however, because all of the Cascabel Watersheds had been exposed 

to low fire severities (Stropki and others 2009). Therefore, 

the respective data sets were combined to evaluate effects of 

the three burning events on the tree overstories.

Initial Survival of Trees

Almost 78 percent of the trees (all species and size classes combined) tallied on the watersheds before the prescribed 

burning treatments and wildfire (Ffolliott and Gottfried 2005, 

Ffolliott and others 2008b) survived the burning events initially (fig. 4). More specifically, 80 percent of the oak trees 

(all three species and all size classes combined) survived the 

burns while 75 percent of the juniper trees (both species and 

all size classes combined) survived. Border pinyon trees and 

the tree form of mesquite were too few in number on the 

watersheds to effectively evaluate their respective survival. 

Oak trees continued to dominate in the overstories on the 

watersheds after the burning events, followed (in descending order) by juniper trees, border pinyon trees, and the tree 

form of mesquite.

Seven tree species were tallied on the watersheds before 

the burning events (Ffolliott and Gottfried 2005, Ffolliott 

and others 2008b) and seven species were present after the 

events. Species richness of the tree overstories, therefore, 

was not impacted by the prescribed burning treatments. One 

species, Emory oak, was especially abundant both before 

and after the burns, however, indicating a limited ecological 

diversity of overstory trees on the watersheds.

Crown Damage

The numbers of oak (three species combined), juniper 

(both species combined), and border pinyon are shown in 

relation to their observed crown damage classes in fig. 5. 

Figure 4. Average numbers of oak trees (three 

species combined) and juniper trees (both 

species combined) per acre and 90 percent 

confidence intervals before (Ffolliott and 

Gottfried 2005, Ffolliott and others 2008b) and 

after the prescribed burning treatments and the 

wildfire on the Cascabel Watersheds. Border 

pinyon trees and the tree form of mesquite were 

too few in numbers to include in the figure.

Figure 5. Levels of crown damage suffered by 

surviving oak trees (three species combined), 

juniper trees (both species combined), 

and border pinyon trees on the Cascabel 

Watersheds as a result of the prescribed 

burning treatments and wildfire. The tree 

form of mesquite was too few in numbers to 

include in the figure.
USDA Forest Service Research Paper RMRS-RP-86. 2011. 7

The crowns of 80 percent of the surviving oak trees were 

top-killed or scorched by the burning events, with 30 percent 

of the trees suffering damage to less than 1/3 of their crowns, 

45 percent with damage to 1/3 to 2/3 of their crowns, and 25 

percent with damage to more than 2/3 of their crowns. The 

crowns of 80 percent of the surviving juniper trees suffered 

damage from the burns, with 37 percent of the trees exhibiting damage to less than 1/3 of their crowns, 53 percent with 

damage to 1/3 to 2/3 of their crowns, and 10 percent with 

damage to more than 2/3 of their crowns. Crown damage 

to border pinyon was evaluated. The crowns of 51 percent 

of the surviving pinyon trees suffered damage, with 64 

percent of the trees exhibiting damage to less than 1/3 of 

their crowns, 32 percent with damage to 1/3 to 2/3 of their 

crowns, and 4 percent with damage to more than 2/3 of their 

crowns. Crown damage to the tree form of mesquite was 

not evaluated in the study because of the limited occurrence 

of those trees on the watersheds. The observed levels 

of crown damage to the trees that survived the burning 

events on the Cascabel Watersheds were what one might 

expect following a low-severity surface fire in the oak 

savannas of the Region.

With all tree species considered together, there were 

no differences in the numbers of saplings or medium 

trees in the respective crown damage classes. However, 

the number of large trees with 1/3 or more of their 

crowns killed or scorched by the burning events was 

greater than the number of large trees that were undamaged or that suffered damage to less than 1/3 of their 

crowns. The more severe damage to the crowns of large 

trees was attributed mostly to the large accumulations 

of flammable fuels at the base of those trees before the 

burns.

Trees with crowns that were killed or severely 

scorched by the burning events (that is, more than 2/3 of 

their crowns damaged) often died eventually. This observation supports the work by Fowler and Sieg (2004) in 

their review of criteria for predicting post-fire tree mortality, that trees in montane forests of the western United 

States with severely scorched crowns frequently suffer 

mortality. However, it is possible that many of the oak 

and juniper trees at Cascabel were not root-killed and, 

therefore, are capable of producing basal sprouts in the 

future.

Densities

As a consequence of the burns, there was a reduction 

of almost 20 percent in the number of trees and of about 

15 percent in the volume of trees (all species and size 

classes combined) in relation to those values before the 

burns (Ffolliott and Gottfried 2005, Ffolliott and others 

2008b). Some tree species and size classes (for example, oak and juniper trees in the two largest size classes) 

appeared to be more resilient to burning than others in 

terms of density reductions.

Numbers of Trees

Paralleling the data prior to burning (Ffolliott and 

Gottfried 2005, Ffolliott and others 2008b), oak (three species combined) dominated the numbers of trees after the 

burning events, followed by juniper trees (both species combined) (as shown in fig. 6). Though not presented in fig. 6, 

the number of juniper trees that survived the burns was 

greater than the number of either border pinyon trees or the 

tree form of mesquite following the burns.

A greater number of large trees (all species combined) 

survived the prescribed burning treatments and the wildfire 

than either the medium or small trees (fig. 6). However, there 

also were a greater number of large trees on the Cascabel 

Watersheds before the burning events occurred (Ffolliott and 

Gottfried 2005, Ffolliott and others 2008b). In terms of percentages of the surviving trees, 86 percent of the large trees, 

Figure 6. Average numbers of trees (all species combined) 

per acre (top) and trees by size classes (bottom) and 90 

percent confidence intervals (both) before and after the 

prescribed burning treatments and the wildfire on the 

Cascabel Watersheds. Border pinyon trees and the tree 

form of mesquite were too few in numbers to include in 

the figure.
 8 USDA Forest Service Research Paper RMRS-RP-86. 2011.

88 percent of the medium trees, and 50 percent of the small 

trees survived the burns.

Volumes of Trees

Following the burning events, a local volume table based 

on values calculated by Chojnacky (1988) was applied in 

converting the numbers of trees per acre to corresponding estimates of cubic-foot volumes. These values also were used 

in estimating the volumes of trees before burning occurred 

(Ffolliott and Gottfried 2005, Ffolliott and others 2008b).

Effects of the burning events on volumes of oak and juniper trees (all size classes combined) in comparison to the 

volumes before the burning events (Ffolliott and Gottfried 

2005, Ffolliott and others 2008b) are shown in the top of 

fig. 7. The dominance of oak relative to juniper trees on the 

Cascabel Watersheds can be seen in this portion of the figure. 

Both of the tree species lost essentially the same percentage 

of their volumes in the burns, however, with oak trees losing 

12 percent and juniper trees losing 13 percent, respectively. 

Volumes of border pinyon trees and the tree form of mesquite lost to the burns were insignificant.

Not surprisingly, the volume of large trees (three oak species and both juniper species combined) after the burning 

events was greater than the volumes of small and medium 

trees, as shown in the bottom of fig. 7. In terms of percentages, the volume of small trees before the burns was reduced 

by 12 percent, the volume of medium trees was reduced by 

11 percent, and the volume of large trees was reduced by 14 

percent. The small differences in the volumes before and after the burning are attributed to the low fire severities 

of these events.

Annual Growth

Stand age and overstory density variables in the 

variable-density table developed for the Cascabel 

Watersheds were modified and used to estimate annual growth of trees (all species and size classes 

combined) following the three burning events and 

to determine change relative to pre-fire values. The 

stand age was increased and the tree overstory density was decreased by the burns. (Site quality values 

remained unchanged in this study, as these values 

are not impacted by short-term disturbances such 

as the occurrence of fire.) The appropriate changes 

in stand age and tree overstory density were made 

accordingly.

The estimated annual growth rate of 0.056 ± 

0.017 ft3/acre (mean ± standard error) of the tree 

overstories following the burning events was statistically similar to the estimate of 0.069 ± 0.023 ft3

/

acre before the burns (Ffolliott and Gottfried 2005, 

Ffolliott and others 2008b). Both of these estimates 

of annual growth were less than 1 percent of the respective tree volumes. This finding was expected, 

however, because trees in the oak ecosystems of 

the Southwestern Borderlands Region grow slowly, 

rarely exceeding a fraction of a cubic-foot annually 

(McPherson 1992, 1997, McClaran and McPherson 

1999, Ffolliott 2002).

Basal Sprouting

Basal sprouting by the same tree species found on 

the Cascabel Watersheds is often observed following 

high-severity burning events in the Region (Niering 

and Lowe 1984, Caprio and Zwolinski 1992, 

McPherson 1992, 1997, Ffolliott 2002). However, 

basal sprouting of these tree species on the Cascabel 

Watersheds was surprisingly limited (relatively) after 

Figure 7. Impacts of the prescribed burning treatments and the 

wildfire on volumes of oak trees (three species combined) and 

juniper trees (both species combined) on the Cascabel Watersheds 

by species (top) and by size classes (bottom). Scales of the Y-axis 

are unique for each portion of the figure. Border pinyon trees and 

the tree form of mesquite were too limited in volume to include in 

the figure.
USDA Forest Service Research Paper RMRS-RP-86. 2011. 9

the prescribed burning treatments and wildfire, perhaps because of the low fire severities of the burns (fig. 8). Basal 

sprouting was observed on 37.4 percent of the surviving 

oak trees (three species combined) and on 11.0 percent of 

the surviving juniper trees (both species combined). Basal 

sprouting by the tree form of mesquite was inconsequential. 

Pinyon does not sprout.

Relationships between basal sprouting by trees after the 

burning events and the size of either the oak or juniper trees 

were inconsistent. While there were no differences in the 

presence or absence of basal sprouting by small and medium 

trees after the burns, basal sprouting by large trees was less 

frequently observed than on the small and medium trees. 

The large and likely older trees might have been less vigorous than the smaller trees before the burns, which may have 

limited their capabilities to sprout following the burns.

Relationships of basal sprouting by trees (all species combined) to the crown damage classes were insignificant, with 

the exception of basal sprouting of trees with greater than 

2/3 of their crowns either killed or scorched by the burns. 

More trees suffering this level of crown damage sprouted 

than did not sprout. Apparently, many of these trees had not 

been root-killed by the burning events and, as a result, were 

capable of reproducing vegetatively by basal sprouting.

The extent that basal sprouting of the trees on the Cascabel 

Watersheds after the burning events will impact the future 

sustainability of the overstories is not known. Developing a 

more complete picture of effects of the burns on the post-fire 

basal sprouting of trees was not possible because information on pre-fire basal sprouting of the trees was not available. 

However, only limited basal sprouting before the burning 

events is possible. Caprio and Zwolinski (1992) found “frequent and vigorous” basal sprouting by Emory oak trees on 

burned sites following a wildfire of unknown severity in the 

Santa Catalina Mountains of southeastern Arizona, while 

basal sprouting by the trees on adjacent unburned sites was 

nil.

Spatial Distributions of Trees

Spatial distribution of trees in the oak savannas is generally more variable than in the more densely stocked oak 

woodlands at higher elevations (Ffolliott and Gottfried 2005, 

Ffolliott and others 2008b). However, the spatial distribution 

of trees (all species and size classes combined) in the overstories on the Cascabel Watersheds was not impacted by the 

burning events (fig. 9). The pre-fire heterogeneous stocking 

of the trees on the watersheds was also unchanged by the 

Figure 8. Basal sprouting by trees on the Cascabel Watersheds, such as this Emory oak, was limited following 

the prescribed burning treatments and wildfire.
 10 USDA Forest Service Research Paper RMRS-RP-86. 2011.

Figure 9. The heterogeneous stocking of tree overstories on the Cascabel Watersheds was not impacted by the 

burning events. This photo view was taken in July 2010 of a watershed burned during the warm season.

burns. Openings of varying sizes, shapes, and orientations 

that were interspersed within tree overstories of the oak savannas remained intact.

Conclusions

The prescribed burning treatments and the wildfire in the 

oak savannas on the Cascabel Watersheds were all low fire 

severities. It was not surprising, therefore, that the effects of 

these burning events on tree overstories of the watersheds were 

similar. Effects of these burns on initial tree survival, crown 

damage to the trees, and basal sprouting were mostly inconsequential in terms of management implications. Changes in 

tree composition, density, and annual growth rate in comparison to pre-fire conditions were also relatively minor.

It is unknown if the effects of fire on tree overstories in 

the oak savannas that are reported in this paper would be 

similar in magnitude with repeated prescribed burning treatments of low fire severity on the Cascabel Watersheds. It 

is also unknown what these effects might be if prescribed 

burning treatments of low fire severities were imposed 

on other sites in the oak ecosystems of the Southwestern 

Borderlands Region. A hotter fire might produce different 

results. However, ignitions of the warm-season prescribed 

burning of May 20, 2008, were approaching the threshold 

for initiating prescribed burning treatments in the Region. 

The results presented in this paper, therefore, should be considered case studies.

Management Implications

The information presented in this paper is useful to managers interested in re-introducing more historical fire regimes 

into the oak savannas of the Southwestern Borderlands 

Region. The overcrowding of tree overstories, concurrent 

decrease in herbage (forage) production, loss of critical 

wildlife habitats, and large accumulations of flammable 

fuels currently found on many sites could be alleviated (to 

some extent) by scheduling prescribed burning treatments 

at regular intervals. Managers might use results from the 

burning events on the Cascabel Watersheds as initial guidelines since there is relatively little information available on 

the effects of prescribed burning treatments on ecosystem 

resources in the oak savannas. However, these managers 
USDA Forest Service Research Paper RMRS-RP-86. 2011. 11

should also recognize that prescribed burning treatments of 

low fire severities on other sites in oak ecosystems of the 

Region might not produce results similar to those on the 

Cascabel Watersheds. Additional evaluations of prescribed 

burning treatments of varying severities and seasonal timing 

on ecosystem resources are needed to formulate appropriate management strategies to achieve the desired benefits. 

A change in protocols for implementing prescribed burning 

treatments might have to be considered in order to facilitate 

successful ignitions under less restrictive weather and fuel 

conditions and to achieve ecosystem improvement goals.

Evaluations of prescribed burning treatments of varying frequencies and timing should also include studies of 

the entire array of ecosystem resources available. In addition to tree overstories, these evaluations should include 

herbaceous plants, wildlife populations and habitats, soil 

resources and sediment production, and flows of water 

from upland watersheds. Such efforts are underway on the 

Cascabel Watersheds (Gottfried and others 2007). It is also 

necessary that management agencies, private organizations, 

and local stakeholders collaborate to obtain more natural fire 

regimes in the Southwestern Borderlands Region.

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