Fire Effects on Herbaceous Plants and 

Shrubs in the Oak Savannas of the 

Southwestern Borderlands

Peter F. Ffolliott, Gerald J. Gottfried, Hui Chen, 

Cody L. Stropki, and Daniel G. Neary

United States Forest Rocky Mountain Research Paper

Department Service Research Station RMRS-RP-95

of Agriculture

July 2012
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Ffolliott, Peter F.; Gottfried, Gerald J.; Chen, Hui; Stropki, Cody L.; Neary, Daniel G. 2012. 

Fire effects on herbaceous plants and shrubs in the oak savannas of the Southwestern 

Borderlands. Res. Pap. RMRS-RP-95. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station. 15 p.

Abstract __________________________________________________

Much has been learned in recent years about the ecological, hydrologic, and environmental characteristics of the oak (encinal) woodlands of the Southwestern Borderlands. Comparable information for the lower-elevation oak savannas, including the impacts of fire on ecosystem resources, is 

also necessary to enhance the knowledge of the oak ecosystems in the region. Oak savannas are 

more open in stand structure than are the oak woodlands and, as a consequence, a higher level 

of herbaceous production might be expected in this ecosystem than in the oak woodlands. The effects of prescribed burning treatments and a wildfire on species compositions, production of grass 

and forb components, growth of shrubs, utilization of forage and browse plants by herbivores, and 

ecological diversity in a oak savanna are described in this paper.

Keywords: herbaceous plants, oak savannas, overstory-understory relationships, production, 

shrubs, southwestern United States, species compositions

Authors_____________________________

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

Resources and the Environment, University of Arizona, Tucson.

Gerald J. Gottfried is a Research Forester, Forest and 

Woodland Ecosystems Program with the Rocky Mountain 

Research Station, U.S. Forest Service, Phoenix, Arizona.

Hui Chen is a Research Assistant with the School of Natural 

Resources and the Environment, University of Arizona, Tucson.

Cody L. Stropki was formerly a Research Associate with the 

School of Natural Resources and the Environment, University 

of Arizona, Tucson, and is currently an Environmental Scientist 

with SWCA, Inc., Environmental Consultants, Albuquerque, 

New Mexico.

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

Leader, Air, Water, and Aquatic Environments with the Rocky 

Mountain Research Station, U.S. Forest Service, Flagstaff, 

Arizona.

Cover: Photograph by Larry Allen, Malpai Borderlands Group. A view of an 

area near the Cascabel Watersheds that was burned in the Whitmire Wildfire. 

The photograph was taken three months after the fire. The white instrument is 

a frequency transect frame used to sample plant species composition.
Contents

Introduction.......................................................................................................................1

Cascabel Watersheds ......................................................................................................1

Prescribed Burning Treatments and Wildfire ................................................................3

Fire Severities...................................................................................................................4

Study Protocols................................................................................................................5

Sampling Basis................................................................................................... 5

Field Measurements........................................................................................... 5

Analytical Methods ............................................................................................. 5

Results and Discussion...................................................................................................5

Species Compositions........................................................................................ 6

Precipitation Record........................................................................................... 7

Production of Grasses........................................................................................ 7

Production of Forbs ............................................................................................ 8

Production of Herbaceous Plants....................................................................... 9

Growth of Shrubs ............................................................................................. 10

Utilization of Forage and Browse Plants ...........................................................11

Tree Overstory-Understory Plant Relationship..................................................11

Ecological Diversity .......................................................................................... 12

Conclusions....................................................................................................................12

Management Implications..............................................................................................13

Acknowledgments..........................................................................................................13

References ......................................................................................................................13

USDA Forest Service Res. Pap. RMRS-RP-95. 2012 1

Introduction

Much has been learned in recent years about the ecological, hydrologic, and environmental characteristics 

of the oak (encinal) woodlands of the Southwestern 

Borderlands. Ecological and hydrologic relationships 

of these woodland communities have been studied by 

a number of investigators (DeBano and others 1995; 

Ffolliott 1999, 2002; Gottfried and others 2005, 2007a; 

McClaran and McPherson 1999; McPherson 1992, 1997; 

and others). However, comparable information for the 

lower-elevation oak savannas is necessary to enhance 

the knowledge of all of the oak ecosystems in the region. 

Oak savannas are situated in the transition (interface) 

between the higher-elevation oak woodlands and lowerelevation desert grasslands and shrub communities. 

While Niering and Lowe (1984), working in the Santa 

Catalina Mountains near Tucson, Arizona, described 

this band of vegetation as “open oak woodlands,” the 

authors prefer the term “oak savannas” in differentiating 

this more open canopy ecosystem from oak woodlands.

Species compositions, the production of grass and forb 

components, growth of shrubs, utilization of forage and 

browse plants by herbivores, and ecological diversities 

before and after prescribed burning treatments and a 

wildfire are described in this paper. This information 

should be helpful in developing management strategies 

for reintroducing a more historically similar fire regime 

into the oak savannas to manage tree densities, control 

invasive plant species, increase productivity, and sustain 

ecosystem resources. 

Cascabel Watersheds

Twelve watersheds on the eastern side of the Peloncillo Mountains in southwestern New Mexico were 

Fire Effects on Herbaceous Plants 

and Shrubs in the Oak Savannas of 

the Southwestern Borderlands

Peter F. Ffolliott, Gerald J. Gottfried, Hui Chen, 

Cody L. Stropki, and Daniel G. Neary

(collectively) the study areas for evaluating the effects 

of the burning events on the resources mentioned. 

These watersheds, ranging from about 20 to almost 

60 acres in size, were established by the U.S. Forest 

Service, Rocky Mountain Research Station, to evaluate the impacts of prescribed burning treatments on 

ecological and hydrologic characteristics of the oak 

savannas in the region (Gottfried and others 2000, 

2005, 2007b; Neary and Gottfried 2004). The aggregate area of the watersheds, called the Cascabel 

Watersheds, is 451 acres. They are representative of 

the oak savannas in the Malpai Borderlands in the 

eastern part of the Coronado National Forest on the 

western edge of the Animas Valley as shown in fig. 1. 

The Malpai Borderlands are found within the larger 

Southwestern Borderlands of the region.

The watersheds are at 5380 to 5590 ft in elevation. The nearest long-term precipitation station at 

the Cascabel Ranch headquarters indicates that annual precipitation in the vicinity of the watersheds 

averages 21.8 ± 1.2 inches, with more than one-half 

falling in the summer monsoonal season from late 

June through early September. However, a prolonged 

drought impacted the Southwestern Borderlands from 

the middle 1990s through the end of the study when 

the data summaries and interpretations presented in 

this paper were collected. The annual precipitation 

in this drought averaged 15.8 inches. The bedrock 

geology of the watersheds is Tertiary rhyolite overlain 

by Oligocene-Miocene conglomerates and sandstone. 

Soils are classified as Lithic Argustolls, Lithic Haplustrolls, or Lithic Ustorthents. These soils are generally 

less than 20 inches to bedrock. Streamflow originating 

in the oak savannas is mostly intermittent in nature, 

although large flows can follow high-intensity rainfall 

events (Gottfried and others 2006).
2 USDA Forest Service Res. Pap. RMRS-RP-95. 2012

Figure 1—The Cascabel Watersheds (arrow) are located within the oak savannas of the Malpai Borderlands, an area of 

approximately 802,750 acres within the larger Southwestern Borderlands Region.
USDA Forest Service Res. Pap. RMRS-RP-95. 2012 3

Further descriptions of the geological, edaphic, vegetative, and hydrologic characteristics of the watersheds 

are found in Ffolliott and others (2008b), Gottfried and 

others (2000, 2005, 2007b), Hendricks (1985), Neary 

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

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

(2001), and others. 

Prescribed Burning 

Treatments and Wildfire

The original objective of the research program on 

the Cascabel Watersheds was to evaluate the effects of 

warm-season (May through October) and cool-season 

(November through April) prescribed burning treatments 

on the ecosystem resources of the watersheds, including 

the herbaceous plants and shrubs. It was anticipated that 

these evaluations would be compared to those obtained 

on control (unburned) watersheds in determining these 

fire effects. Four of the watersheds (Watersheds C, H, K, 

and N) were burned by prescribed fire during the coolseason on March 4 and March 11 2008 (fig. 2). Three 

of the four watersheds (Watersheds A, E, and F) to be 

burned in the warm-season were ignited on May 20, 

2008, but burning of the fourth watershed (Watershed I) 

was delayed because of the shifting weather conditions. 

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

May 21, 2008, blew firebrands onto Watershed I and the 

four control watersheds (Watersheds B, J, M, and G). 

The resulting wildfire, designated the Whitmire Wildfire, spread beyond the watershed boundaries to burn 

nearly 4000 acres. Therefore, the original objective of 

the research on the Cascabel Watersheds was modified 

to evaluate the effects of cool-season and warm-season 

prescribed burning treatments and wildfire on the herbaceous plants and shrubs of the watersheds.

Average atmospheric conditions during the prescribed 

burning treatments and wildfire are presented in table 1. 

This information was obtained from a weather station 

located in the middle of the watersheds (Ffolliott and 

others 2011).

Figure 2—The original study design on the Cascabel Watersheds was to burn four watersheds by 

prescribed fire in the cool season, to burn four watersheds in the warm season, and to retain four 

watersheds as controls. However, the study design was changed following the Whitmire Wildfire, as 

shown in this figure. Watershed I was originally designated to be burned in the warm season but instead 

burned as a result of the wildfire.
4 USDA Forest Service Res. Pap. RMRS-RP-95. 2012

Fire Severities

A system that relates fire severities 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 421 

plots on the watersheds. The post-fire soil conditions are 

related to classes of severity ranging from low to medium 

to high in this system. Further details of this system are 

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

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

of fire severity at the plots were then 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.

It was determined that 85 percent of the four watersheds experiencing the cool-season prescribed burn had 

been exposed to a low-severity fire; 5 percent had been 

exposed to a moderate-severity fire, and the remaining 

10 percent were unburned (Stropki and others 2009). 

Spatial distributions of fire severities on the watersheds 

experiencing the warm-season prescribed burn and 

wildfire were similar to the distributions of fire severities for 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. These low fire severities were attributed largely 

to the discontinuous and generally limited accumulations of flammable fuels before the burns (Ffolliott 

and others 2008a) and the relatively high wind speeds 

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 this table was obtained from a weather station located in the middle 

of the watersheds.

Temperature Relative Wind speed

Treatment Date (°F) humidity (%) (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 3—Fire severities of the 

cool-season and warm-season 

prescribed burning treatments 

and the Whitmire Wildfire on the 

Cascabel Watersheds based 

on the percent of plots sampled 

(from Stropki and others 2008). 

Fire severities shown in this figure 

are based on a classification 

system relating fire severities 

to the soil response to burning 

(Hungerford 1996). These values 

were extrapolated to a watershed 

basis.
USDA Forest Service Res. Pap. RMRS-RP-95. 2012 5

during burning events (M. Harrington 2010, personal 

correspondence).

Wide-spread high-severity fire was not observed on 

the plots. However, there were scattered and localized 

sites on the watersheds where high fire severities occurred as a consequence of “heavy accumulations” of 

litter, duff, and other organic debris building up before 

the burning events occurred (Neary and others 2008).

Study Protocols

Sampling Basis

Each of the Cascabel Watersheds contains between 

35 and 45 permanent plots that were established along 

transects located perpendicular to the main stream 

channels and situated from ridge to ridge to measure 

ecological resources and determine hydrologic functioning. The interval between the plots varied depending 

on the size and configuration (shape) of the watershed 

sampled. A total of 421 plots were established on the 

watersheds. Estimates of the production of herbaceous 

plants and growth of shrubs were obtained on 9.6 ft2

plots centered over the previously established plots. 

Sampling was conducted in the spring and fall of each 

year.

Field Measurements

Estimates of the production of grasses and forbs and 

shrub growth were obtained by the weight-estimate 

procedure originally outlined by Pechanec and Pickford 

(1937) and later by Cook and Stubbendieck (1986). Samples of the herbaceous plants and shrubs were collected 

on 20 to 30 temporary 9.6 ft2 plots located adjacent to a 

sub-sample of the primary plots. The collections were 

obtained at the same time that the estimates of herbage 

production and the growth of shrubs were made in order 

to develop the corrections factors necessary to convert 

the field estimates of green weights to actual (oven-dried) 

weights. The estimates of herbage production and shrub 

growth were then expressed in pounds per acre.

Species compositions and seasonal (spring and fall) 

estimates of the production (standing biomass) of the 

grass and forb components of herbaceous plants and 

seasonal growth of the shrubs were obtained before the 

prescribed burning treatments and wildfire from the 

spring of 2003 through the fall of 2007 (Ffolliott and 

others 2008b) and from the spring of 2008 through the 

fall of 2010 following the burns. The spring estimates 

reflect the production of early growing herbaceous plants 

and early growth of shrubs, while the fall estimates 

represent the production of late growing herbaceous 

plants and the late growth of shrubs (McPherson 1992, 

1997). Temperature and antecedent soil water derived 

largely from late fall and winter precipitation events are 

the primary factors favorable to early growing plants. 

Late growing plant species are more responsive to the 

summer monsoonal rains.

Utilization of forage and browse plants by herbivores 

was determined ocularly (Cook and Stubbendieck 1986; 

Holechek and others 2004) at the same times that seasonal 

herbage production and shrub growth were estimated 

in the watersheds. No differentiation was made of the 

herbivores involved in utilizing the plants.

Analytical Methods

The data sets obtained on the Cascabel Watersheds 

before the early season and late season prescribed burning 

tests and wildfire were statistically similar for all pre-fire 

sampling dates and, therefore, pooled for analysis. The 

data sets on the watersheds experiencing the individual 

burning events were also statistically similar for all of 

the sampling dates. Furthermore, there were no statistical differences among the data sets for the burns and 

wildfire for the sampling dates. Therefore, the post-fire 

data sets also were pooled to evaluate the effects of the 

burning events on the herbaceous plants and shrubs.

The tests of significance in determining statistical 

differences in the data sets were evaluated at a 0.10 level 

of significance. However, because the grass and forb 

components were nested within the overall tests of all of 

the herbaceous plants, the individual tests of these two 

herbaceous components were evaluated separately at a 

0.05 level of significance to maintain the overall 0.10 

level of significance in accordance with a Bonferroni 

adjustment.

The number of herbaceous and shrub species present 

(species richness) and how equally abundant the species 

are (species evenness)—two commonly used measures 

of ecological diversity (Magurran 1988, 2004)—before 

and after the burning events were indicative of the effects 

of the burns on the ecological diversity of the species.

Results and Discussion

Species compositions of the herbaceous plants and 

shrubs on the Cascabel Watersheds were similar on 

the watersheds both before and after the prescribed 

burning treatments and wildfire. However, while the 

estimates of production obtained on the watersheds 
6 USDA Forest Service Res. Pap. RMRS-RP-95. 2012

before the burns were statistically similar and, therefore, 

combined (Ffolliott and others 2008b), the effects of the 

burning events on the production of grasses and forbs 

and the growth of shrubs differed on the watersheds 

that experienced the three burning events. As a result, 

the watersheds were grouped into those experiencing 

prescribed burning in the cool season, those treated 

by prescribed burning in the warm season, and those 

exposed to the wildfire for analyses. Differences among 

the watersheds within each of these three groupings 

were insignificant.

Species Compositions

Species compositions of herbaceous plants and 

shrubs following the prescribed burning treatments and 

wildfire were largely similar to the compositions before the burning events (fig. 4). Perennial grasses after 

the burns included blue (Bouteloua gracilis), sideoats 

(B. curtipendula), slender (B. repens), and hairy (B. hirsuta) grama; common wolftail (Lycurus phleoides); 

bullgrass (Muhlenbergia emersleyi); and Texas bluestem 

(Schizachyrium cirratum). Species of lupine (Lupinus

spp.), mariposa lily (Calochortus spp.), and verbena 

(Verbena spp.) were the primary plants in the relatively 

minor forb component of the herbaceous plants.

Beargrass or sacahuista (Nolina microcarpa), common sotol (Dasylirion wheeleri), fairyduster (Calliandra eriophylla), pointleaf manzanita (Arctostaphylos 

pungens), Mexican cliffrose (Purshia mexicana), and 

Fendler’s ceanothus (Ceanothus fendleri) were scattered half-shrubs and shrubs. Palmer’s agave (Agave 

palmeri) and banana yucca (Yucca baccata) were 

occasionally found on well-drained rocky slopes. 

Shrub-forms of Emory (Quercus emoryi), Arizona 

white (Q. arizonica), and Toumey (Q. toumeyi) oak 

and mesquite (Prosopis glandulosa var. torreyana) 

were present after the burns as they were before. Annual plants were largely absent on the watersheds.

Figure 4—Species compositions of the herbaceous plants and shrubs on the Cascabel Watersheds were 

not significantly changed by the prescribed burning treatments or wildfire. This is a view of the cool-season 

prescribed fire on Watershed K.
USDA Forest Service Res. Pap. RMRS-RP-95. 2012 7

Precipitation Record

The production of herbaceous vegetation is correlated with the seasonal distribution of precipitation 

and available soil water in arid ecosystems (Mulroy 

and Rundel 1977). Data from the Cascabel J2 weather 

station, located in the center of the experimental area 

(fig. 2), provide information about precipitation during 

the study period (fig. 5). The totals sum the cool-season 

and warm-season records for the precipitation year and 

differ from the annual totals.

Average cool-season precipitation for the eight years 

with standard deviation was 6.0 ± 3.3 inches, while average warm-season precipitation was 10.9 ± 3.8 inches. 

The total average was 16.9 ± 4.4 inches. Approximately 

64 percent of the moisture occurred in the warm season. 

Natural production of many important species coincides 

with the warm-season monsoon rains. The relative importance of the seasonal precipitation to early growing 

and late growing herbaceous plant production before 

and after the fire treatments is apparent in the subsequent 

figures. Although rigorous tests were not conducted, it 

does not appear that seasonal precipitation was different 

before and after treatments.

Production of Grasses

Averages and 90 percent confidence intervals for the 

production of early growing and late growing grasses 

before and after the prescribed burning treatments and 

wildfire are presented in fig. 6. The production of grasses 

in both the spring and fall was significantly greater following the burning events than before these burns, as 

illustrated by fig. 6. Depending on the event, there was 

a five- to seven-fold increase in the production of early 

growing grass species. Increases in the production of late 

growing grasses following the burns were significant but

smaller in magnitude than the increases in early growing grasses. It must be remembered, however, that the 

effects of these burning events were estimated during a 

period of prolonged drought. It is also possible that the 

reported increases in the production of grass species 

will decline in magnitude if the drought continues. 

Figure 5. Precipitation records from the Cascabel J2 weather station. Cool-season precipitation 

includes monthly records from late fall of the previous year through April of the year of record. 

The cool-season precipitation for 2003 for example includes November and December 2002 

through April 2003. The warm-season for 2003 includes May through October 2003.
8 USDA Forest Service Res. Pap. RMRS-RP-95. 2012

The greater production of grasses following the burning 

events was attributed to a combination of factors. One 

cause was likely the reduction of trees in the overstories. 

While only 20 percent of the trees were lost to burning 

(Ffolliott and others 2011), these trees competed with 

herbaceous plants for soil water. Therefore, the reduction of even this small number of trees was likely to 

make more of the limited soil water on the Cascabel 

Watersheds available for grass production. The consumption of portions of the litter and duff layers by the 

burns reduced interception of precipitation and could 

have also provided more soil moisture for the growth 

of the grasses. Still, another reason for the increase in 

grass production after the burning events could be that 

a pool of nutrients was formed where vegetation, litter, 

A

B

Figure 6—Averages and 90 percent confidence intervals for the production of early 

growing (A) and late growing (B) grasses on the Cascabel Watersheds before and after 

the prescribed burning treatments and wildfire. 

and duff were combusted by the burns. In general, one 

might anticipate a flush of available nutrients following 

fire (Bond and van Wilgen 1996; DeBano and others 

1995; Whelan 1997).

Production of Forbs

The production of forbs was significantly less than the 

production of grasses before and after the prescribed 

burning treatments and wildfire. However, the effects of 

the burning events on forb production were inconsistent, 

as shown in fig. 7. Neither the production of early growing forbs nor the production of late growing forbs was 

altered by the cool-season burning treatment. However, 
USDA Forest Service Res. Pap. RMRS-RP-95. 2012 9

the production of the early growing forbs increased following the warm-season prescribed burning treatment 

and the wildfire. These increases were attributed to the 

fact that the two burning events occurred at the same 

time and that both of the events resulted in low fire 

severities. There was no change in the production of 

late growing forbs after warm-season prescribed burn 

or the wildfire.

The reasons for the inconsistencies in the effects of the 

burning events on the production of forbs are unknown. 

However, varying combinations of the fewer trees and 

the release of available nutrients following the burns 

likely affected the growth of individual forb species 

in different ways and at different times throughout the 

growing season. Increased evaporation of soil water 

because of the loss of shade could have reduced soil 

water content in the summer and, as a consequence, 

affected the response of forbs to the burning events.

Production of Herbaceous Plants

Averages and 90 percent confidence intervals for the 

production of early and late growing herbaceous plants 

(grasses and forbs combined) before and after the prescribed burning treatments and wildfire are presented 

in fig. 8. The production of these plants paralleled the 

pattern of the seasonal production of grasses. That is, 

the production of herbaceous plants in both the spring 

and fall was greater following the burning events than 

before these burns. This finding was expected because 

the large proportion of the herbaceous plants on the 

Cascabel Watersheds was grass species.

Figure 7—Averages and 90 percent confidence intervals for the production of early growing 

(A) and late growing (B) forbs on the Cascabel Watersheds before and after the prescribed 

burning treatments and wildfire. Scales of the Y-axis are unique for each portion of the figure.

A

B
10 USDA Forest Service Res. Pap. RMRS-RP-95. 2012

The reduction in trees; combustion of vegetation, litter, 

and duff; and a pool of nutrients were among the factors again assumed to have contributed to the estimated 

increases in herbaceous plants after the burning events. 

Thus, the reintroduction of fire into the Cascabel Watersheds achieved a beneficial increase in the production 

of herbaceous species, a primary ecosystem component.

Growth of Shrubs

Shrubs contributed little to the production of understory plants on the Cascabel Watersheds both before and 

after the prescribed burning treatments and wildfire. 

Furthermore, the growth of shrubs was not generally 

affected by these burning events, as shown in fig. 9. 

One exception was the late season growth of shrubs 

on the watersheds burned by the wildfire. The growth 

of shrubs, and particularly the growth of the basal 

sprouts of oak trees that dominated the shrub biomass, 

was significantly greater before than after the wildfire. 

This finding contrasts with the “frequent and vigorous” 

basal sprouting by oak trees on burned sites following a 

wildfire in the Santa Catalina Mountains of southeastern 

Arizona (Caprio and Zwolinski 1992). Basal sprouting 

by oak trees on adjacent unburned sites in the Santa 

Catalina Mountains was nil.

A

B

Figure 8—Averages and 90 percent confidence intervals for the production of early growing 

(A) and late growing (B) herbaceous plants on the Cascabel Watersheds before and after 

the prescribed burning treatments and wildfire.
USDA Forest Service Res. Pap. RMRS-RP-95. 2012 11

Utilization of Forage and Browse Plants

Utilization of forage and browse plants by herbivores 

was less than 5 percent both before (Ffolliott and others 

2008b) and after the burning events. The local rancher 

removed the cattle from the Cascabel Watersheds in 

the summer of 2004 because of the prevailing drought 

conditions (Gottfried and others 2007b), and cattle were 

not allowed back onto the watersheds throughout the 

remainder of the study. It was speculated that the Coues 

white-tailed deer (Odocoileus virginianus var. couesi), 

an indigenous ungulate found on the watersheds, move 

to the higher elevations of the Peloncillo Mountains 

in the warmest months of the year to escape the high 

temperatures and lack of water (Ffolliott and others 

2012). The low utilization of forage and browse plants 

by herbivores, therefore, was anticipated.

Tree Overstory-Understory Plant Relationship

Analyses of the frequently reported relationships of 

increasing production of understory plants with decreasing densities of a tree overstory (Ffolliott and Clary 

1982; Bartlett and Betters 1983) indicated that there 

were no significant correlations between the production 

of herbaceous plants or the growth of shrubs and the 

range in densities of the tree overstories on the Cascabel 

Watersheds either before (Ffolliott and others 2008b) or 

after the burning events. A lack of correlation between 

the production of understory plants and the densities 

Figure 9—Averages and 90 percent confidence intervals for the early growth (A) and late 

growth (B) of shrubs on the Cascabel Watersheds before and after the prescribed burning 

treatments and wildfire. Scales of the Y-axis are unique for each portion of the figure.

A

B
12 USDA Forest Service Res. Pap. RMRS-RP-95. 2012

of tree overstories has also been found in the higherelevation oak woodlands (Gottfried and Ffolliott 2002; 

Ffolliott and Gottfried 2005).

It is likely, therefore, that while the reduction in numbers of trees might have influenced the production of 

herbaceous plants, the densities of the tree overstories 

are not a significant factor in influencing the total production of understory plants. To some extent, however, 

precipitation is a factor influencing the production of 

herbaceous plants. An earlier study on the Cascabel 

Watersheds reported “high correlations” between the 

production of herbaceous plants and the precipitation 

amounts “favorable” to the seasonal production of these 

plants (Ffolliott and others 2008b). Depending on the 

plant component (grasses, forbs, or total herbage) and 

the season when its production was estimated, up to 64 

percent of the variation in these estimates of production 

was attributed to the seasonal precipitation. Relationships between production of the herbaceous plants and 

air temperatures or relative humidity were insignificant.

In contrast to the situation on the Cascabel Watersheds, forage production beneath overstories of blue oak 

(Quercus douglasii) trees was significantly greater than 

the production of forage in the open grasslands on the 

San Joaquin Experimental Range in the foothills of the 

Sierra Nevada Mountains in central California (Frost and 

others 1991, 1997). Most of the forage production in this 

region occurs in March, April, and May when the soil 

moisture stored beneath the tree overstories is available 

for forage growth. However, soil moisture stored in the 

open grasslands at this time of the year is less because 

of the warm temperatures and high evapotranspiration 

rates.

Ecological Diversity

Diversity is a frequent theme of ecological studies 

because it is a measure of the “well being” of an ecosystem. Ecological diversity of the oak ecosystems in 

the Southwestern Borderlands Region is a reflection 

of the inherent structure and functioning, past and 

present land-use activities, and historical fire regimes 

(Bahre 1991, 1995; Bahre and Shelton 1996; Sayre 1999; 

Swetnam and Baisan 1996a, b). This ecological parameter 

embodies two fundamental indices no matter how it is 

measured (Magurran 1988, 2004). These indices are 

species richness (the number of species) and species 

evenness (how equally abundant the species are). High 

species evenness, that is, when the species inhabiting an 

area are virtually equal in their abundance, is equated 

with high diversity. These two indices were the basis 

for determining the effects of the prescribed burning 

treatments and wildfire on the ecological diversities of 

the understory plants on the Cascabel Watersheds.

The numbers of herbaceous and shrub species tallied 

on the Cascabel Watersheds before and after the burnings were essentially the same. Therefore, the ecological 

diversity of understory plants in terms of species richness was not affected by the burning events (fig. 10). 

However, the frequency of occurrences of these species 

was not measured either before or after the burns. As 

a consequence, a “direct measure” of species evenness 

was not possible. But, assuming that the herbaceous 

species included in the estimates of the production of 

herbaceous plants, and the shrub species included in the 

estimates of the growth of shrubs, can be a proxy for the 

frequencies of these species, the ecological diversity of 

understory plants by this measurement is also similar 

both before and after the burning events. It has been 

concluded, therefore, that the burns did not affect the 

general ecological diversity of understory plants on the 

watersheds.

Conclusions

Assuming that the findings presented in this paper 

reflect the more general situation in the oak savannas of 

the Southwestern Borderlands, it is suggested that burning events of low severity have little effect on species 

compositions. However, these burns can significantly 

increase the production of both early and late growing 

grasses in this ecotype. The effect of burning on the 

production of forb species is inconsistent, and the effect 

Figure 10—Species richness of understory plants on the 

Cascabel Watersheds was not changed by the prescribed 

burning treatments or wildfire on the watersheds. View of an 

area burned by the Whitmire Fire in August 2008, about three 

months after the fire is illustrated by the photograph by Larry 

Allen, Malpai Borderlands Group.
USDA Forest Service Res. Pap. RMRS-RP-95. 2012 13

of burning on the growth of shrubs is inconsequential. 

Ecological diversity appears to be little changed by such 

burns. It is unknown if the effects of burning on understory plants reported in this paper would be similar in 

magnitude following repeated prescribed burning events 

of low severities. It is also unknown how low-severity 

burns imposed on other sites in the oak savannas might 

affect the occurrence of herbaceous plants and shrubs. 

Furthermore, a hotter fire might produce different 

findings. Therefore, the results presented in this paper 

should be considered case studies.

Management Implications

The information in this paper should be useful to 

managers interested in reintroducing a more natural fire 

regime into the oak savannas. For example, managers can 

use the results obtained on the Cascabel Watersheds as 

initial guidelines on the effects of burning on herbaceous 

plants and shrubs in the oak savannas since information 

on these effects are limited. However, managers should 

also recognize the prescribed burning treatments of low 

severities on other sites in these ecosystems might not 

produce results that are similar to those obtained on the 

Cascabel Watersheds. Additional evaluations of treatments of varying burning severities and seasonal timing 

are needed to more completely formulate management 

strategies to achieve the desired benefits.

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

the array of ecosystem resources and services available. 

Such efforts are underway on the Cascabel Watersheds 

(Gottfried and others 2007b). It is also important that 

management agencies, private organizations, and local 

stakeholders continue to collaborate in efforts to obtain more natural fire regimes and derive the resulting 

benefits.

Acknowledgments

This research and the preparation of this paper were 

supported by the Southwestern Borderlands Ecosystem 

Management Project of the Rocky Mountain Research 

Station, U.S. Forest Service, Phoenix, Arizona; U.S. 

Forest Service National Fire Plan; the Coronado National Forest, Douglas Ranger District; and the Arizona 

Agricultural Experiment Station, University of Arizona, 

Tucson. The cooperation of the Diamond A Ranch, 

Animas, NM is appreciated.

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