United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical

Report RMRS-GTR-81

October 2001

Mechanisms of Range Expansion

and Removal of Mesquite in Desert

Grasslands of the Southwestern

United States

Thomas B. Wilson

Robert H. Webb

Thomas L. Thompson
The Authors ________________________________________

Thomas B. Wilson, Department of Soil, Water and Environmental Science, University of Arizona, Tucson,

AZ 85721.

Robert H. Webb, U.S. Geological Survey, 1675 W. Anklam Road, Tucson, AZ 85745.

Thomas L. Thompson, Department of Soil, Water and Environmental Science, University of Arizona,

Tucson, AZ 85721.

Acknowledgments ___________________________________

The funding for this project was provided by the Southwestern Borderlands Ecosystem Research

Program of the USDA Forest Service, Rocky Mountain Research Station. The authors particularly thank

Gerry Gottfried for his support of this project. Sheridan Stone of Fort Huachuca provided considerable

support for our on-base work. Janice E. Bowers and Peter G. Griffiths of the U.S. Geological Survey and

Thomas Biggs of the University of Virginia provided data for this report. John H. Brock, Peter F. Ffolliott,

Gerald J. Gottfried, and Lorne G. Wilson critically reviewed the manuscript.

Abstract ___________________________________________

Wilson, Thomas B.; Webb, Robert H.; Thompson, Thomas L. 2001. Mechanisms of range expansion

and removal of mesquite in desert grasslands of the Southwestern United States. Gen. Tech.

Rep. RMRS-GTR-81. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain

Research Station. 23 p.

During the last 150 years, two species of mesquite trees in the Southwestern United States have

become increasingly common in what formerly was desert grassland. These trees have spread from

nearby watercourses onto relatively xeric upland areas, decreasing rangeland grass production.

Management attempts to limit or reverse this spread have been largely unsuccessful. This paper

reviews studies regarding mesquite natural history and management strategies, emphasizing studies

published during the past decade. Mesquite possess a deep root system and are capable of fixing

atmospheric N, rendering them capable of accessing resources unavailable to other plants in open

rangeland. Their seeds, which remain viable for decades, have a hard exocarp and require scarification

before germination. Consumption by cattle provides a means of scarification and seed dispersal, and

is a major factor contributing to the spread of mesquite in open rangelands. Increases in atmospheric

CO2 and winter precipitation during the past century also contribute to enhanced seed germination.

Removal techniques have included herbicides, prescribed burning, grazing reduction, and mechanical

removal. For increased effectiveness of these techniques, management goals must be clearly

articulated; these goals include complete removal, no removal, and limited removal. Of these, limited

removal appears the most feasible, using an initial herbicide application followed by periodic prescribed

burning.

Keywords: mesquite, biological invasions, rangeland ecology, soil nitrogen, rangeland management

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CAUTION:

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Pesticide Precautionary Statement

This publication reports research involving pesticides. It does not

contain recommendations for their use, nor does it imply that the uses

discussed here have been registered. All uses of pesticides must be

registered by appropriate State and/or Federal agencies before they can

be recommended.

CAUTION: Pesticides can be injurious to humans, domestic animals,

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Table of Contents

Page

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

Mesquite Natural History ..................................................... 1

Floristics .......................................................................... 1

Morphology ...................................................................... 8

Phenology and Germination ............................................ 8

Mesquite and Nitrogen .................................................... 9

Historical Range Expansion of Mesquite .......................... 10

Mechanisms of Mesquite Range Expansion ..................... 11

Fire ................................................................................ 11

Seed Dispersal .............................................................. 12

Livestock Grazing .......................................................... 14

Climate Fluctuations and Atmospheric Gases .............. 14

Mesquite Management Practices ...................................... 17

Prescribed Burning ........................................................ 17

Herbicide Application ..................................................... 18

Mechanical Removal ..................................................... 19

Mesquite Management Objectives .................................... 19

Objective 1: Complete Mesquite Removal

From Desert Grassland. ........................................... 19

Objective 2: Continued Mesquite

Establishment in Desert Grassland. ......................... 20

Objective 3: Limited Mesquite Removal

From Desert Grassland. ........................................... 20

Conclusions ...................................................................... 20

References ........................................................................21

Page

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 1

Introduction ____________________

Rangelands in the Southwestern United States

have been used for settlement, agriculture, and livestock ranching during the last 150 years. Because of

the gentle topography, sufficient available water, and

abundant forage, use of this rangeland—especially in

southeastern Arizona and southwestern New Mexico

(fig. 1)—has been intensive. As a result, much of this

area has been transformed to a mixed-phase woodland or shrubland, with no sign of this trend diminishing (Archer and others 1988; Buffington and

Herbel 1965; Hastings and Turner 1965; McClaran

and Van Devender 1995). Figures 2–6 show repeat

photography that document this change at selected

sites in the region. The alteration of grassland to

shrubland has created considerable concern as land

use managers recognize a corresponding decline in

available livestock forage. The mesquite tree (Prosopis

spp.) has played a major role in this change, as it has

expanded its range from more sheltered and mesic

desert grassland drainage systems and riparian zones

to open rangelands.

While the causes that account for this altered distribution—climate change (Grover and Music 1990),

livestock grazing (Archer and others 1988; Bahr and

Shelton 1993; Schlesinger and others 1990), suppression of wildfires (Bahre 1985; Biggs 1997; Humphrey

1958), or rodent effects (Cox and others 1993; Reynolds

and Glendening 1949)—have been investigated for

several decades, no single effective strategy has been

offered as a means of slowing or reversing mesquite

encroachment in desert grasslands. Removal tactics

have included fire (Britton and Wright 1970; Wright

and others 1976), herbicides (Gibbens and others

1986; Jacoby and others 1982; Warren and others

1996), and mechanical removal (Dodd and Holtz 1971).

These have resulted in varying degrees of success,

mainly short term.

One key to understanding why mesquite has expanded its range, and why it may be at times impervious to attempts at removal, is through an examination

Mechanisms of Range Expansion

and Removal of Mesquite in Desert

Grasslands of the Southwestern

United States

Thomas B. Wilson

Robert H. Webb

Thomas L. Thompson

of its natural history. Though this has been studied in

the past, many studies concerning intraspecific competition (Ansley and others 1998; Kramp and others

1998), seed germination requirements (Bush and

Van Auken 1990; Cox and others 1993), root systems

(Ansley and others 1990, 1991; Stromberg 1993), and

nitrogen fixation (Johnson and Mayeux 1990; Zitzer

and others 1996) have taken place only within the last

10 years.

In this study, we especially refer to the desert grasslands of southeastern Arizona and southwestern New

Mexico in the area shown in figure 1. Previous workers

(Brown and Lowe 1980) referred to two types of grassland in this region—semidesert grassland and plains

and Great Basin Desert grassland—but we prefer the

simpler classification of “desert grassland” as used by

McClaran and Van Devender (1995). We subdivide the

desert grasslands into the desert grasslands of southeastern Arizona, which mostly surround the Santa

Rita and Huachuca Mountains, and the desert grasslands of the western Chihuahuan Desert, which spans

the area from the Sulfur Springs Valley of southeastern Arizona to the Bootheel of southwestern New

Mexico.

The objectives of this study are (1) to describe the

natural history of the mesquite species that have

increased their distribution within the Southwestern

United States, focusing on those attributes that could

account for this range expansion, (2) to review the

existing research that has addressed this expansion,

with an emphasis on research produced within the last

decade, (3) to describe management goals, practices,

and results, and (4) to propose a management strategy

that accounts for regional differences while providing

long-term effectiveness.

Mesquite Natural History _________

Floristics

Three species of mesquite tree are currently recognized as indigenous to the Southwestern United
2 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001

Figure 1—Map showing biotic communities of the region of southwestern New Mexico and southeastern

Arizona that contain desert grasslands (from Brown and Lowe 1980). This area includes what we refer to as

Sonoran and Chihuahuan Desert grasslands, defined by Brown and Lowe (1980) as semidesert grassland

and plains and Great Basin grassland.

States: screwbean mesquite (Prosopis pubescens

Benth.), velvet mesquite (Prosopis velutina Woot.),

and honey mesquite (Prosopis glandulosaTorr.). Two

subspecies of honey mesquite are found in this area:

Texas honey mesquite (Prosopis glandulosa var.

glandulosa Torr.) and western honey mesquite

(Prosopis glandulosa Torr. var.torreyana [L. Benson]

M.C. Johnston). They are members of the legume

family (Fabaceae) and are characterized by pinnate

leaves, spiny branches, seedpods, and a central taproot. However, individual variation in physical characteristics as well as hybridization makes taxonomic

classification at the species level problematic. In

general, P. glandulosa can be differentiated from

P. velutina because the former has glabrous leaflets

up to 4 cm long; the latter has pubescent leaflets up

to 1.3 cm long. The two varieties of P. glandulosa are

distinguished by their spines; P. glandulosa var.

torreyana has paired thorns, while P. glandulosa var.

glandulosa has solitary thorns (Isely 1998).

Two of these species, P. glandulosa and P. velutina,

have had the most significant expansion into desert

grasslands. While P. velutinais restricted in the United

States to southern and central Arizona, P. glandulosa
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 3

Figure 2—The Altar Valley east of the Coyote Mountains (approximately 884 m). (A) October 8,

1903. In 1903, much of the Altar Valley south of what is now known as Three Points, AZ, was open

desert grassland with scattered cacti and shrubs. Even at this early date, scattered small

mesquite (P. velutina) shrubs, along with littleleaf palo verde (Cercidium microphyllum), are on

the open rangeland. Cattle grazing was pervasive in the Altar Valley in large ranches (photograph

by David Griffiths, number 2098, courtesy of the National Archives). (B) February 20, 1999. The

Altar Valley is now mostly a mesquite-palo verde woodland with little remaining perennial grass.

At this site, creosote bush (Larrea tridentata) is also prominent in the foreground along with cholla

cacti (Opuntia spp.) (photograph by Dominic Oldershaw, stake 3805, courtesy of the Desert

Laboratory Photograph Collection).

A

B
4 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001

Figure 3—Railroad Pass on the Munk Ranch north of Willcox, southeastern Arizona

(1,250 m). (A) Between 1901 and 1920. Much of the rangeland east of Willcox was

grassland punctuated with scattered mesquite trees and yuccas (Yucca elata) at the turn

of the century. This type of environment was probably maintained by periodic wildfires

before the advent of widespread cattle grazing and fire suppression (photograph by J. A.

Munk, number A.141.86, courtesy of the Southwest Museum). (B) June 19, 1995.

Rangeland east of Willcox now is mostly shrubland, dominated by snakeweed (Gutierrezia

sarothrae) with locally abundant mesquite (both P. velutina and P. glandulosa var.

glandulosa) and yuccas (photograph by Dominic Oldershaw, stake 3350, courtesy of the

Desert Laboratory Photograph Collection).

A

B
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 5

Figure 4—Hooker Ranch in the Dos Cabeza Mountains, southeastern Arizona

(1,433 m). (A) Between 1901 and 1919. The headquarters of the Hooker Ranch

is in a valley south of the peaks of the Dos Cabeza Mountains. Around the turn

of the century, this valley had vegetation similar to that shown in figure 2A just

east of Willcox. (photograph by J. A. Munk, number A.142.143, courtesy of the

Southwest Museum). (B) June 24, 1995. Prosopis velutina is now a dominant tree

on the lower slope rangelands in the view. Perennial grasses remain between the

trees at this higher-elevation site, however (photograph by Dominic Oldershaw,

stake 3355, courtesy of the Desert Laboratory Photograph Collection).

A

B
6 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001

Figure 5—Lincoln Mine Camp in the Sierrita Mountains of southern Arizona (1,091 m).

(A) February 25, 1906. The Lincoln Mine is in the foothills of the Sierrita Mountains on the

east side of the Altar Valley in southern Arizona. As shown in this view of the former mine

headquarters, the valley supported a grassland with scattered mesquite (P. velutina) and

succulents, including ocotillo (Fouquieria splendens) and prickly pear cactus (Opuntia

phaeacantha) (photograph by D. T. MacDougal, number A1-58, courtesy of the Desert

Laboratory Photograph Collection). (B) 1994. Woody vegetation now dominates this

valley. Littleleaf paloverde (Cercidium microphyllum) has increased as well as mesquite

(P. velutina), shrubs, and prickly pear cactus. Perennial grasses are difficult to find in

most years of the late 20th century (photograph by Robert H. Webb, stake 3114,

courtesy of the Desert Laboratory Photograph Collection).

A

B
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 7

Figure 6—Munk Draw on the Munk Ranch north of Willcox, southeastern Arizona (1,364 m).

(A) Between 1901 and 1919. Scattered trees, mostly mesquite, line the shallow

watercourse in this photograph of the active Munk Ranch north of Willcox. Note the cattle

in the background (photograph by J.A. Munk, number A.141.96, courtesy of the Southwest

Museum). (B) June 16, 1995. Woody vegetation, primarily mesquite, has increased along

this ephemeral watercourse in the 20th century (photograph by Dominic Oldershaw, stake

3336, courtesy of the Desert Laboratory Photograph Collection).

A

B
8 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001

occurs from Texas to California. P. glandulosa var.

glandulosa is found mainly in Texas east of the Pecos

river and north into Oklahoma, while P. glandulosa

var. torreyanais found west of the Pecos river, throughout southern New Mexico and adjacent Arizona, and

in southern California, northwestern Arizona, southern Nevada, and southwestern Utah. P. glandulosa

var. glandulosa can also be found along Interstate 10

through New Mexico and Arizona east of Tucson due

to cattle transport with subsequent seed dispersal. In

this paper we assume that Prosopis velutina and the

two subspecies of P. glandulosa are sufficiently similar in characteristics to be treated together.

Morphology

Mesquite attains its maximum height of over 12 m

in sheltered riparian habitats, yet is opportunistic

enough to grow on exposed desert slopes where reproductively mature individuals may only be 1.5 m tall. In

xeric upland sites, their taproot may reach only 1 m in

depth where solid bedrock prevents deeper penetration; however, taproots of P. velutina have been recorded at over 53 m depth (Philips 1963) under more

favorable conditions.

While mesquite trees are characterized by a massive

central taproot, they also possess an extensive lateral

root system (Heitschmidt and others 1988). For this

reason they are classified as facultative phreatophytes,

able to extract permanent ground water when available but also able to extract soil moisture (Ansley and

others 1990). Ansley and others (1990, 1991) severed

the lateral roots of four P. glandulosa var. glandulosa

trees at two sites in western Texas. They found that

stomatal conductance was reduced during the first

growing season on the site with less available subsurface water; this reduction disappeared during

the second growing season, probably due to root

regeneration.

In riparian habitats and desert grasslands, P. glandulosa and P. velutina are typically single stemmed

with a central trunk up to 35 cm in diameter. However,

multistemmed individuals are not uncommon, especially as a result of resprouting following fire. Conversely, multistemmed plants are most common in

more arid habitats such as the Arizona upland regions of the Sonoran Desert. Aboveground form also

appears to be related to subsurface characteristics.

For example, Stromberg and others (1993) compared

P. velutina aboveground characteristics to available

water and found that the height of stands in riparian

areas was inversely related to depth of the water table;

trees were 8 m tall where the ground water depth was

greater than 15 m, but grew to 12 m where the ground

water was less than 15 m.

Whether single or multistemmed, mesquite trees

produce branches that may form a canopy with a

diameter twice the height of the plant. Both stems and

larger branches may be either smooth or rough textured with a fair degree of individual variation.

Branches are typically spiny with considerable variation in thorn length (from 0.5 to over 5 cm) and density.

Finn and others (1994) identified annual growth

rings in the wood of P. glandulosa at five sites in

Texas. Their samples included trees that resprouted

vegetatively following cutting; for these plants they

noted that ring count would signify age of regrowth,

not necessarily the tree itself. They found stems ranging from 7 to 30 years in age, but their data in conjunction with Archer’s (1989) growth rate study suggest

that the age tree stems may exceed 200 years. Using

repeat photography, Bowers and others (1995) documented P. glandulosalongevities more than 100 years.

Phenology and Germination

Though climatic conditions vary considerably throughout the ranges of P. glandulosa var. glandulosa and

P. velutina, their growing seasons are generally in the

spring and in the late summer. This is typical of a C3

species but not the C4 grass species that dominate

open rangelands. Mesquite trees are typically deciduous during the winter months, but may also lose their

leaves during summer drought (Ansley and others

1992). Leaves produced during the spring may be

larger than in late summer (Nilsen 1986), perhaps in

response to light intensity and transpiration potential. These leaves form a hydrophobic waxy cuticle

that can accumulate during the growing season, providing mesquite some resistance to foliar-applied herbicides (Jacoby and others 1990c).

Flowering occurs in the late spring and typically

attracts a variety of flying insect-pollinators, including bees, wasps, and beetles. Seed pods exceeding

30 cm in length are produced through the summer.

The seeds have a generally high mortality rate from

bruchid beetles (Algarobius prosopis and Neltumius

arizonensis), which bore holes into the fruit and deposit eggs on the seed. Germination occurs in both fall

and spring, coincident with peaks in annual precipitation (Brown and Archer 1987).

The seeds have a hard exocarp and must undergo

scarification prior to germination. This contributes to

their long-term viability, which may exceed 20 years

(Martin 1948, 1970). Kramp and others (1998) evaluated the effectiveness of deer, cattle, and coyotes as

scarification agents and vectors for P. glandulosa var.

glandulosa seed dispersal. They found that seeds

continued to germinate in fecal samples of all species

2 years after deposition, and that an average of 7.8,

5.2, and 4.5 seedlings emerged from deer, cattle, and

coyote feces, respectively. These authors concluded

that cattle were most effective as seed vectors because

cattle feces contained more seed. Brown and Archer
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 9

(1989) found that honey mesquite had a survival rate

of 74 to 97 percent for individuals that survived 2 weeks

after germination in cattle fecal sites. Ingestion by

herbivores has the added benefit of eliminating bruchid

beetles from the fruit (Burkhart 1976).

Under laboratory conditions, P. glandulosa seedlings in a clay loam soil emerged from planting depths

between 0.5 and 1.5 cm after 3 to 6 days, with a rootto-shoot ratio of 5:1 within this period (Scifres and

Brock 1971). In a similar study of P. velutinaseedlings,

Cox and others (1993) found that a planting depth of

2 to 4 cm in a sandy loam resulted in the highest

percent germination; they also determined that seeds

planted at this depth were unaffected by fire. Brock

(1986) found that unrestricted root growth is critical

for the survival of velvet mesquite seedlings. In a

laboratory study, he found that the greatest root

development and growth rate during the first 30 days

was in sandy loam and clay loam soils; after this time

aboveground growth exceeded root development.

Emerging seedlings readily survive losses of top

growth due to herbivory (Weltzin and others 1998). In

a growth chamber experiment, Weltzin and others

(1998) evaluated the relative tolerances of seedlings

aged 18 and 33 days to repeated clipping above either

the first or fourth nodes. They found that the younger

seedlings had a 75 percent survival rate after repeated

clipping above the first node, verses a 38 percent

survival rate for older seedlings.

Insufficient light may limit the survival of emerging

seedlings. Bush and Van Auken (1990) observed that

seedlings rarely emerge under the canopy of mature

P. glandulosa trees, despite elevated soil nutrient

concentrations. When these authors compared seedling emergence and survival, they found that the

largest seedlings were found in areas with full light

and herbaceous plant exclusion, while the smallest

seedlings were found under adult trees with herbaceous plant competition. Conversely, Brown and Archer

(1989) found P. glandulosaseed germination and establishment to be independent of herbaceous biomass or

moisture conditions, though their survival rate increased with supplemental watering.

Intraspecific competition can limit population densities of either seedlings or reproductively mature

individuals. In a survey of establishment rates for

Prosopis glandulosa var. glandulosa in cattle fecal

sites in northern Texas, Kramp and others (1998)

found that all sites where mesquite had germinated

eventually had either a single established plant or

none. Ansley and others (1998) noted that access to

soil water appears to be the competitive mechanism

limiting population densities and individual tree size

for this species in areas where lateral root systems

provide the main source of water uptake.

Mesquite and Nitrogen

Nitrogen availability is considered to be a major

factor limiting plant growth in regions where mesquite occurs in the Southwestern United States (West

and Klemmedson 1978). However, several studies

have indicated that mesquite in these regions represent a source of nitrogen rather than a sink. In a 3-year

study, Barth and Klemmedson (1983) found that soil

under P. velutina canopies accumulated 11.2 g N/m2

per meter of tree height; this concentration was inversely related to distance from the trunk and decreased away from the canopy edge. Virginia and

Jarrell (1983) found similar results in a study of soil

properties under P. glandulosa var. glandulosa canopies; total N, NH4

+

, NO3

–

, organic C, plant-available

P, and K were significantly (P ≤ 0.05) higher beneath

canopies than in adjacent open areas. In California,

Rundel and others (1982) found 1,020 g soil total N/m2

under Prosopis glandulosa var. torreyana canopies,

160 g soil total N/m2 total N between canopies, and

45 g soil total N/m2 in adjacent open areas. At the

Fort Huachuca Military Reservation in southeastern

Arizona, shallow soil under mesquite canopies had an

average of 17 mg NO3

–

-N/kg and 2.05 mg/kg in adjacent open areas (Biggs 1997).

The main mechanism responsible for elevated soil

nutrient concentrations under mesquite is biological

nitrogen fixation by soil Rhizobia followed by litterfall.

In a symbiotic relationship, mesquite will transport

abundant C into their root systems at a high energy

cost to facilitate the establishment of Rhizobia bacteria within the rhizosphere. Subsequently, the roots

form nodules around the Rhizobia which proceed to

convert N2 from the soil atmosphere to a form usable

by the host plant (NH3). Eventually this fixed atmospheric N is returned to the soil via litterfall. Though

plants resorb leaf nutrients to varying degrees before

leaf senescence, fallen P. glandulosa leaves can contain 2.6 percent N, a retention rate of over 74 percent

(Killingbeck and Whitford 1996). This represents an

annually renewed source of soil N which is replenished

mainly during the late fall when mesquite become

deciduous. Rundel and others (1982) have estimated

that 25 to 30 kg N/ha accumulates annually under

canopies of P. glandulosa var. torreyana as a result of

fixation, whereas Virginia (1986) found that mesquite litterfall annually returns 135 kg N/ha to the

soil. Since litter deposition occurs mainly under the

mesquite canopy, adjacent open areas may remain

unaffected.

Despite empirical evidence, explicit identification of

N fixation by mesquite has remained elusive. Shearer

and others (1983) found a lower isotope ratio of 15N/14N

in leaf tissue of P. glandulosa var. torreyana compared

to similar tissue of unrelated reference plants. This
10 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001

indicated that mesquite were using atmospheric N,

which is naturally more abundant in 14N, rather than

mineralized soil N, which is more abundant in 15N.

However, these authors could not quantify the amount

of N fixed using this method, due to lack of replication

and the absence of appropriate non-N-fixing reference

plants.

Other researchers have attempted to locate N-fixing

nodules in the roots of mesquite. Johnson and Mayeux

(1990) recovered brown, leathery textured nodules in

the roots of P. glandulosa var. glandulosa in eastern

Texas. They found that 11 of 19 trees possessed nodules within 10 cm of the soil surface; the average

length of each nodule was 1.7 mm. These nodules

increased in frequency with increasing depth below

the surface to their maximum sampling depth of 3 m.

The nodules were weakly attached to roots less than

0.5 mm in diameter. These authors also found abundant insect frass and body parts in the vicinity of the

roots and suggested that nodule predation by insects

may have occurred. Virginia (1986) found that soil

Rhizobia populations increased with soil depth to a

maximum concentration at the upper margins of

ground water. Johnson and Mayeux (1990) suggest

that since soil NO3 can prevent N fixation, nodule

formation may occur in deeper soils with lower NO3

concentrations. Jenkins and others (1988, 1989) found

seasonally fluctuating populations of Rhizobia at 0 to

9 m depth in soil columns collected in playas, dunes,

arroyos, and grasslands in California and New Mexico.

Their data suggest that soil salinity, age of the host

plant, and ground water depth and fluctuations determine Rhizobia distribution and type. Presumably, the

location and density of nodulation in the mesquite root

zone correspond to these populations.

Considerable logistical difficulties must be overcome for field identification of nodules, given their

fragility, small size, and the depths at which they are

found. Johnson and Mayeux (1990) failed to find nodules under mesquite in western Texas, New Mexico,

and Arizona despite δ15N analysis that suggested

that these mesquite were utilizing atmospheric N.

These authors speculated that they may have simply

failed to locate the soil nodules since nodule formation

may be of an ephemeral nature, as has been suggested

by Bailey (1976) and Zitzer and others (1996).

Conversely, mesquite seedlings can form nodules

under controlled conditions. Bailey (1976) identified

root nodules produced by P. glandulosa seedlings

grown in a growth chamber. Frequency of nodulation

was statistically correlated with soil texture, water,

and the abundance of Rhizobia; wet sandy soils produced the most nodules, while dry clayey soils produced the least. Felker and Clark (1980) also found

nodules formed when both P. glandulosa varieties

were grown in vermiculite, furnished with a nutrient

solution free of N, and inoculated with soil Rhizobia

collected from the nodules of a mesquite grown in a

separate nodulation experiment (Eskew and Ting

1978).

Historical Range Expansion of

Mesquite_______________________

Much of the information on mesquite expansion in

the Southwestern United States comes from anecdotal

observations of ranchers and other rangeland users,

published reports, and repeat photography (figs. 2–6).

In one of the earliest scientific reports on the subject,

Griffiths (1910) reported his observations of increasing mesquite populations on the Santa Rita Experimental Range south of Tucson, AZ, and speculated

that overgrazing was one of the primary contributing

factors. Hastings and Turner (1965) present repeat

photography that documents increases in P. velutina

on southern Arizona rangelands. They found that

much of the increase in mesquite occurred after 1880.

A recent update of Hastings and Turner (1965) shows

that mesquite has also increased between the mid1960s and mid-1990s, indicating that the expansion

has not ceased (R.M. Turner, written communication,

1999).

Estimates of the rate of recent mesquite establishment in open rangelands vary widely. Hennessy and

others (1983) determined that a black grama (Bouteloua

eriopoda) grassland on the Jornada Experimental Range

in southern New Mexico was completely converted to

mesquite woodland between 1934 and 1980. Gibbens

and others (1992) compared mesquite density on the

Jornada in herbicide-treated areas in 1976 and in

1988 and noted an increase from 67 to 494 plants/ha.

These authors also estimated that on the College

Ranch in New Mexico, P. glandulosa increased from

130 plants/ha in 1982 to 147 plants/ha in 1988. Using

data provided by Glendening (1952), Gibbens and

others (1992) estimated that P. velutina increased in

a 17-year period on the Santa Rita Experimental Range

at the rate of 9.2 plants/ha/year in areas with livestock

grazing, and at the rate of 15.7 plants/ha/year in areas

where livestock and rodents were excluded. Felker

and others (1990) noted that P. glandulosa stands can

regenerate to a density of 10,000 stems/ha, with an

average trunk diameter of 4 cm, 10 years after land

has been cleared. To predict future growth of P. velutina

stands, Chojnacky (1991) developed a model using

basal area, stems per stand, trees per stand, and

average tree height data from 47 locations in southern

Arizona. This model is mainly applicable to short-term

projections, since it does not account for mesquite

mortality. Using the model with a 30-year projection,

this author noted that initial basal area was probably
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 11

the most important variable influencing growth rate;

for a stand with an initial basal area of 18.5 m2/ha the

stand volume increased from approximately 62 to

80 m3/ha within 10 years, followed by a reduction to

the initial volume in the last 20 years. Implicit in this

model is the idea that mesquite stands that occur in

locations with abundant resources will experience a

rapid increase in size, then decline through intraspecific competition.

Mechanisms of Mesquite Range

Expansion _____________________

Mesquite have the ability to access ground water

using a deep root system, grow in a variety of soil types

and in the variety of climatic conditions found in the

Southwestern United States, and can indirectly access atmospheric N. Therefore, mesquite can avoid the

resource limitations that restrict the establishment

and maintenance of competing plants in the majority

of the habitats in which they are found. With this

relative independence, why have mesquite moved

into open grasslands from adjacent riparian or ephemeral channels only within the past century? While

much research has been focused on the mechanisms

of recent mesquite dispersal into open grasslands

(Archer and others 1988; Bahre 1985; Bahre and

Shelton 1993; Cox and others 1993; Grover and Musick

1990; Humphrey 1958; Reynolds and Glendening 1949;

Schlesinger and others 1990), little attention has

focused on why mesquite had not occupied these areas

prior to the past 150 years.

Fire

Mesquite range expansion has been attributed to

altered land-management practices associated with

cattle ranching (Bahre and Shelton 1993). Fires have

been actively suppressed during the past century to

minimize damage to vegetation, livestock, and property; in addition, cattle grazing has reduced herbaceous fuel loads, thereby reducing fire frequency

(Van Auken and Bush 1997). Before widespread settlement by Euro-Americans, Native Americans regularly set wildfires in desert grasslands (Bahre 1985).

Over long periods, these fires presumably minimized

mesquite seedling establishment in open areas while

allowing the continued presence of mesquite in sheltered drainage and riparian areas. Now, occasional

fires occur with uncertain long-term effects (fig. 7).

Figure 7—June 1999. Effect of rangeland fire on Prosopis velutina woodland on

eastern foothills of the Huachuca Mountains, southeastern Arizona. Note the mesquite

tree (P. velutina) in the right foreground appears to be largely undamaged by fire

(photograph by Thomas Wilson).
12 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001

At the Fort Huachuca Military Reservation, we

located three adjacent sites of different fire frequency

but equivalent topography and soil type (fig. 8). These

sites are separated by a network of roads that function

as firebreaks. The site with less than one fire per

decade for the past 30 years had 4.75 kg/m2 of Prosopis velutina aboveground biomass; the site with an

average of two fires per decade for this time period had

0.52 kg/m2; and the site with an average of four fires

per decade had 0.078 kg/m2 (Biggs 1997). The ratios of

P. velutina biomass to grass biomass were 116:1 on the

unburned site, 3:1 on the site with two fires per decade,

and 1:2 on the site with four fires per decade.

Seed Dispersal

Mechanisms of seed dispersal are likely to play a key

role in the dynamics of mesquite distribution (Brown

and Archer 1989). Janzen and Martin (1982) suggested that mesquite thrived in open grasslands as

recently as 10,000 years ago. During the Pleistocene,

horses, elephants, and ground sloths served as the

A

B

Figure 8—Aerial photographs of Woodcutter’s Canyon in the

southeastern gunnery ranges at Fort Huachuca Military

Reservation in southeastern Arizona. Note that the road system

changes through time but the topography is consistent among

these photos. The letters A, B, and C refer to plots intensively

studied by Biggs (1997), Biggs and others (1999), and Wilson

and others (1999) for the effects of fire frequency on soilnutrient concentrations and mesquite stand densities. (A) 1935.

Individual mesquite (P. velutina) are mostly confined to washes

at the upper right, center, and lower left. A prominent line from

upper right to middle left is a fence line, a residual of livestock

grazing in the area that ended in the early 1940s. Note that no

mesquite occur on sites A, B, and C. (B) November 17, 1956.

By 1956, mesquite trees are scattered throughout the view,

although large sections of grasslands remain without trees,

particularly at right. The modern road system, which also

serves as a firebreak system, was established just before this

time. The stand density of mesquite on sites A, B, and C is

approximately equal; all three sites had disturbances related to

World War II training exercises. (C) January 18, 1975. Mesquite

C

Fig. 8 (con.)
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 13

stands have become quite dense in some portions of the view, particularly at lower left. Mesquites have continued

to increase on site A, which did not burn in the 20th century. Site C burned shortly before this photograph, resulting

in its light color in this photograph, and mesquite trees are not very visible on this site in 1975. (D) January 20, 1985.

Mesquite stand densities are highest in 1985, particularly in the rangelands at left. Site C, which had a fire frequency

of four fires per decade between the early 1970s and 1989 (Biggs 1997), remains free of significant mesquite. In

the same period, site B had a fire frequency of two fires per decade, and mesquite trees are scattered uniformly

at a lower density than sites at lower left. Mesquite density on site A continued to increase because of lack of fires.

The difference between the burned sites and site A is reflected by differences in albedo. (E) March 31, 1994.

Because of wildfires set during military exercises, mesquite stand densities may be significantly lower on much of

the rangeland, particularly at lower left. However, the size of individual plants is much lower, reflecting resprouting

of trees damaged by fires. Because of the lack of fires, site A consequently has a high stand density and large size

of individual mesquite trees (Biggs and others 1999). For these reasons, site A clearly is distinguishable from sites

B and C as well as the rest of the rangeland. For these reasons, the Southern Gunnery Range at Fort Huachuca

serves as a good example of our management objective C, the limited control of mesquite on rangelands.

D E

primary seed-dispersal agents, ingesting mesquite

fruit and subsequently depositing scarified seed at

fecal sites. Martin and Burney (1999) also suggest that

many of the characteristics of mesquite—sharp spines,

waxy leaves, sweet-tasting fruit, and the ability to

readily resprout following herbivory—evolved with

ancestral megafauna. This prevented overgrazing

while promoting fruit consumption and seed dispersal.

With the extinction of these large mammalian herbivores, mesquite were found less frequently in open

areas but persisted in drainages and riparian areas,

where they were more resistant to environmental

perturbations such as drought or fire, and where their

seeds became dispersed through surface-water transport (Grover and Musick 1990).

Adjacent drainages and riparian areas offer more

favorable growing conditions compared to open grasslands: improved moisture and nutrient availability

through surface-water transport, reduced maximum

air temperatures, and some protection from wind and

fire. However, mesquite are currently thriving on

open grasslands, and are providing an environment

favorable for the establishment of other woody shrubs

such as prickly pear cactus (Opuntia lindheimeri),

desert hackberry (Celtis pallida), hog plum (Colubrina

texensis)(Archer and others 1988), and perennial grasses

such as bush muhly (Muhlenbergia porteri) (Haque

and others 1991). Birds, rodents, and insects acting

as seed dispersal agents for shrub species may be

attracted to mesquite canopies for shade and reduced

visibility, incidentally transporting seed to the soil

below (Archer and others 1988). Consequently, the

shrubs germinating from these seeds might become

established because areas under mesquite canopies

provide most of the same resources and protection

from the elements as riparian areas.

Figure 8 (Con.)
14 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001

Livestock Grazing

Cattle grazing is believed to be a major cause of the

contemporary range expansion of mesquite trees.

Through grazing, cattle limit herbaceous competition

for germinating mesquite seedlings, creating more

open areas with access to sunlight. Cattle ingest mesquite fruit, consequently transporting the seed, scarifying it, then depositing it in a moist, nutrient-rich

microenvironment. Brown and Archer (1987) compared seedling emergence on sites with and without

cattle. They found 12 to 15 seedlings/m in the former

case and no seedlings in the latter. Brown and Archer

(1989) maintain that as seed dispersers, cattle are

surrogate megafauna, resuming relationship between

mesquite and large mammals that last existed in the

latest Pleistocene.

Cattle indirectly facilitate the establishment of

other shrubs under mesquite by augmenting soil nutrient concentrations. Cattle tend to congregate under

the shade of mesquite trees, where they deposit abundant feces, providing supplemental N to the soil that

adds to the elevated N concentrations that result from

litterfall. Livestock therefore contribute to the redistribution of nutrients from open rangelands to beneath

mesquite canopies.

Livestock grazing facilitates mesquite establishment

in open grassland ranges but reduces the biomass of

native grasses by at least three mechanisms. First, as

mesquite woodlands comprise a larger portion of the

aboveground biomass within an area, the grazing

intensity on the remaining grasses increases because

shrubs and trees are less palatable than grasses (Grover

and Musick 1990). Second, mesquite woodlands can

form a canopy that inhibits native grass seed germination as suitable microclimates become more spatially restricted. Third, mesquite trees sequester

nutrients, making intertree areas relatively nutrient

poor (Biggs 1997).

Eventually, mesquite growth may inhibit grazing.

Archer and others (1988) used aerial photographs

taken in 1941, 1960, and 1983 to monitor closedcanopy woodlands and woody plant groupings centered around P. glandulosa within grasslands on the

Rio Grande Plains of Texas. Within the grassland,

they found that half of the groupings were less than

5 m apart and 95 percent were within 15 m. These

authors predicted that these gaps would diminish as

individual canopies grew because the mean area for

each grouping increased from 494 m2 to 717 m2 (+70

percent) between 1941 and 1983. Seedlings, composed

mainly of P. glandulosa, occupied 85 percent of the

open areas (Archer and others 1988). Eventually these

trees can form an impenetrable thicket, restricting

livestock traffic and increasing the cost of livestock

management (Teague and others 1997).

Mesquite stands may continue to increase in size on

open ranges after cattle grazing has been eliminated

(Brown 1950; Brown and Archer 1989; Glendening

1952). Once seed has been dispersed into open areas

and mesquites have become established, a constantly

renewed seed bank and seed viability that can last for

20 years (Martin 1970) ensure that mesquite could

persist on open rangelands. For example, at the Fort

Huachuca Military Reservation in southeastern Arizona, cattle grazing has been eliminated since the

mid-1940s. However, aerial photographs of the region

from 1935 to 1994 document the continued increase of

mesquite in open rangeland 50 years after cattle

grazing was terminated, despite periodic grassland

fires that appear to affect average canopy size (fig. 8).

While maximum stand densities continue to exist

within riparian areas, populations of established trees

and seedlings in grasslands persist to form woodlands

of greater than 80 trees/ha at Fort Huachuca (Wilson

2001).

Climate Fluctuations and Atmospheric

Gases

An increase in the relative contribution of winter

rains to annual precipitation can favor the germination and establishment of certain shrub species, including mesquite (Sala and others 1997). Mesquite

and many common shrubs in the Southwestern United

States have a C3 photosynthetic pathway, deciduous

habit, a moderate transpiration rate, drought avoidance, low herbivory, high seed production, many germination requirements, high survival following disturbance, and slow recruitment (Diaz and Cabido

1997). Grasses in this region are C4 plants with a high

leaf area, a shallow dense fine root system, a high

transpiration rate, low to moderate drought resistance, moderate herbivory, moderate seed production,

few germination requirements, and low survival after

disturbance but rapid reestablishment. Changes in

the seasonal pattern of soil-water potential is reflected

in the relative abundance of C3 shrubs verses C4

grasslands in a region (Sala and others 1997). Shrubs

favor winter precipitation for seed germination and

vegetative growth, while grasses are more responsive

to warm-season rainfall, suggesting that changes in

seasonal precipitation may drive rangeland conversions from grasses to shrubs.

Changes in climate in the Southwestern United

States during the 20th century have been documented (Hastings and Turner 1965) and disputed

(Bahre 1991). To evaluate climatic variation in southwestern New Mexico and southeastern Arizona, we

standardized seasonal precipitation using the technique of Hereford and Webb (1992). We identified

the two ecological precipitation seasons of summer
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 15

(July through September) and winter (November

through March). For winter precipitation, November and December totals were considered part of the

following year. For each climate station that we

used (tables 1–2), we calculated the standardized

seasonal precipitation, Ps, by

Ps = ∑{∑[(xij – μi)/σi]/k}/n, (1)

where xi,j = monthly precipitation for climate station i

in month j (mm); μi = the mean and σi = the standard

deviation of monthly precipitation for climate station

i (mm); k = the number of months in the season; and

n = the number of climate stations with data. The

value of n varies through the 20th century due to the

varying number of available climate stations (for

example, tables 1–2) and whether or not data are

missing.

Annual and seasonal precipitation in southwestern

New Mexico and southeastern Arizona has varied during the 20th century (figs. 9–11). From 1980 through

1998, 15 and 12 years had above-average annual

precipitation in southwestern New Mexico and southeastern Arizona, respectively (fig. 9). Annual precipitation from about 1940 to 1980 was generally below

average, particularly in the mid-1950s. The results

displayed in figure 9 confirm previous work that suggests that annual precipitation in the Southwestern

United States is nonstationary (Hereford and Webb

1992; Webb and Betancourt 1992).

Seasonal precipitation also varies between years. In

southwestern New Mexico and southeastern Arizona,

12 years had above-average winter precipitation between 1980 and 1998, compared to 12 years of aboveaverage winter precipitation during the previous 40

years (figs. 10–11). Between 1900 and 1998 in southeastern Arizona, winter or summer precipitation (but

not both) was above average for 51 years; of these, 28

years had above-average winter precipitation with

below-average summer precipitation. The period from

1895 to 1906 included only 3 years of above-average

precipitation, and those only occurred during the summer (fig. 11A). However, 10 of the following 11 years

had above-average winter precipitation, coinciding

with the range expansion of mesquite observed by

Griffiths (1910). These weather patterns may have

significantly contributed to this expansion by optimizing environmental conditions for enhanced mesquite

seed germination. More recent increases in winter

precipitation could be manifested in an accelerated

rate of mesquite establishment during the past 20

years as observed by ranchers in the Southwestern

United States.

The 27 percent increase in atmospheric CO2 from

1900 to the present has been cited as a contributing

Table 1—Climate stations in the western Chihuahuan Desert grassland.

State Location Elevation Period of record Annual precipitation

m mm

Arizona Bowie 1,149 1-1-1899 to 12-31-1998 273

Arizona Chiricahua National Monument 1,616 1-1-1909 to 12-31-1998 493

Arizona Clifton 1,058 1-1-1893 to 12-31-1998 328

Arizona Douglas 1,232 7-1-1948 to 2-28-1994 371

Arizona Douglas FAA Airport 1,250 7-2-1948 to 12-31-1998 333

Arizona Douglas Smelter 1,210 12-1-1903 to 3-20-1973 312

Arizona Duncan 1,116 5-21-1901 to 12-31-1998 276

Arizona Paradise 1,655 1-1-1906 to 8-31-1937 484

Arizona Portal 1,524 1-1-1914 to 3-31-1955 442

Arizona Portal 4SW 1,643 3-1-1965 to 12-31-1998 546

Arizona Safford 884 8-1-1998 to 6-30-1973 226

Arizona Safford Agricultural Station 899 8-1-1948 to 12-31-1998 234

Arizona San Simon 1,101 3-1-1898 to 12-31-1998 244

Arizona San Simon 9ESE 1,183 7-19-1962 to 7-31-1986 251

Arizona Willcox 1,274 6-1-1898 to 12-31-1998 311

New Mexico Animas 1,345 9-1-1923 to 12-31-1998 280

New Mexico Columbus 1,268 1-1-1925 to 12-31-1998 242

New Mexico Deming 1,311 1-1-1914 to 12-31-1998 240

New Mexico Eicks Ranch 1,619 1-1-1933 to 10-31-1961 373

New Mexico Hachita 1,375 1-1-1914 to 12-31-1998 269

New Mexico Lordsburg 1,296 1-1-1914 to 12-31-1998 275

New Mexico Redrock 1,265 1-1-1914 to 12-31-1998 324

New Mexico Rodeo 1,256 1-1-1914 to 12-31-1998 285

New Mexico White Signal 1,851 11-1-1948 to 12-31-1998 381
16 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001

Table 2—Climate Stations in the southeastern Arizona Desert grassland.

State Location Elevation Period of record Annual precipitation

m mm

Arizona Apache Powder Company 1,125 7-1-1923 to 4-30-1990 335

Arizona Arivaca 1,105 1-1-1956 to 12-31-1998 455

Arizona Benson 1,119 6-1-1894 to 5-31-1975 288

Arizona Elgin 1,494 10-1-1912 to 12-31-1969 383

Arizona Fort Huachuca 1,424 2-1-1900 to 12-31-1981 397

Arizona Nogales 1,162 7-1-1948 to 6-30-1983 421

Arizona Nogales 6N 1,085 10-1-1952 to 12-31-1999 450

Arizona Old Nogales 1,189 12-1-1892 to 6-30-1948 399

Arizona Patagonia 1,233 7-1-1921 to 12-31-1998 454

Arizona San Rafael Ranch 1,445 12-1-1892 to 3-31-1968 439

Arizona Santa Rita Experimental Station 1,311 7-1-1916 to 12-31-1998 566

Arizona Sierra Vista 1,402 3-1-1982 to 12-31-1998 374

Arizona Tombstone 1,405 7-1-1893 to 12-31-1998 354

Sonora Agua Prieta 1,189 2-1-1961 to 12-31-1986 379

Sonora Cananea 1,607 1-1-1923 to 10-31-1991 539

Sonora Naco 1,404 1-1-1923 to 12-1-1995 368

Figure 9—Standardized anomalies of annual

precipitation. (A) Grasslands of the western

Chihuahuan Desert. (B) Grasslands of southeastern Arizona.

Figure 10—Standardized anomalies of seasonal

precipitation in grasslands of the western

Chihuahuan Desert (table 1). (A) Summer precipitation (July through September). (B) Winter

precipitation (November through March).

A

B

A

B
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 17

factor in mesquite range expansion. Polley and others

(1994) observed increased nodulation and N fixation

when greenhouse-grown P. glandulosa seedlings were

exposed to elevated CO2 concentrations. These seedlings also produced more belowground biomass, and

their N and water-use efficiencies increased linearly

with increasing atmospheric CO2 concentrations. Conversely, mesquite seedlings grown with little bluestem

(Schizachyrium scoparium), a C4 perennial grass,

manifested none of these characteristics. The authors

suggest competition with grasses attenuates the beneficial effects of increased atmospheric CO2 on mesquite. In the absence of this competition, mesquite

seedlings can establish more rapidly.

Mesquite Management

Practices ______________________

Because of mesquite expansion into open rangelands, with resulting reductions in livestock forage,

land-use managers have employed a variety of tactics—fire, herbicide, and physical removal—to completely remove existing trees and limit further spread.

These practices all have something in common: logistical difficulties and side effects potentially harmful to

habitat restoration. For example, ranchers at the

Waggoner Ranch (202,000 ha) in northern Texas

have attempted to remove mesquite since the 1950s

without effect, and it continues to regenerate in

treated areas and spread further. Attempts at control

have been largely terminated due to increasing costs

(Teague and others 1997).

Prescribed Burning

Fire has been used as a management tool to control

mesquite distribution for decades (Humphrey 1949).

However, mesquite trees can survive fire while still

relatively young. Wright and others (1976) determined

that while P. glandulosa seedlings up to 1.5 years of

age were easy to kill with medium intensity fires,

2.5-year-old trees survived with severe damage, and

3.5-year-old trees were tolerant; these authors recommended against using fire as a management tool in

overgrazed areas. Archer and others (1988) found

similar results. Martin (1983) determined that within

5 years of a fire in southern Arizona P. velutina

biomass had attained preburn levels. In these cases

cattle grazing could increase mesquite survival rates

by not only lowering fire frequency through fuel removal (Van Auken and Bush 1997), but fire intensity

as well. Hobbs and others (1991) found that grazing

reduced fire temperature and energy release in a

tallgrass prairie.

Livestock grazing alters rangeland fire characteristics in the following ways. Consumption and trampling by livestock can lower the density and average

height of herbaceous biomass. In turn, this can affect

the average height and duration of a fire; in many

cases, flames may not reach the canopies of reproductively mature mesquite. After a fire, mesquite below a

certain size class can be reduced in numbers; however,

given the survival rate of larger reproductively mature trees, recruitment would continue as the larger

trees replenish the mesquite seed bank.

Since mesquite trees appear largely impervious to

individual fires, repeated burning has been recommended (Wright and others 1976). In this case, the

fires would need to occur at a frequency that would

prevent the recovery from damage inflicted by previous fires, and it assumes that sufficient fuel would

accumulate in the periods between fires. While this

might provide an effective management tool to limit

the distribution of mesquite of certain size classes, it

could also provide the conditions that facilitate the

establishment of exotic plant species better adapted

Figure 11—Standardized anomalies of seasonal

precipitation in the desert grasslands of southeastern

Arizona (table 2). (A) Summer precipitation (July

through September). (B) Winter precipitation

(November through March).

A

B
18 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001

than native grass species to cope with frequent disturbance, such as Lehmann lovegrass (Eragrostis

lehmanniana) (Martin 1983; Wilson and others 1999).

This is an important point to consider when attempting to restore desert grasslands to historic species

compositions.

Herbicide Application

Herbicides have also been employed to control the

spread of mesquite. In a 3-year study, aircraft applied

1.12 kg/ha of 3,6-dichloropicolinic acid (clopyralid)

(Jacoby and others 1981). Subsequent monitoring

over a 2-year period showed that this agent was

significantly more effective at killing mesquite than

2,4,5-T and picloram applied at the same rate, with

60 to 68 percent mortality. Jacoby and others (1981)

observed that clopyralid was not effective in killing

associated shrub and succulent plants such as saltbush (Atriplex sp.) or prickly pear (Opuntia sp.). In

another study, Jacoby and others (1982) determined

that dead mesquite serves to protect germinating

forage plants from grazing, allowing significant increases in forage production.

Subsequent studies have attempted to quantify the

effects of herbicide treatment based upon mesquite

morphology. Jacoby and others (1990b) determined

that mesquite with multiple stems were significantly

more resistant to mortality from a variety of herbicides including clopyralid, triclopyr, picloram, and

2,4,5-T. Though they determined that clopyralid was

the most effective herbicide, mortality decreased from

55 percent for single-stemmed trees, 35 percent for

three- to five-stemmed trees, and 19 percent for trees

with greater than five stems. These authors suggest

that the greater canopy coverage for multiple-stemmed

trees is more effective in preventing herbicides from

penetrating the soil under the canopies. In contrast,

they did not find a significant relationship between

tree height and herbicide-induced mortality (Jacoby

and others 1990a).

The epicuticular wax found on mesquite leaves can

minimize the effects of foliage-absorbed herbicides.

Mayeux and Wilkinson (1990) suggest that herbicides

applied in an oil spray are more effective than aqueous

sprays. Jacoby and others (1990c) found that leaf

epicuticular wax increased through the growing season, then stabilized or decreased in late summer.

These authors also found individual differences in the

amount of leaf epicuticular wax within populations

and suggest that over time these trees could represent

a greater portion of the population as the selective

pressure of foliage-absorbed herbicide applications

eliminated trees with less leaf epicuticular wax.

Jacoby and Ansley (1991) evaluated the effects of

herbicide applications during various seasons. They

found the most effective herbicide—clopyralid—induced the greatest mortality (92 percent) when applied during September. These authors suggested that

herbicide applications during the late summer are

most effective, and nearby agricultural areas are less

susceptible to treatment effects at that time.

One negative effect of herbicide treatment is the

potentially deleterious effects on the environment and

biodiversity within open rangeland. Nolte and

Fulbright (1997) monitored plant and animal biomass

for 2 years following treatment of mesquite with a

mixture of picloram and triclopyr. They found no

differences in vegetation, rodent, and avian species

richness between control plots and treatment areas,

although they noted that annual precipitation was 16

percent above average during this period, possibly

attenuating the deleterious effects of the herbicide.

This study and the ones previously mentioned (Jacoby

and others 1981, 1982, 1990a,b,c, 1991) were conducted within a 10-year period or less, and only monitored the effects of a single herbicide application. To

completely remove mesquite or at least limit its spread

in open rangeland using herbicides only, multiple

treatments are required; otherwise, the long-term

viability of mesquite seeds and their abundance within

the seed bank would ensure continual recruitment.

These multiple applications could create adverse

side effects to rangeland species diversity and biomass

not recognized from the results of a single treatment.

In a study that evaluated impacts of repeated herbicide applications on biodiversity, Gibbens and others

(1986) applied 2,4,5-T to Prosopis glandulosa within

dunelands in southern New Mexico during 3 consecutive years. They found mesquite mortality to be 54

percent, without specifying if any specific age class of

mesquite were particularly susceptible. These authors also determined that residual 2,4,5-T in the soil

dissipated rapidly, and was not detected 131 days

after treatment. While they found lower indices of

microbial activity on treated areas, they attributed

this to lower carbon availability as the mesquite trees

were defoliated. Overall, the authors found a decrease

in avian species diversity on treated areas but no

differences in arthropod or rodent diversity. Since this

was a 3-year study, no data were presented on the rate

of mesquite recruitment following the herbicide treatment. Although these results appear promising, mesquite mortality after 3 consecutive years of herbicide

applications accounted for just over half of the trees in

the study area. This implies that these treatments

would have to continue at a fairly regular interval for

many years to create a significant long-term reduction

in mesquite populations. With the attendant costs of

herbicides and aerial application over large areas, a

viable long-term management strategy using only

herbicides may be impractical.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 19

Mechanical Removal

Mesquite physical removal is another management

tactic. Roller chopping and shredding can reduce plant

density, stature, and canopy cover immediately following treatment, but subsequent basal resprouting

can create greater stem densities (Dodd and Holtz

1971). Other techniques include root plowing, chaining, and dozing. While these tactics may kill existing

mesquite, the woody debris is often left in place,

therefore impeding navigation through these areas

and limiting available space for livestock forage

(McPherson and Wright 1986). In addition, mechanical removal may negatively affect other plant and

animal species. For example, driving large mechanical

equipment through rangeland can cause soil compaction, crush animals, destroy animal burrows, and

uproot desirable plant species such as perennial

grasses. The extent of these effects remains unknown.

Mesquite Management

Objectives _____________________

This paper has mainly discussed the negative impacts of mesquite population expansion onto open

rangeland. However, mesquite wood is prized as a

material for furniture, and the wood is also used for

cooking, with an estimated value of $400/ton as

barbeque chips (Felker 1996). Some characteristics of

the wood, such as hardness and shrinkage, are considered equivalent or superior to mahogany, oak, and

walnut (Felker and others 1990), with lower rates of

expansion and contraction due to changes in ambient

humidity. While still nascent, the mesquite lumber

industry in Texas generates $5 to 10 million per year

(Felker 1996).

Mesquite seed pods are high in carbohydrates, providing a rich energy source for wildlife, livestock, and

humans (Felker 1998). Mesquite trees also provide

habitat for birds, small mammals, reptiles, and insects, increasing biodiversity and recreational opportunities (McPherson 1997). Therefore, completely removing mesquite trees from open rangeland may not

only be an unrealistic objective, it may actually lower

the profitability of this rangeland to land-use managers. For this reason, management objectives and their

implications should be clearly articulated when formulating a management strategy.

For example, if the management objective is the

complete removal of mesquite trees from open rangeland, the long-term commitment of resources to this

task might not be a realistic goal for many land-use

managers. Instead, a compromise may have to be

reached, whereby the density of mesquite stands are

reduced to the point that land-use activities such as

cattle ranching may be efficiently practiced, while

recognizing that these trees will continue to exist on

open rangeland. Periodic burning of this open rangeland may therefore not entirely remove mesquite, but

will lower its density to the point that cattle may find

sufficient forage, and ranchers may avoid navigating

within dense woodland. Overall, land-use managers

have a choice of three management objectives. They

are described as follows, with the specific procedure or

combination of procedures that may be most effective.

Objective 1: Complete Mesquite Removal

From Desert Grassland

This option is considered by those who wish to

restore desert grasslands to their historic (within the

past 150 years) species compositions. To completely

remove mesquite trees, two points need consideration.

First, one has to consider the prodigious commitment

of resources in terms of personnel, materials, time,

and expense. Second, complete removal of mesquite

does not necessarily result in the restoration of desert

grassland. If mesquite range expansion represents a

form of disturbance to the grassland plant community,

then efforts to remove it represent another. Exotic

plant species better adapted to various disturbance

regimes may have a competitive advantage over native grasses in exploiting this disturbance. For this

reason, a management strategy would have to include

continual monitoring and persistent removal of mesquite and other undesirable plant species. Where

possible, documentation of the rate and extent of

mesquite spread in the management area could help

determine the extent of management efforts necessary for removal. This documentation would include

dated aerial photographs and interviews with local

residents.

A suggested management tactic is a combination of

mechanical removal and/or herbicide treatment followed by prescribed burning. If prescribed burning is

scheduled after allowing the mechanically displaced

woody debris to dry sufficiently for combustion, then

this may allow the surviving mesquite to resprout.

These resprouts could then be killed back by the

subsequent fire. McPherson and Wright (1986) recommended that areas to be burned should be suspended

from livestock grazing for at least a previous growing

season so that herbaceous dry matter could accumulate. They also recommended conducting a small test

burn to verify the efficacy of a prescribed burn within

a given time period. Over time, periodic prescribed

fires could be used to destroy emerging mesquite

seedlings. The rate of mesquite reestablishment in

this area would depend on the size of the existing seed

bank, the density and distribution of mesquite stands

in adjacent areas, and the mechanisms of seed dispersal from those adjacent areas.
20 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001

To satisfy this objective, land-use managers could

identify and protect rangelands that are currently

devoid of mesquite, such as the desert grassland north

of Sonata in southeastern Arizona (fig. 1). Since the

seed bank in these areas would be largely devoid of

mesquite, potential seed dispersers would have to be

monitored and controlled. Due to the high rate of mesquite establishment in rangelands, these mesquite-free

areas are becoming more limited in size. Preventing

mesquite establishment is less difficult than removing

it, though this may be unrealistic in areas that contain

livestock.

Objective 2: Continued Mesquite

Establishment in Desert Grassland

This objective may be implemented by ranchers who

lack the resources necessary for range control or by

researchers who wish to study the current and future

rate and extent of mesquite establishment on open

rangelands. Because the N-fixing capability of mesquite can elevate soil N concentrations within the

rangeland, exotic plants that have higher N requirements could become established. Even a management

objective of nonintervention would have to include

continued monitoring of rangeland for potentially invasive species.

The implementation of any of the other management objectives will become more difficult with time as

the mesquite alter soil-nutrient concentrations and

create a long-lasting seed bank. Therefore, a longrange forecast of land use in these rangelands is

advisable before implementing this objective.

Objective 3: Limited Mesquite Removal

From Desert Grassland

This objective is probably the most feasible alternative for most land-use managers, including those who

have limited resources for rangeland control, and/or

those who wish to profit from the beneficial aspects of

limited mesquite establishment on rangeland. Prescribed fires combined with limited grazing and an

initial herbicide application are recommended management tactics in this case. Ansley and others (1996)

determined that while intense fires may kill all

aboveground portions of mesquite, basal resprouting

may quickly produce an impenetrable grove. These

authors suggested that lower intensity fires may kill

small mesquite while allowing larger individuals to

survive with intact upper canopies; this can create a

park effect with enhanced visibility beneath stand

canopies. Since the main stems of the larger mesquite

remain intact, resprouting would be minimized.

By minimizing multistemmed resprouting, the average stem diameter of mesquite stands would be

larger, increasing the commercial lumber value.

Ansley and others (1998) thinned sections of a dense

mesquite woodland from 300 to 80 trees/ha; within

5 years of thinning, trees within this section had

significantly greater basal stem diameters, canopy

diameters, and leaf area. Felker and others (1990)

have suggested that limited removal of mesquite from

rangeland may actually increase its value as lumber,

as stands below maximum density tend to produce

larger individuals with thicker main stems. They

predicted that a spacing of 10 m per tree would produce an optimum yield. Limited grazing intensity

would slow the rate of mesquite seed establishment

and would also allow grasses to retain viable populations between mesquite canopies.

The parklike stands resulting from periodic prescribed fires would become more aesthetically appealing, creating opportunities for recreation and tourism

while maintaining an increased biodiversity of wildlife. Ultimately, this land management system could

increase the commercial value of rangeland for ranchers; commercial utilization of these areas for mesquite

lumber and tourism and recreation would offset any

losses in revenue from limited livestock grazing. Since

this management tactic is relatively inexpensive to

implement (Teague and others 1997), it can be used

over a long period to ensure that these thinned mesquite stands remain stable in density.

Conclusions____________________

After Euro-American settlement in rangelands of

the Southwestern United States, mesquite moved

from predominantly riparian and ephemeral-channel

locations to adjacent xeric grasslands. The exact causes

for this range expansion are complex and may never be

fully determined. Although mesquite is native to the

geographic region, its expansion into more xeric habitats resembles that of exotic invasive species, and

warrants an inspection of its life history strategies, as

discussed in this paper.

Although many plant species lack common features

that determine whether they are invasive or potentially invasive, mesquite is uniquely qualified to occupy areas that are resource-limited to many other C3

plants. As C3 plants, they can utilize winter precipitation that is unused by C4 grasses, and their deep root

system ensures that they have access to soil moisture

unavailable to shallow-rooted grasses. Mesquite can

indirectly access atmospheric N, a resource that is

severely limited in xeric environments.

Two interdependent factors may account for increased mesquite distribution in open rangelands

within the last 150 years. Seed dispersal has increased

due to ingestion by livestock, and to a lesser extent by

rodents. Germination and establishment of seeds has
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-81. 2001 21

been enhanced by increases in winter precipitation,

particularly since about 1960, and decreases in interspecific competition due to livestock grazing. Ultimately, mesquite stands may increase until mesquites are only competing with each other for resources.

In the absence of a land-management strategy, intraspecific competition may be the main mechanism

limiting their densities in this habitat.

It is ironic that livestock grazing is one of the main

dispersal mechanisms for seed, while at the same time

increasing mesquite stand densities limits future grazing potential of this region. For continued use of these

areas for livestock grazing, a land-management strategy is therefore mandated. This strategy must contain

clearly defined objectives after determining how these

areas will be used.

Of the three management objectives offered, complete removal of mesquite from open rangelands would

be the most difficult to accomplish and to maintain. In

contrast, nonintervention would be the easiest to implement. However, this could have undesirable consequences as exotic plant species may become established, and the biodiversity of native plants become

diminished. Limited removal of mesquite from these

rangelands is potentially the most realistic objective

for most land use managers, as it may be relatively

cheap to implement (Teague and others 1997), while

actually increasing the commercial potential of this

landscape.

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