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The Influence Of Climate, Soil And Grazing
On the Distribution
Of Lehmann Lovegrass
Jerry R. Cox1 and George B. Ruyle 2
ABSTRACT
Overstocking of the range at the end of the nineteenth and
beginning of the twentieth centuries led to a decline in perennial
grasses. To replace those grasses, new species were introduced
from Africa and Asia. The purpose of this paper is to describe
factors which have contributed to the spread of Lehmann's lovegrass
and to discuss relationships between soils and climate which may
influence the long-term persistence of the grass in Arizona.
INTRODUCTION
Between 1860 and 1930, large numbers of cattle and sheep were stocked on southwestern
USA rangelands (Cox et al. 1983). Overstocking and drought caused a decline in perennial grasses
and a subsequent decrease in livestock numbers. Because rangeland productivity could not be
restored with native species, Lehmann love grass [Eragrostis lehmanniana (LL)], a perennial
warm-season bunch grass from southern Africa, was introduced to Arizona (Cox and Ruyle 1986).
In 1932, F. J. Crider, Director of the Boyce Thompson Southwestern Arboretum at Superior,
Arizona received LL seed collected in the Griqualand West Region of South Africa (Crider 1945).
Crider planted LL seed on the arboretum grounds and observed that seedlings produced seed heads
during the first year or growth. In 1935, Crider joined the USDA-Soil Conservation Service (currently
named the USDA-Natural Resources Conservation Service) and organized a series of screening tests
at the Plant Materials Nursery in Tucson. Of the many LL accessions tested, Crider selected one that
matured quickly and produced abundant seed under irrigation. The accession was named 11 A-68 11 ,
and the USDA-Natural Resources Conservation Service (NRCS) initiated a seed production program
in 1937.
Between 1937 and 1950, approximately 136 kg of LL seed were produced at Tucson and
distributed to soil conservationists and scientists within USDA-NRCS for field plantings. By 1940, LL
seed had been sown in Arizona, New Mexico, and Texas (Cox et al 1982). Between 1940 and
1950, the grass began to appear on areas which had not been seeded.
Early seeding successes sparked interest and the demand for LL exceeded what NRCS could
supply. Between 1951 and 1984, the USDA-NRCS provided 1,197 kg of seed to commercial seed
growers, and growers produced 75,069 kg (Cox and Ruyle 1986). Approximately 70% of the
1
2
Texas A&M, College Station TX.
School of Renewable Natural Resources, University of Arizona, Tucson AZ.
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commercially available seed was sown on rangeland and along highways in Arizona, New Mexico and
Texas. The majority of the remaining seed was transported to Mexico and planted in the northern
frontier states of Chihuahua, Coahuila, and Sonora (Cox et al. 1982). As more and more stands
were established, Cable (1971) predicted that LL wold spread naturally to adjacent rangelands.
With this natural spread of LL, it was suspected that a subsequent decline in native grasses
would occur. Both of these predictions have come to pass. Since about 1975, this scenario has
occurred on an alarmingly large acreage in southeastern Arizona.
DISTRIBUTION FACTORS
In 1986, we hypothesized that five major factors have significantly contributed to the spread of
LL in southeastern Arizona (Cox and Ruyle 1986). Since then, several studies have been published,
both supporting and rejecting our original tenets.
Of the direct factors contributing to the spread of LL, mechanical soil disturbance and sowing of
LL seed are perhaps the most obvious. These seedlings have provided the seed source and the
multiple loci largely responsible for setting the stage for LL invasion (Moody and Mack 1988,
McClaran and Anable 1992).
Approximately 69,000 ha of shrub land have been cleared and sown to LL in Arizona (Cox and
Ruyle 1986). Mechanical treatments were used to reduce shrub competition, increase water
infiltration and prepare a seedbed to enhance LL seed germination and seedling growth.
Mechanical soil disturbances due to highway, pipeline, and power line construction began to
accelerate after 1960 in southeastern Arizona. Because of soil erosion on disturbed areas the
Arizona Department of Transportation began to seed LL in 1965. The majority of the seeded area
(75%) was along Interstate 10 between Tucson and the New Mexico border, and along Interstate 19
between Tucson and No gales. Approximately 3,060 kg of LL seed have been sown along highways
in southeastern Arizona.
The total land area successfully sown to improve rangeland is more than 31 times greater than
that area sown along highway, pipeline and powerline right-of-ways because highway, pipeline and
powerline plantings established continuous corridors into rangelands, and traverse many
environmental gradients, whereas field plantings were made in rectangular shapes and were
confined to localized areas (Cox and Ruyle 1986). Additionally, vehicular use of these corridors
likely increases seed dispersal from existing stands into new areas (Robinett 1992a).
Seeding of LL continues, both along disturbed right-of-ways and on privately held rangelands for
forage production, although higher elevation seeding is discouraged. Seeding the exotic lovegrass
on public lands is no longer routinely approved. It was also previously suggested that, on suitable
sites, brush control without soil disturbance can influence LL invasion (Cox and Ruyle 1986). Cable
and Tschirley (1961) reduced mesquite (Prosopis juliflora) competition with a herbicide and aerially
applied LL seed to treated and untreated areas. LL production was nil in the treated and untreated
areas in fall 1954. LL production increased from 125 kg'ha in fall 1955 to 4 75 kglha in fall 1959
on treated areas, and from 20 kg'ha in fall 1955 to 210 kgtha in 1959 on untreated areas. Native
grass production during the same period varied from 350 to 910 kgtha on the treated, and from
130 to 7 40 kgtha on the untreated areas. Cable (1975) reevaluated the study 21 years post
treatment and found that LL had replaced native grasses on both treated and untreated areas. Cable
(1976) speculated that LL would continue to invade native grasslands at elevations between 1,200
and 1,500 m with or without chemical or mechanical brush control.
It appears that brush control is not the primary factor influencing LL movement onto a site. The
presence of native perennial grasses on the site and the availability of a nearby LL seed source
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interact with management and climatic influences to determine subsequent invasiveness of LL. Cox
and Ruyle (1986) also suggested several indirect factors that may contribute to LL invasion. Of these
factors, drought and fire appear to be most important.
Drought conditions severe enough to kill both native perennial grasses and LL periodically occur
in southeastern Arizona. Droughts of this magnitude occurred during the 1950's and 1960's.
Anecdotal accounts made subsequent to these drought cycles indicated a substantial decline in
both native grass and LL populations. When growing conditions improved, especially after the later
dry cycle, native grasses did not re-establish as quickly or completely as did LL. Studies to
document the influence of drought on LL are few, but mostly support this premise (Cable 1971,
Robinett 1992a and b).
LL plants growing on shallow soils are particularly susceptible to drought. About 90% of the
plant population on shallow soils died during a mid-summer drought in South Africa, but seedlings
quickly re-occupied the site when soil moisture improved (Fourie & Roberts 1977).
The loss of perennial grasses including LL was documented due to drought conditions in 1988
and 1989 in southern Arizona (Robinett 1992a and b). By the fall of 1990, the dead patches had
filled in completely with mature LL plants. Native species reduced were primarily black gram a
(Bouteloua eriopoda, and three awn (Aristida) species but also included hairy gram a and spruce top
gram a (B. hirsuta and B. chondrosioides). However, McClaran and Anable (1992) found consistent
increases in LL over time, on the Santa Rita Experimental Range, including a dry period during 1979
and 1980, perhaps indicating that drought did not hasten the spread of LL.
Natural or man-caused fires that occur prior to the summer growing-season, when the soil profile
is dry, are known to kill mature LL plants (Humphrey and Everson 1951, Cable 1965 and 1967).
However, plant populations do not decline because new seedlings from available seed, which
germinate during the summer growing-season, quickly replace dead plants (Cable 1965, Ruyle et al.
1988b). Where native grasses occur, fire creates bare areas that are quickly colonized by LL.
Since 1986, numerous studies have supported this claim. For example, Robinett (1994) found
that LL increases as fire frequency increased on certain range sites. The increase in LL was at the
expense of native grasses. Biedenbender and Roundy (1996) determined that LL seedling
establishment increased after burning as did studies by Ruyle et al. (1988b) and Summrall et al.
(1991). Increased fire-fuels in Lehmann love grass stands seem to have increased the fire on sites
formerly dominated by native grasses (Anable et al. 1992). Evidence exists that a fire-induced
positive feedback pattern may develop where heavier stands of LL lead to higher fire frequencies
leading to still heavier stands of LL (Anable et al. 1992).
Cattle grazing has also been suggested as a mechanism that enhances the spread of LL. In
pastures where LL occurs with native grasses, selective cattle grazing may favor the establishment
and spread of LL. Under conventional year-long grazing management, cattle prefer native grasses
during the summer growing season and lightly graze LL (Martin 1983, Ruyle et al 1988a). In
contrast, cattle utilize LL in fall, winter and spring because the foliage remains green longer than the
native grasses (Cable and Bohning 1959). This seasonal pattern of animal selectivity reduces
native-grass vigor, because plants are repeatedly grazed during active plant growth. Consequently, LL
may obtain a competitive advantage.
Further study only partially supports this assumption. McClaran and Anable (1992) have
demonstrated that the adventive spread of LL does not require livestock grazing. However,
increased grazing intensity increased the proportion of lovegrass in the total grass population, albeit
due to decreases in native grass densities and not from absolute increases in LL density with grazing
intensity. Additionally kangaroo rats (Dipolomys spp.) appear to be critical to increased LL stands in
Chihuahuan Desert shrub habitat (Brown and Heske 1990).
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RELATIONSHIPS BETWEEN SOILS, TEMPERATURE,
PRECIPITATION AND THE SPREAD OF LEHMANN LOVEGRASS
Surface soils at 33 sites where LL was successfully established in southeastern Arizona are sand
to sandy loams, and soil depth varies from 0 to 120 em (Cox and Ruyle 1986). At one-third of the
sites, LL has spread from the seeding site to surrounding rangeland. Where the plant spreads, the
sand to sandy loam surface soils are at least 15 em deep, the soil profile is greater than 50 em
deep and soils are well drained. Soils are classified as either Cornaro, Forest, Sonoita, Tubac, or
Whitehouse series (Soil Taxonomy 1975, Richmond 1976, Richardson et al. 1979.) Surface soils at
the remaining sites are sandy, but soil depth is either less than 6 em or greater than 30 em. At
these sites, LL has been slower to spread to surrounding rangeland.
Elevation, mean annual winter temperature, and mean summer precipitation for the 33 planting
sites was evaluated by Cox and Ruyle (1986). Elevation is less than 1,000 m at 6 sites, greater than
1,000, and less than 1,500 m at 21 sites, and greater than 1,500 at 6 sites. Annual winter
temperatures are less than 12 C at 8 sites, greater than 12, and less than 14 C at 21 sites, and
greater than 14 C at 4 sites. Total summer precipitation is less than 200 mm at 13 sites, greater
than 200, and less than 250 mm at 15 sites, and greater than 250 mm at 5 sites.
CONCLUSIONS
It is commonly believed that LL will continue to spread in the southwestern USA because the
plant is adapted to a wide range of climatic and soil conditions (Cox 1984, Cox and Martin 1984,
Martin and Cox 1984, Frasier et al. 1984 and 1987, Cox et al. 1990, Ruyle et al. 1988b, Roundy
et al. 1993). In southeastern Arizona, approximately 90% of the area where summer rainfall ranges
between 200 and 300 mm is currently occupied by LL. Because of shallow soils, dense shrub
stands, and low summer rainfall, we do not believe LL is likely to invade at elevations below 1,000
m, except in localized cases. We suspect that subsequent population increases will largely be
through increased stand densities and continued invasion of areas above 1,500 m.
There is little to indicate that LL will decline under natural successional processes where it has
dominated favored sites. None-the-less attempts to reduce LL stands and favor native perennial
grasses continue. Spring burning, moving or heavy grazing, herbicide treatments after the onset of
summer rains and LL seedling emergence, followed by seeding native grasses may be the best
suggestions to date to accomplish the above objective (Biedenbender and Roundy 1996).
As Anable et al. (1992) and Robinett (1992a) suggest, seed arrival appears to be a major
influence on LL invasion at elevations above 1,000 m in southern Arizona regardless of perturbation
levels or management practices. Where seed sources are available we believe that the primary
factors influencing continued invasion of LL are drought and a positive feed-back response to fire.
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