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Full text of "Preliminary notes on important vegetative species of Region 8 : plant-soil correlation contribution"

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Historic, Archive Document 

Do not assume content reflects current 
scientific knowledge, policies, or practices. 

Hugh Gr. Calkins 
Regional Conservator 


Regional Bulletin No. 23 
Soil Sorios No. 2 
May 2h, 1933 







Even L, Flory, Ecologist and D. F, Trussell, Assoc, Soil Scientist 

The range, once tho history of its vegetation is under- 
stood, can be held, by judicious management, in the most desirable 
st&ge of its development for maximum feed production and effective 
erosion control. Why the shrubby and weedy species invade an over- 
grazed range or why these species in turn give way to grasses under 
proper management, is clearly revealed by examination of the root 
systems of the various species under different degrees of usage. 

The rhizomes and finely divided roots of properly utilized 


native grasses so thoroughly occupy the upper soil mass that un- 
palatable shrubby and weedy species cannot invade or compete with 
them. Heavy overgrazing of palatable species, particularly during 
the growing season, will shorten and diminish their root systems 
and decrease the stand, creating a favorable condition for the en- 
croachment of invaders, A continuation of this process will mean 
the eventual displacement of grass by shrubs and weeds that are 
less palatable and less erosion resistant. 

Many depleted and overgrazed ranges may be restored to 
their former condition by judicious management, allowing the re- 
maining grasses a chance to regain their vigor and increase their 
density. The finely branched grass roots occupy the surface soil 
so thoroughly and absorb moisture so effectively that very little 
penetrates deeply. The deeply rooted shrubs and weedy species are 
quite unfitted to compete wi ih the grasses in the surface soil and 
are consequently killed out. 


Black grama ( Boutoloua eriopoda) is indigenous to the semi- 
desert grasslands of the Southwest. It occupies tha t part of these 
grasslands bordering the desert scrub formation. The zone occupied 
by this species has a higher precipitation, slightly lower tempera- 

This species occupies soils which show a relatively mature 
development. Weathering during the development of ihose soils has 
resulted in the greater proportion of the clays and colloidal ma- 
terial being leached to the depth of the average maximum penetration 
of moisture. This action has given rise to a heavy- textured soil 
horizon at this point. Ihis layer of soil, ("3" horizon), because 
of its heavy texture, is able to store large quantities of moisture. 
The overlying soil layer, because of the removal of the finer par- 
ticles, is relatively coarse- textured, open, friable, allows water 
to penetrate readily, and protects the subsoil from excessive loss 
of moisture. The heavy- textured M B M horizon under the climatic con- 
ditions where black grama occurs begins at depths of 5 "to 9 inches 
below the surface, depending upon the percentage and mobility of the 
clay resulting from disintegration of the parent material and also 
upon the precipitation. A large percentage of the rainfall comes in 
light showers which penetrate only a few inches. The high tempera- 
tures and high rate of evaporation in "this habitat will dissipate 
the moisture in a very few hours after the showers. Black grama has 
developed a root system (Figure 1) that can absorb much of this mois- 
ture before it is lost. A species, in order to thrive in such a 
habitat, must have numerous very finely branched roots close to the 


surface so that this moisture can quickly be taken up by the plant. 
Black grama is especially well equipped in this respect. In addi- 
tion to the shallow roots, black grama also has deeper penetrating 
roots which draw upon stored moisture to maintain its growth through 
the frequent sustained drought periods. Occasionally rains occur 
which penetrate to and moisten the heavier textured ,f B" horizon* 

Large areas once dominated by black grama have been largely 
denuded by the devitalizing effects of overgrazing. Black grama 
plants when properly utilized maintain their full vigor as shown by 
the plant to the left in Figure 1. Under such conditions the leaves 
are allowed to develop and remain oxposod to sunlight sufficiently 
long for the manufacture of food to extend and maintain a normal 
root system and develop viable seed. Black grama plants may be 
maintained in this condition by grazing in the dormant season, or 
by judicious grazing during "the growing season with provision made 
for an occasional rest from grazing during a growing season. Ranges 
where the grass still retains this degree of vigor are very rare and 
difficult to find. A range maintained in this condition will guar- 
antee the stockman sustained high yields as long as the vigor of 
the grass is maintained. The soil mass is so thoroughly occupied 
by finely divided grass roots that less soil will be removed by 
erosion than accumulates or forms. Weeds and other less desirable 
forage or erosion control species are not able to obtain a foothold, 

Tho center plant represents the condition of black grama 
plants on the average range in the Southwest. Iho number and extent 
of roots has been materially impaired by the consistent removal of 



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Fig. 1 - Response of black grama to different degrees of utilization. 

Fig. 2 - Common invaders on overgrazed semi-arid grasslands. 


leaves. An insufficient photo synthetic area remains exposed to the 
sunlight for maintaining the vigor of the plant and the root system 
is restricted to conform to the lessened food supply. Less thorough 
occupation of the surface soil and the destruction of some of the 
grass plants render the surface soil more vulnerable to the action 
of water which will now attain some velocity due to less obstruction 
by decreased amounts of leaves, stems and vegetative debris. Sheet 
erosion is evident in varying degrees on all ranges in this condition. 
Less desirable forage and erosion control invaders now have a chance 
to compote with the weakened grama grass and they continue to assume 
a more dominant role as 1he stock do not weaken them by grazing but 
continue to overgraze the grama. 

The root systems of three of the most common invaders of 
overgrazed black grama ranges are shown in Figure 2. It can be read- 
ily seen why the deeper, coarser, less finely branched roots of the 
invaders are much less effective in erosion control than black grama. 

It is fbrtunate, however, that the root systems of the in- 
vaders on overgrazed black grama ranges are of such a character or 
they would retain complete possession of the habitat once they es- 
tablished dominance. When black grama is givon an opportunity to 
regain its vigor and again approach its former density in overgrazed 
areas, the extensive occupation of the surface few inches of soil by 
finely branchod grama roots shuts off the moisture of all except 
heavy rains from the deeper roots of the invaders. When a severe 
drought occurs the invaders are not able to compote with the more 
efficient absorbing system of black grama and the invaders are kill- 
ed. Under the arid conditions where black grama is found, the above 

restoration of a good black grama range free from the less desirablo 
species must necessarily involve a period of several years. The 
period of time vail depend largely upon the soil type, degree of 
depletion and climatic conditions. 

Black grama in the condition shown by the middle plant is 
still able to maintain a fair stand where a relatively heavy textured 
subsoil lies within roach of most of the deeper penetrating roots, 
but the grass stand will be materially diminished if the upper limits 
of the heavier textured subsoil is doeper than 8 or 10 inches* Where 
the soil is relatively coarse textured throughout or the heavier 
textured subsoil lies deeper than the above limits, there is not 
sufficient water storage capacity wi thin the absorbing zone of the 
restricted root system to maintain the grama through protracted 
drought periods. Many such areas formerly covered by a heavy grama 
stand are now barren and outside of local hcresay, the only evidence 
we have of former conditions is the numerous root relics still 
readily perceptible in the subsoil* 

In many areas these root relics not only indicate the former 
presence of black grama, but also its abundance and the depth to which 
roots extended. In areas where black grama still persists in this 
overgrazed and weakened condition, root relics not only indicate the 
former extent and penetration of the roots but also the density of 
stand. The plant to the right in Figure 1 represents the condition 
black grama may be reduced to when very heavily overgrazed through 
successive growing seasons. Black grama will remain alive in such a 
weakened condition only during a period of adequate rainfall or in 
those locations whore a fino textured subsoil exists within a depth 
of U to 6 inches from the surface. When a drought occurs there is 
not sufficient storage of moisture within reach of the weakened and 

shortened root systens to keep the grass alive if a coarse soil ex- 
tends below the above limits, This phenomenon may be observed in many 
locations, but the most outstanding occurrance of this sort was in 
the vicinity of Bonita, Arizona, In the severe drought of 193h> sev«- 
eral hundred acres of black and blue grama grass died out completely. 

Grazing history of the area reveals that the grass has been 
heavily utilized throughout the growing season for many years, The 
constant removal of green foliage before sufficient food could be 
manufactured to sustain the vigor of the plants has resulted in a 
gradual weakening of the plants and a restriction of the root system 
to conform to the diminished food supply until the general feeding 
and absorbing level is about 6 inches deep as shown by the plants to 
the right in Figures 1 and 3» Where the heavy textured subsoil was 
only 2 to If. inches below the surface, sufficient moisture was stored 
within reach of the restricted grass roots to maintain the plants 
through the drought period. TiVhere the heavy textured subsoil was more 
than 6 inches below the surface the grass was all killed. 

Depletion of this range has taken place in a relatively short 
time and species which commonly invade with overgrazing have not had 
time to establish themselves since the vigor and density of the grasses 
have been diminished. The area where the grass has die/i is without a 
perennial cover with the exception of scattered individuals of two 
species of three awn (Aristida). Substantial reductions in grass den- 
sities during pronounced dry periods throughout the Southwest have 
doubtless boon due to the above cause. In most cases, however, weedy 
and shrubby invaders have gradually constituted a higher proportion of 
the total vegetative density as the grasses were weakened by selective 
grazing until the grasses largely disappeared in a drought after which 
the invader dominated the habitat. 


Blue grama ( Bouteloua graoillis ) occupies that part of the 
semi-desert grasslands which has a higher precipitation, slightly low- 
er temperature and a lower rate of evaporation than the part occupied 
by black grama. In the broad contact zone where the two species occur 
together, blue grama will be found on the more mature soils, north 
slopes, depressions and -those places which have the more favorable 
soil moisture conditions. 

Blue grama in vigorous condition (plant to left in Figure 3) 
has a general feeding level of about 18 inches. The major portion of 
its root system, however, is confined to the upper 6 or 8 inches of 
soil* It is indigenous, therefore, to these locations where the major 
portion of soil moisture lies within theso limits. Blue grama is very 
drought resistant and has the ability to remain dormant in what would 
normally bo a growing season if sufficient moisture is not available 
for the resumption of growth. During prolonged drought periods the 
deeper penetrating roots absorb enough moisture to sustain life. 

Blue grama makes its growth in a period of 60 to 70 days after 
the advent of the summer rains, which is usually about the middle of 
July. 3he forage matures well on the ground and is excellent feed 
throughout the dormant period. Seed produced is usually of question- 
able viability except in infrequent years when moisture conditions are 
especially favorable and then only when plants are in a vigorous con- 
diti on. 

On those areas where blue grama is best suited to soil and 
other factors of tho habitat and when grazed in a manner to insure the 
vigor of its plants blue grama will produce heavy yields of forage, 



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Fig. 3 ™ Response of blue grama to different degrees of 


protect the surface from erosion by a dense vegetative cover and so 
completely utilize -the factors of the habitat that other species "will 
not be able to invade or compete with it. 

Blue grama, because of its high palatability and occurrance 
in readily accessible locations, has been generally very heavily grazed. 
!Ihe condition of blue grama plants as found under general grazing con- 
ditions of this region is shown by the center plant of Figure 3» In 
this condition and in the still further weakened condition as shown to 
the right, deeper rooted invaders and weeds easily obtain a foothold* 

The lower ranges of blue grama with lighter rainfall and those 
characterized by the major portion of the annual precipitation occur- 
ring within the growing season have been heavily invaded by the less 
desirable erosion control (Figure 2) and non-palatable, snakeweed, 
three awn, rabbit brush and annual weeds. The overgrazed blue grama 
on higher elevations where the annual precipitation is distributed 




over the dormant as well as the growing seasons has been heavily in- 
vaded by sagebrush. In many of the most seriously overgrazed grama 
ranges, sagebrush has almost completely replaced grama* 

3lue grama in the very weakened condition as portrayed by the 
plant to the right in Figure 3* nay still be alive on some areas where 
rainfall has continued to be adequate. Grama kept in such a depleted 
condition by continuous overgrazing cannot withstand a serious drought. 
The grass cover has been known to die out in a single dry season when 
weakened to this extent by heavy continuous overgrazing. On some of 
the more favorable grass soils blue grama has been able to live through 
extreme droughts even with its root systems reduced to this extent. 
This will happen when the surface is permeable so as to absorb a max- 
imum amount of water and a heavy textured subsoil capable of holding 
large amounts of water comes to within a few inches of the surface. 

An examination of -the blue grama root systems as they occur 
under 1he different degrees of utilization makes it apparent that the 
properly utilized plant is much more effective in keeping the soil in 
place as it is knit very closely together by the numerous finely 
divided roots that come to within less than an inch of the surface. 

Numerous experiments have shown that the properly utilized 
plant produces as much as seven times more foliage than very heavily 
utilized ones. The reason for this is readily apparent when the re- 
stricted leaf area of heavily grazed plants for the manufacture of 
food, and the fractional soil mass which their diminished root systems 
can draw upon for moisture and nutrients as compared to vigorous 
plants is considered. 




GALLETA (hiiaria jamssii) 

Gallet a (Hi iaria jamesii ) is the dominant species on the plains 
of northwestern New Mexico where it often forms practically pure stands 
many miles in extent. It is abundant over extensive areas in the upper 
plains and in the lower limits of the ponderosa pine belt at eleva- 
tions of 5*000 to 7*000 feet in Arizona and New Mexico. It occurs 
chiefly in scattered stands in Utah and Colorado on mesas, plains and 
deserts, usually in association with blue grama in sagebrush* It at- 
tains its major development on the heavier textured soils which are 
calcareous in the surface or slightl y saline. The species is assoc- 
iated with shale outcrops and shale derived soils v/hich are slightly 
saline. Where the alkali content increases or the leached surface is 
removed by erosion, the species gives way to alkali sacaton and salt- 

1/tfhere the vigor of -the plants has not boon impaired by heavy 
overgrazing, the tough, woody rootstalks are sometimes as long as 6 
feet. The grass is so heavily overgrazed in most instances that the 
vigor of the plants is lessened to such an extent thr.t. -the rootstalks 
do not develop beyond a few inches in length and the grass is restrict- 
ed to a bunchlike form. The strong, scaly rootstalks and stems which 
are very numerous in the surface few inches of soil are more effective 
in the prevention of erosion than the roots which branch off the root- 
stalks and generally extend downward from a slightly oblique to a 
vertical dir ection. 

The tough rootstalks constitute the best protection of the 
species against trampling and overgrazing, but the root systems are 
shortened and restricted as in the gramas by continuous removal of the 
foliage. The average condition of the range is represented by the 
center plant of Figure Former condition of the grass plants is