Download Ecotones Between Artemisia nova and Plant Communities in the A. tridentata

Ecotones Between Artemisia nova and
A. tridentata Plant Communities in the
Buckskin Mountains of Western Nevada
James A. Young
Charlie C. Clements
Abstract—The habitat restrictions between the larger woody sagebrush (Artemisia) species and the lower growing species have long
intrigued ecologists in the Great Basin. In the Buckskin Mountains
of west-central Nevada, ecotones between big sagebrush (A. tridentata) and black sagebrush (A. nova) communities are extremely
abrupt and distinct, with no intermixing of the two types. These
differences are conditioned by different soils and are reversible with
changing edaphic development through erosion or deposition.
The relative distribution of the taller, woody sagebrush
(Artemisia) species in comparison to the shorter or dwarf
woody species has long intrigued plant ecologists. As typified by big sagebrush (A. tridentata), the taller woody species
of sagebrush are often landscape characterizing in their
distribution. The shorter or dwarf woody species usually
have more limited distribution and are often associated with
specific edaphic situations (Zamora and Tueller 1973). It
was proposed by the soil scientist Harry Summerfield, that
the dwarf sagebrush species low (A. arbuscula) and black
sagebrush (A. nova) often occupied sites where the surface
soil horizons had been removed by erosion on a recent
geologic time scale (Young and others 1996). The woody
sagebrush species occupy such a variety of habitats over
such a large and climatically diverse geographical area,
there are many exceptions to this hypothesis, but it provides
an innovative approach for considering the ecotones between black and big sagebrush communities.
Our purpose was to investigate ecotones between big and
black sagebrush to provide insight into the distribution of
the two species in the Buckskin Mountains of west-central
Nevada.
Study Area _____________________
The Buckskin Mountains are located 100 km southeast of
Reno, Nevada in the rain shadow of the Sierra Nevada and
the Pine Nut Mountains (Morrison 1964). This is the portion
of the Carson Desert in which Billings (1945) suggested that
shadscale (Atriplex confertifolia)-dominated salt desert
shrub communities occurred because of atmospheric
In: McArthur, E. Durant; Ostler, W. Kent; Wambolt, Carl L., comps. 1999.
Proceedings: shrubland ecotones; 1998 August 12–14; Ephraim, UT. Proc.
RMRS-P-11. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Rocky Mountain Research Station.
James A. Young is a range scientist and Charlie D. Clements a wildlife
biologist for U.S. Department of Agriculture, Agricultural Research ServIce,
920 Valley Road, Reno, NV 89512.
USDA Forest Service Proceedings RMRS-P-11. 1999
drought rather than the concentration of soluble salts in the
soil.
The Buckskin Mountains are a very structurally complex
fault-block mountain range typical of the Great Basin
(Hudson and Oriel 1979). The bulk of the range is composed
of repeated flows of volcanic tephra. The incandescent volcanic ash flows moved across the landscape like water as they
were deposited. The heat from each new flow converted the
surface of previous landscapes to an indurate tuff (Proffett
and Proffett 1976). These hardened strata tend to form a
series of cliffs above eroding talus slopes.
The landscape of the Buckskin Mountains has existed for
a sufficient period of time for considerable soil development
to have occurred (Blank and others 1996). On north facing
slopes, clay-textured soils occur at the soil surface. Apparently, previously existing surface horizons have eroded over
geologic time. This same relationship is apparent on many
older alluvial fans that spill from the mountain slopes and
canyons. The areas with clay-textured surface soils support
black sagebrush plant communities (Young and Palmquist
1992).
South facing slopes have soils with no horizonation or only
color changes indicating weak horizonation. Shadscalehorsebrush (Tetradymia glabrata)/desert needlegrass
(Achnatherum speciosum) plant communities form the vegetation on the south slopes. Big sagebrush communities do
not occur on the residual soils of the Buckskin Mountains.
Erosion and Deposition __________
Degrading System
On the north end of the Buckskin Mountains, a fairly
substantial drainage has eroded into the stony clay-textured
soils of an alluvial fan complex. Under current climatic
conditions, the drainage would only have running surface
flow for brief periods immediately after exceptional high
intensity short-duration summer thunderstorms. Sometime
in the recent geologic past, there was sufficient flow to erode
a flat bottomed arroyo 3 m deep into the clay. The width of
the arroyo averages 15 m. The erosion surface is partially
filled with irregularly stratified erosional products reflecting the mixed volcanic make up of the watershed. The fill
ranges in texture from sand to substantially sized boulders.
There is no evidence of soil development.
The ecotone between the black sagebrush communities of
the clay textured fans and the big sagebrush community of
the arroyo fill is very striking because of the difference in
stature of the two sagebrush species. The ecotone between
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the two communities is absolutely abrupt, reflecting the
soils change. Most of the shrub species in the arroyo community are at least as tall as big sagebrush. Antelope bitterbrush (Purshia tridentata), gray rabbitbrush (Chrysothamnus nauseosus), and true sage (Salvia dorrii) occur
with big sagebrush in the arroyo.
It is obvious from the ecotone between the two communities that black sagebrush in the Buckskin Mountains is
exclusively adapted to the clay-textured soils where it is the
only woody species except for a diminutive form of green
rabbitbrush (Chrysothamnus viscidiflorus), occasional plants
of Nevada ephedra (Ephedra nevadennsis), and horsebrush
(Young and Palmquist 1992).
Extensive excavation, with mechanical equipment to the
bottom of the arroyo fill, failed to provide any evidence of
prolonged subsurface flow of moisture in the fill. The sandtextured nature of the fine material in the fill certainly offers
contrasting moisture retention characteristics compared to
the predominantly clay-textured soils of the older fans.
Mutual Exclusion of Artemisia _____
The bulk of the Buckskin Mountain range is tilting to the
west (Hudson and Oriel 1979). The mouths of the canyons on
the east side of the range (long axis running north and south)
spill alluvium on the top of old fans rather than eroding
incised arroyos into the old surfaces. The broad basin on the
eastern flank of the range, known as Lincoln Flat, supports
both big and black sagebrush plant communities. The black
sagebrush communities occur on the erosional surfaces with
clay-textured surface horizons. The big sagebrush communities, without antelope bitterbrush or true sage, occur on
the recent alluvial surfaces.
Fulstone Canyon spills on to Lincoln Flat along the northeastern portion of the Buckskin Range. It has a very large
watershed, made larger by the capture of several adjoining
canyons along a fault strike that parallels the main axis of
the range. At the mouth of the canyon, there is an area of
several hectares where coarse-textured recent alluvium has
buried older erosional surfaces with black sagebrush communities. There is a succession of big sagebrush communities from seedling dominance to maturing plants on this
deposition area. There are no black sagebrush plants, seedling or adult, in these recent deposition areas despite the
clay-textured horizon being present under moderately or
thinly bedded recent erosional products.
Big sagebrush plants thrive on the clay-textured soils as
long as there is a surface horizon of coarser-textured material. This suggests that seedbed quality may be the factor
excluding big sagebrush from the black sagebrush communities. In the Buckskin Mountains we know very little about
the seedling ecology of black sagebrush plants, because
seedlings are very rare in natural communities (Young and
Palmquist 1992). The black sagebrush plants grow on
mounds, which is a common feature for many shrubs in the
Great Basin (Charley and West 1975) . Young and Palmquist
(1992) suggested, based on a very small sample, that in the
Buckskin Mountains black sagebrush seedlings establish in
the interspace between mounds and new mounds build
around the seedlings. Obviously, because of litter deposition
and nutrient cycling, the old mounds would appear to be the
most desirable seedbed in the black sagebrush communities,
but when black sagebrush plants become senescent and die
the mound becomes densely colonized by the native perennial grass squirreltail (Elymus elymoides) and by the exotic
annual cheatgrass (Bromus tectorum). These herbaceous
species preempt the environmental potential released by the
death of the shrub, making it very difficult for black sagebrush to become established on the mound. Eventually the
herbaceous species exhaust the nitrogen-enriched soils of
the old mound and the mound erodes.
Considering the great linear extent of the ecotones between big and black sagebrush it would seem logical that
occasionally plants of one species would establish in the
community dominated by the other species. This seems
never to happen in the Buckskin Mountains. Big sagebrush
plants would be highly visible in black sagebrush communities. This near total lack of the dominant species crossing of
the soils ecotones suggests the seedbed potential of the
contrasting soils and the inherent potential of the seeds of
the species must be mutually exclusive. The environmental
parameters that control establishment and persistence of
woody sagebrush species on an extensive array of environmental potentials is an intriguing subject for further study
in the Intermountain Area.
In the case of the sagebrush communities in the Buckskin
Mountains, the hypothesis that erosion of surface soil horizons is related to dominance of dwarf species of sagebrush
may have merit and deserves investigation in other areas of
the Great Basin.
Lack of Hybridization ____________
References _____________________
Artemisia and Chrysothamnus are the two woody genera
with species common to both communities across the abrupt
ecotonal boundary. They occur exclusively as distinct species with no evidence of hybrid swarms. Perhaps the environmental gradient is so abrupt and occurs over such a short
distance there is no environmental potential suitable for
intermediate forms even if the genetic make up and breeding
system of the species permitted hybridization.
Billings, W. D. 1945. The plant associations of the Carson desert
region, western Nevada, BuIter Univer. Botanical Studies. 7:
89-123.
Blank, R. R.; Young, J. A.; Lugaski, T. 1996. Podogenesis on talus
slopes, the Buckskin Range, Nevada, USA. Eeoderma. 71:
121-142.
Charley, J. L.; West, N. E. 1975. Plant-induced soil chemical
patterns in some shrub-dominated semi-desert ecosystems of
Utah. J. Plant Ecology. 63: 945-964.
Agrading System
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USDA Forest Service Proceedings RMRS-P-11. 1999
Hudson, D. M.; Oriel, W. M. 1979. Geologic map of the Buckskin
Mountain Range, Nevada. Map 64. Nevada Bureau of Mines,
Reno,NV.
Morrison, R. B. 1964. Lake Lahonton: Geology of southern Carson
desert, Nevada. Prof. Paper 401. Geological Survey, Washington,
DC. 477 p.
Proffett, J. M., Jr.; Proffett, B. H. 1976. Stratigraphy of the Tertiary
ash flow tuffs in the Yerington district, Nevada. Report 27.
Nevada Bureau of Mines, Reno, NV. 64 p.
USDA Forest Service Proceedings RMRS-P-11. 1999
Young, J. A.; Palmquist, D. E. 1992. Plant age/size distribution in
black sagebrush (Artemisia nova): effects on community structure. Great Basin Naturalist. 52:313-320.
Young, J. A., Longland, W. S.; Blank, R. R. 1996. Management of
woody species of Artemisia in western North America. In: Caligari,
P. D. S.; Hind, D. J. N. (eds.) Compositae: Biology and Utilization.
Proc. International Compositae Conference, Kew. Kew, UK.
Royal Botanical Gardens: 277-290.
Zamora, B. T.; Tueller, P. T. 1973. Artemisia arbuscula, A. longiloba,
and A. nova habitat types in northern Nevada. Great Basin
Naturalist. 33: 225-242
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