WILDLAND SHRUB DIEOFFS
FOLLOWING EXCESSIVELY
WET PERIODS: A SYNTHESIS
Arthur Wallace
David L. Nelson
ABSTRACT
SEQUENTIAL ADDITIVITY AND
SYNERGISMS
Many studies have been made of the recent shrub dieoffs
in the western United States following successive wet years.
Major implicated factors are increased soil salinity, anaerobiosis in the root zone, and soilborne disease-inducing organisms. Several other possible contributing factors are
reported. It is further suggested that plants in desert ecosystems are already under the usual stresses associated
with desert climates. Added to these stresses are several
more associated with excess water such as the possible increased soil bicarbonate that induces iron deficiency, the
parasite dodder, and loss of drought tolerance because of
the wet years. Seventeen identified stress factors operating
together under sequential additivity could decrease plant
vigor to very low levels. If any of the new stresses were synergistic in nature, the result could easily be death of the
shrubs.
In the 1987 shrub symposium, Wallace (1989) discussed
how various stress factors interact; many stress factors
acting together result in fragile and delicate ecosystems.
A desert plant ecosystem is a major example of one that
is already under many continuous stresses without added
ones because of continuous high-precipitation periods on
vegetation adjusted to dry conditions. To the stresses suggested in the papers of the two symposia must be added
those generally associated with cold deserts, such as summer heat, winter cold, wind, animals, lime soil, sodic soil,
irregular precipitation, salt, low fertility, and very little
soil organic matter, hence poor soil structure. The shrub
dieoff phenomenon then is preceded by several harsh factors nearly always present that already cause stress.
Researchers' reports in the two symposia have not covered all possible stresses caused by the wet years. Incidentally, there was not an extensive shrub dieoff at the
Nevada Test Site around 1984-85 as in other places, even
though 1983-84 was very wet compared with the average
years (Romney and others 1989). When A. Wallace started
his work at the Nevada Test Site in the middle of the
1960's, there was talk of recent widespread dieoff, particularly of Atriplex canescens, and past investigations revealed some interesting phenomena. Following the highrainfall year of 1978, we observed considerable shrub
dieoff, possibly because of a dodder (Cuscuta nevadensis)
infestation (Wallace and others 1980). Secondary causes
could be involved. We observe a continuous dieoff at
the Nevada Test Site because of gophers (Hunter and
others 1980a). Perhaps the most important herbivore
at the Nevada Test Site is the rabbit (Hunter and others
1980a,b,c). It is not known how mammal activities interact with several years of high precipitation.
A possibly very important parameter in shrub dieoff
could be the increased soil bicarbonate brought into solution by the high soil-moisture condition. Although the
desert plants obviously have considerable resistance
to lime-induced (bicarbonate-induced) iron chlorosis
(Coulombe and others 1984; Nelson and Jolley 1989),
those most susceptible to "high-water" shrub dieoffmay
be subject to high bicarbonate chlorosis. Resistance to
iron deficiency in dicotyledonous plants is usually associated with the production of hydrogen ions by plant roots.
A high bicarbonate level can completely swamp the capacity ofroots to produce hydrogen ions so that the dieoffmay
at least in part be associated with iron deficiency.
INTRODUCTION
After experiencing successive years of above-average precipitation (1977-86), in many western areas there was considerable dieoff of many species of native shrubs, especially
shadscale. A very detailed description of the problem was
given at the 1987 shrub symposium in Logan, UT (Nelson
and others 1989).
At the 1989 symposium in Las Vegas, NV, the problem
was addressed from many points of view: salinity, excess
soil moisture and high water table, anaerobiosis, cold hardiness, plant disease, insects, competition from cheatgrass
(Bromus tectorum), leaky roots, grazing pressure (these proceedings: Dobrowolski and Ewing; Harper and Wagstaff;
Haws and others; Nelson, C. R. and others; Nelson, D. L.
and others; Walser and others; Weber and ethers).
Perhaps the same general conclusions held in 1989 as in
1987: increased salinity, anaerobioses, and soilborne microorganisms, which can cause plant disease, were the factors
primarily related with others having an effect, especially
on predisposition to disease development.
Paper invited as a followup to the Symposium on Cheatgrass Invasion,
Shrub Die-Off, and Other Aspects of Shrub Biology and Management, Las
Vegas, NV, April 5-7, 1989.
Arthur Wallace is Research Plant Physiologist, Laboratory of Biomedical and Environmental Sciences, University of California at Los Angeles,
900 Veteran Avenue, Los Angeles, CA 90024-1786, operated for the U.S.
Department of Energy by the University of California under contract No.
DE-AC03-76-SF00012. David L. Nelson is Plant Pathologist, Intermountain Research Station, Forest Service, U.S. Department of Agriculture,
Shrub Sciences Laboratory, Provo, UT 84606.
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This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
During successive wet years, plants may become adjusted to mesic rather than to xeric conditions. When desert plants are cultured in the glasshouse with abundant
water, they must be "hardened" before they "behave"
like desert plants. Field dieoffmay occur on plants that
have not experienced a hardening process when the lowmoisture environment of deserts resumes.
In the 1984-85 dieoff situation we may have the usual
stresses ofheat, cold, wind, animals, salinity, poor-nutrient
soil, poorly structured soil, and then additionally, more
salinity, pathogens, insects, root anaerobiosis, high-water
table, and other factors such as competition from invading
plant species, more nematodes, dodder, high soil bicarbonate, and loss of adjustment to drought. Listed are 17 possible stress factors, although all may not occur simultaneously. The relative value of each stress is not known, but
supposing that each has the capacity to decrease plant
vigor by 10 percent, all17 combined would decrease it
by 83 percent if all operated on the principle of sequential
additivity (Wallace 1989a; 1990a,b). If, however, synergistic factors were included, the decrease in plant vigor could
easily drop to zero or to death. Certainly anaerobiosis is
synergistic with other factors.
There are several examples in the biological world that
are similar to the shrub dieoff (Wallace 1989, 1990b).
Many years ago in Utah there were lawsuits over whether
or not sheep had been killed by radiation from one of the
nuclear tests conducted in Nevada by the Atomic Energy
Commission in the 1950's. Documents showed that the
test was conducted in a low rainfall year, sheep were near
starvation, had been grazed on halogeton (a toxic plant),
and were obviously in weak condition when exposed to the
fallout (Life Magazine 1980). Had the sheep been healthy,
the radiation would probably have had little effect.
IRREGULAR RAINFALL
A member of Wallace's research team, E. M. Romney,
has observed while making several trips each year since
1955 to work at the Nevada Test Site, an interesting
pattern of stem dieback on Larrea tridentata shrubs in
the northern Mojave Desert. The route of travel involves
Interstate Highway I-15 from Los Angeles to Las Vegas
and U.S. Highway 95 from Las Vegas to Mercury.
Seasonal precipitation from storms moving from west to
east across this observation area is influenced by El Nino
weather patterns such that two to three successive wet
years and two to three successive dry years occur at
roughly 10- to 12-year intervals. During the dry side of
the cycle, the precipitation pattern varies such that some
rainfall is'laid down in west- to east-oriented strips varying from 10 to 30 miles in width with gaps in between of
similar width wherein little or no rainfall occurs. Larrea
tridentata shrubs in these dry strips undergo severe moisture stress over extended periods (9 to 12 months) resulting in virtual defoliation of the normally evergreen plants.
Mter this occurs, the shrubs do not produce new foliage on
many of the old standing branches, but rather produce new
shoots from crown sprouting. The final result of El Nino
is a 60 to 80 percent stem dieback, and the dead stemwood
remains standing within the shrub biomass for several
years, since its decomposition is a slow process in the desert environment.
When we first began investigating the cause of the
extensive dead biomass in L. tridentata shrubs at the
Nevada Test Site, it was presumed to be an insect or parasite problem. Indeed, we saw some evidence of this, especially during years of higher rainfall (Wallace and others
1972), but we now realize that most of the dieback phenomenon resulted from defoliation and death of branches
when they are subjected to long periods of intense moisture
stress.
EL NINOS, PREDISPOSITION, AND
PLANT DISEASE
Plant disease is unique among the 17 mostly physical
stress factors considered here. Plants generally recover
when physical stress ceases. Biotic or parasitic plant
disease (an injurious physiological process) is genetic
and epidemic in nature and generally results in death
once the disease is well in progress. Prolonged abiotic
stress can also induce the disease process, but the plant
can recover if the deficiency or toxicity ends.
Plant disease induced by biotic agents involves an incredibly intricate and complex interaction between pathogen, host, and their environment. The genetics of a
parasitic pathogen imparts a characteristic epidemiology,
pathogenicit';, and virulence. When confronted by biotic
pathogenic agents, a host plant counters, depending on its
genetic characteristics, with a varied array of resistance,
tolerance, and escape mechanisms. Environment, both
physical and biological, acting upon the host, when favorable, can result in a vigorous health that is an advantageous state against facultative parasites (those inducing
rots, wilts, blights), but a healthy state is conducive to
obligate parasites (those inducing rusts, smuts, mildews).
When environment is stressful, poor health can result,
thereby possibly predisposing the plant to disease development induced by facultative parasites. The environment,
again both physical and biological, acting upon the pathogen determines the sequential development of an epidemic.
Parasite population dynamics pulses in response to biological factors of the environment such as antagonisms from
other microorganisms, host phenology, host predisposition
from various stress factors, host/nonhost species diversity,
host population density, and host genetic uniformity in
susceptibility as well as favorability of the physical environment and the duration thereof(moisture, temperature,
wind, humidity) to propagule dormancy survival, germination, infection of the host, vegetative multiplication, sexual
sporulation, and propagule dispersal.
Based on the symptomatology and other evidence presented in these symposium proceedings, the sequential
additivity and synergisms relevant to plant disease and
the extensive shadscale dieoff would be in the arena of
predisposition to disease development. TheEl Ninos of
1982 to 1983 and 1986 with the resulting extended high
precipitation over much of the Great Basin could have
resulted in an abrupt increase in the population of certain
fungi such as water molds that can cause small rootlet
mortality. This predisposition, of biological origin, could
lead to death from vascular wilt and root rot induced by
more virulent fungi. Thus, a fluctuating yet balanced
Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Research Station: 119-135.
Romney, Evan M.; Wallace, Arthur; Hunter, Richard B.
1989. Pulse establishment of woody shrubs on denuded
Mojave Desert land. In: Wallace, Arthur; McArthur,
E. Durant; Haferkamp, Marshall R., compilers.
Proceedings-symposium on shrub ecophysiology
and biotechnology; 1987 June 30..July 2; Logan, UT.
Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Research Station: 54-57.
Wallace, Arthur. 1989. Interaction of stresses in the behavior of native shrub populations. In: Wallace, Arthur;
McArthur, E. Durant; Haferkamp, Marshall R., compilers. Proceedings-symposium on shrub ecophysiology
and biotechnology; 1987 June 30-July 2; Logan, UT.
Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Research Station: 144-146.
Wallace, Arthur. 1990a. [In press]. Crop improvement
through multidisciplinary approaches to different
types of stresses-Law of the Maximum. Journal of
Plant Nutrition. 12.
Wallace, Arthur. 1990b. [In press]. Interactions of two
parameters in crop production and in general biology:
sequential additivity, synergism, antagonism. Journal
of Plant Nutrition. 12.
Wallace, Arthur; Romney, Evan M. 1972. Radioecology
and ecophysiology of desert plants at the Nevada Test
Site. TID-25954. Washington, DC: Atomic Energy Commission, Biology and Medicine. 439 p.
Wallace, Arthur; Romney, Evan M.; Hunter, Richard B.
1980. Regulative effect of dodder (Cuscuta neuadensis
JTN .) on the vegetation of the northern Mojave Desert.
Great Basin Naturalist Memoirs. 4:98-99.
endemic plant disease triangle or system (host-pathogenenvironment) could undergo a fluctuation of high amplitude (epidemic) in response to the abrupt changes caused
by the El Nino effect on plant ecosystems in semiarid
climatic zones.
REFERENCES
Coulombe, B. A.; Chaney, Rufus L.; Wiebold, W. J. 1984.
Bicarbonate directly induces Fe-chlorosis in susceptible
soybean cultivars. Soil Science Society of America Journal. 48: 1297-1301.
Haws, B. Austin; Roe, A. H.; Nelson, David L. 1988. Index
to information on insects associated with western wildland shrubs. Gen. Tech. Rep. INT-248. Ogden, UT: U.S.
Department of Agriculture, Forest Service, Intermountain Research Station. 296 p.
Hunter, Richard B.; Romney, Evan M.; Wallace, Arthur.
1980a. Rodent-denuded areas of the northern Mojave
Desert. Great Basin Naturalist Memoirs. 4: 208-211.
Hunter, Richard B.; Wallace, Arthur; Romney, Evan M.
1980b. Fencing enhances shrub survival and growth
for Mojave Desert revegetation. Great Basin Naturalist
Memoirs. 4: 212-215.
Life Magazine. 1980. The downwind people. June. 3(6):
32-44.
Nelson, David L.; Harper, Kimball T.; Boyer, Kenneth C.;
Weber, Darrell J.; Haws, B. Austin; Marble, James R.
1989. Wildland shrub die-offs in Utah: an approach
to understanding the cause. In: Wallace, Arthur;
McArthur, E. Durant; Haferkamp, Marshall R., compilers. Proceedings-symposium on shrub ecophysiology
and biotechnology; 1987 June 30-July 2; Logan, UT.
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