The Holocene sand dunes on the north shore of Mono Lake are dominated by the two shrub species,
rabbitbrush
(Chrysothamnus nauseosus) and greasewood (Sarcobatus vermiculatus).
The mid-dune slopes and lower ridges on the lakeward side of the parabolic
dunes are preferred by rabbitbrush, seen in flowering here in August, with
greasewood occurring on the higher dune ridges. This area is prime bee-fly
habitat; we began our long-term demographic study of rabbitbrush with this
marked population of shrubs, which we monitored annually to estimate food
resources for adult bee flies. In 1983 and 1984, we marked 
125
rabbitbrush individuals over 30 cm in canopy diameter in a 40 m by 120
m plot; with uninterrupted annual censuses, we have followed this population
for 17 consecutive seasons. In the late 1980's, water diversions combined
with a significant drought pushed the water table to its lowest point during
the study. The 1987 season was a bottleneck for this population: 25% of
the original marked shrubs died, with smaller shrubs significantly more
likely to die than the larger ones. The high mortality gave us an opportunity
to test whether the shrubs died in a "thinning" pattern, that is, was a
shrub more likely to die the nearer it was to a neighbor? In other
words, do rabbitbrush compete for water, when water is clearly a limiting
resource for their growth and survival? The answer was no: shrubs died
randomly with respect to a shrub's llocation relative to its neighbors.
Shrubs were dispersed randomly both before and after the mortality caused
by lack of water. Desert dogma has been that desert perennial shrubs become
uniformly distributed through a classical thinning process wherein shrubs
compete with their nearest neighbors for water. Our data from this long-term
demographic study dispute the "Desert Dogma" thinning hypothesis in adult
rabbitbrush.
Perhaps thinning occurs but not at this stage. Data from the long-term
study suggested that once a phreatophytic shrub sends its long tap root
to deep soil moisture, shrubs no longer compete for water because the ground
water is limitless provided the shrub is able to track it by growing its
tap root. 
However,
when seedlings establish in spring and early summer, they are dependent
on precipitation because they do not have a tap root long enough yet to
reach deep soil moisture, something that they cannot achieve until the
fall (Donovan and Ehleringer). We extended our study by establishing 5
more permanent plots on the lakeward gradient. This plot is on a lakeward
slope of the parabolic dune, facing one of the lagoons; here rabbitbrush
is rapidly recruiting juveniles into the population. We marked a series
of smaller seedling plots within the adults' plots; here David Brown (master's
student) and Claudia Funari (undergraduate assistant) are marking rabbitbrush
seedlings in a 10 m by 10 m plot. We found that for both rabbitbrush and
greasewood, seedlings are highly aggregated. Non-mutually exclusive hypotheses
include a microtopographic effect with low points more likely to gather
seeds, a spatial heterogeneity effect with some microlocations being somehow
more favorable for seedling growth and establishment, or a parent or nurse
plant effect. 
Mortality
of seedlings occurred in the direction predicted under the thinning hypothesis,
although dispersion of established adults stops at a random pattern and
thinning does not continue (as established above) until a uniform
dispersion is reached. In other words, no adult population is uniformly
distributed; at most shrubs are randomly distributed although intershrub
distances may be large as you see in this picture and as is usual in deserts.
We tested these hypotheses with 6 permanent plots, 2 with both rabbitbrush
and greasewood and 4 with only greasewood, in which we marked and mapped
all shrubs within the plot, totalling over 2500 individual shrubs. This
work was possible with the use of an electronic distance measuring device
in a "total station", which allowed us to "shoot" the locations of 500
shrubs easily in one day to the nearest half cm. With highly accurate maps,
we could determine which individuals were missing, versus which had lost
tags, allowing us to get precise estimates of mortality and nearest neighbor
distances.
This work was funded by NSF-FAW GER 90-23789 to Catherine Toft and USDA 92-37101-7419 to Catherine Toft, Deborah Elliott-Fisk and James H. Richards.
Publications:
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Toft, C.A. 1995. A ten-year demographic study of rabbitbrush (Chrysothamnus nauseosus): Growth, survival and water limitation. Oecologia 101:1-12. PDF
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Toft, C.A. and T. Fraizer. 2003. Spatial dispersion and density dependence in a perennial desert shrub (Chrysothamnus nauseosus : Asteraceae). Ecological Monographs 73: 605-624. PDF
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. Benard, R.B. and
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. Benard, R.B. and
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Benard, R.B. and C.A. Toft, in preparation. Seedling establishment in a desert perennial shrub: causes and consequences of spatial aggregation
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Floyd, K.W. and C.A. Toft, in preparation. Spatial demography of greasewood (Sarcobatus vermiculatus: Chenopodiaceae) in the Mono Lake Basin, California.
Related studies:
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Donovan, L.A., J.H. Richards, and M.M. Muller. 1996. Water relations and leaf chemistry of Chrysothamnus nauseosus ssp. consimilis(Asteraceae) and Sarcobatus vermiculatus (Chenopodiaceae). American Journal of Botany 83: 1637-1646
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Donovan, L.A., J.H. Richards, and E.J. Schaber. 1997 Nutrient relations of the halophytic shrub, Sarcobatus vermiculatus, along a soil salinity gradient. Plant and Soil 190: 105-117.
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Donovan, L.A., D.J. Grise, J.B. West, R.A. Pappert, N.N. Alder, and J.H. Richards. 1999. Predawn disequilibrium between plant and soil water potentials in two desert shrubs. Oecologia 120:209-217.
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Dodd, G.L. and L.A. Donovan. 1999. Water potential and ionic effects on germination and seedling growth of two cold desert shrubs. American Journal of Botany 86(6):1146-1153.
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Donovan, Lisa, A. and J. H. Richards. Differences in stress tolerance, but no competition or facilitation, for juveniles of two desert shrubs along a stress gradient. Journal of Ecology 87:1-17.
 Toft
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