Transcript Overcoming Photoinhibtion Chasing Foxtail Chess: A Look

Independent Project
J.C. Sylvan
SEE-U 2001
OVERCOMING PHOTOINHIBITION
A Look into the Relationship Between Bromus rubens and
Prosopis velutina
Introduction
Even though Velvet mesquite (Prosopis velutina) is native to
the Sonoran desert, it is considered something of an invader in the
desert grasslands of the American Southwest (Schlesinger 1990).
Taking advantage of human disturbances—fire suppression,
overgrazing, land development and rising carbon dioxide levels
which favor C3 photosynthesizers—, P. velutina has advanced into
these ecosystems and had an irreversible effect on flora and fauna
populations as well as the chemical and physical properties of the
soil (Wright and Honea 1986). Desert grasslands are slowly
transformed into mixed shrub savanna—a change, which given the
mesquite’s 400 year life span, becomes permanent (Burgess 1995).
One of the most apparent changes comes in terms of the fire
ecology of these areas (McPherson 1995). As mesquite becomes
more prevalent in wake of destructive overgrazing practices, for
example, the productivity of less shade-tolerant summer ephemerals
in these grasslands is reduced. Species that normally inhibit the
growth of mesquite seedlings are pushed back by the spreading
canopy cover, and mesquite seedlings have a better chance of
reseeding themselves. Once the mesquite trees have reached a
certain maturity they more resilient to fire, reshooting quickly
(McPherson 1995). As the standing crop of summer-germinating C4
grasses, a major fuel source for fires, is reduced, the probability of
fires is further reduced thus further ensuring the stability of an
ecosystem that favor mesquite.
Which is why it is ironic that velvet mesquite also seems to
be instrumental in the promulgation of another, true invasive
species, and a major cause of fires in desert grasslands: Bromus
rubens (red brome or foxtail chess).
Foxtail chess is an exotic species to the desert grasslands the Southwest. This cold-season Mediterranean annual was
probably introduced inadvertently into the Sonoran area in the middle of the last century when the area was first heavily
grazed by cattle (Newman Unpublished). Foxtail chess is a C3 type grass; it germinates in the winter, and has an advantage
over summer germinating native ephemerals in years of greater winter precipitation. Thomas Van Devender and Mark Dimmit:
“The roots of the introduced annual grasses, including mouse barley (Hordeum murinum), red brome (Bromus rubens), and
wild oats (Avena fatua), are active at relatively cool soil temperatures, accelerating their growth compared to native annuals.
Often they are so prolific that few nutrients remain for summer ephemerals” (2000). But B. rubens’ native Mediterranean
climate has adapted it to winter moisture and lower light levels. In other words, for all its success in American grasslands, it
may not be perfectly suited to the climate of desert grasslands. According to Newman (Unpublished), its lack of genetic
variability, its susceptibility to crowding, and, perhaps its sensitivity to extreme light levels make it a weak competitor in these
ecosystems. This may explain B. rubens affinity for the shade of velvet mesquite. As mesquite intrudes on the desert
grasslands it may be bringing an ideal microclimate for the grass, facilitating its viability in the shade, nitrogen, and moisture
poor environment of the Sonoran desert. This facilitating relationship between P. velutina and B. rubens is the subject of my
research.
Bromus rubens
1993 fire in Sonoran desertscrub near Sugarloaf Mountain near Phoenix, Arizona. This area along the Beeline Highway
between Phoenix and Payson is being converted from paloverde-saguaro desertscrub to a mesquite and acacia savanna,
with an understory of Mediterranean grasses, primarily red brome. Photo by Cecil Schwalbe.
www.werc.usgs.gov/invasivespecies/sonoran1a.html
Curious about the possible relationship of
these two species, I made a survey of the
hillsides around the Biosphere 2 Center near
Oracle, AZ. I looked under the canopies of 364
velvet mesquites at a range of sites: an arroyo,
northern, eastern and western slopes,
ridgelines, embankments and streambeds. And
there it was. Sometimes it grew in abundance,
sometimes it was sparse, but at all of the 8
sites I could find Bromus rubens growing
beneath at least 30% (and often more) of the
canopies of Prosopis velutina. The frequency of
its occurrence there also tended to follow a
pattern (Figure 1): red brome was more
common on north- and northwest- facing slopes
(slopes with an aspect between 240˚ and 360˚
(without declination), towards the middle of a
hill, and less common on south-facing slopes,
especially on ridgelines. This was true of the
annual grass, even when it was not generally
apparent on the hillside in the open spaces
between trees.
100%
1.000
0.973
0.909
90%
0.813
80%
Average % Occurrence
A Preliminary
Survey
FIGURE 1: AVG% Occurrence of Bromus rubens vs Aspect
70%
60%
y = 2E-05x 2 - 0.0058x + 0.9534
R2 = 0.7445
0.563
0.571
50%
40%
30%
0.259
20%
% of Occurrence
10%
Poly. (% of Occurrence)
0%
0˚
90˚
180˚
270˚
360˚
Aspect (˚)
In this paper I have taken a closer look and
have determined that this pattern is not random,
that it happens for reasons that have ultimately to
do with the abiotic factors that shape desert
grasslands.
Some Background on the Ecology of Fertile Islands:
It is a well documented phenomenon in the literature that larger desert
shrubs harbor beneath their canopies an array of smaller herbaceous plants.
Muller submits that “each Franseria shrub appears to be a miniature flower
garden by virtue of there blooming beneath it” scores of smaller plants (1953).
Wind and rain and erosion turn desert shrubs like the creosotebush and velvet
mesquite depositories of litter and loose organic material (often at the expense
of the surrounding open ground) (Cox et al. 1983). This concentration of
resources beneath the canopies of desert shrubs reinforces the insular character
the mixed shrub savanna, giving plants which can tolerate the shade of the
under-canopy greater access to vital nutrients. When resources are as limited as
they are in the desert, and when there is virtually no limit on the amount of raw
energy (in the form of sunlight) available to plants, it stands to reason plants
which are adapted to collect and concentrate natural resources and
withstanding intense temperatures and light levels would become functionally
dominant in the landscape.
The velvet mesquite may provide an island of fertility for the winter
germinating red brome, allowing the grass to establish itself before other spring
germinating native annuals (Newman Unpublished). According to this theory,
mesquite provides the grass with three crucial materials: nitrogen, water and
shade.
Mesquite as Nitrogen Source
In drier climates the limiting factor in plant production may be nitrogen; and in xeric climates plant phytotransport may
be the most reliable conduit of nitrogen for smaller plants. (source). Cox et al. (1983) have seen a relationship between
creosote [Larrea tridentate (DC.) Co.] and Lehmann’s lovegrass [Eragrostis lehmanniana Nees].Creosote, a shrub common in
higher elevations of the savanna, translocates soil nutrients through its roots and returns them to the soil beneath its
canopy.
A similar trend seems to hold for mesquite. According to one study, some grasses are 5 times more productive under
mesquite canopies than in open ground in the same area (Tiedemann 1970). Rundel et al. (1982) found that stands of honey
mesquite (Prosopis glandulosa) in the Sonoran desert may be able to fixate as much as 150 kg N ha-1/ yr-1 from deeper in the
soil profile. Furthermore this attribute may account for a disparity between surface layer nitrogen levels in the open areas
between trees and those directly beneath mesquite canopies (Wright and Honea 1986). The nitrogen fixing characteristics of
Prosopis could have decisive beneficial effects for red brome. B. rubens responds well to nitrogen fertilizers ( Hulber 1955), and
Newman (Unpublished) suggests that given its shallow root system, the plant is at a disadvantage in competition for nutrients
deeper in the soil; in dry soils of the desert, where N may be the single most important limiting factor in plant development
and productivity, a steady source of nitrogen is a definite advantage.
Mesquite as Moisture Source
Other studies on islands of fertility propose that Prosopis tamarundo may increase the moisture levels
of the soil beneath its canopy as a result of hydraulic lift (Caldwell and Dawson 1998). The long dry periods
in Sonoran climate mean that the moisture is only available to plants with deeper root structures, like velvet
mesquite. Increased moisture levels in upper soil layers may also facilitate the absorption of nutrients by
shallow rooted plants. Since nutrient ion mobility is greatly reduced as soil layers become desiccated
(Caldwell and Dawson 1998).
According to Muller and Muller (1956) canopies of desert shrubs, by virtue of their relatively large
stature in the desert savanna landscape, are able to collect and concentrate materials like water and litter
and nitrogen beneath their canopies. This is especially important during wet years when red brome’s
performance can be critical to its long-term persistence in the savanna. In these years, red brome must
regenerate a depleted seed-bank, and increase its distribution across open grass areas from one island of
fertility to another. “Bromus is a drought escaper that relies on seed production to persist in an episodic
environment characterized by drought” (Huxman and Smith, 2001). Hufstader, writing about red brome in
California climates claims that water may not be important for grasses as was believed. Precipitation
provides red brome with more than enough moisture, which suggests that even sporadic rainfall--provided it
comes—is all the water red brome needs to sustain itself through it’s life cycle. Perhaps velvet mesquite
provides red brome a stable, an annual microclimate, a haven the population can retreat to in between
periods of rain.
Mesquite as Shade Source
Another theory, concomitant with these, is the functional role of Prosopis as a shadegiving plant. Generally, the canopies of nurse plants reduce the photosynthetically active
radiation available to the plant growing beneath them, and thus in turn reduce CO2 uptake. One
study examined the relationship between the saguaro cactus, Carnegiea gigantean, and its
nurse plant, Ambrosia deltoidea (Franco and Nobel 1989). Indeed, young saguaros in the
Sonoran desert are frequently seen emerging from the shelter of taller desert shrubs. Franco
and Nobel (1989) reported a reduction of 90% in CO2 uptake in the saguaro beneath nurse
plants compared with saguaros growing in open, unshaded areas. There may be an analogous
relationship between velvet mesquite and red brome. This would suggest that there should be
reduced productivity in red brome beneath mesquite canopies.
But mesquite is summer-green: it tends have a thinner canopy during winter months.
Red brome is a winter germinator, growing most quickly during the months when the mesquite
is without its leaves. It could be that the mesquite provides red brome with the vital amount of
shade it needs to germinate (Naveh and Whittaker 1979). This shading may increases the
frequency and abundance of red brome by raising the soil temperature during the winter long
enough for this Mediterranean grass to germinate and reducing the temperature during
summer months to enhance seed production as is true in the case of the barrel cactus,
Ferocactus acanthodes (Nobel 1984a). Franco and Nobel also discovered that daily variations in
soil temperatures tend to be less under the canopy of nurse plants (1989). Indeed, the
microhabitat that nurse plants provide seems to be most beneficial in hot years, when soil
temperature is the limiting factor in the establishment of seedlings. But once they are
established, saguaro and barrel cactus seedlings, do much better in open areas; in cooler years
seedlings are more likely to establish themselves in exposed areas. Less is known about optimal
temperature conditions for the germination in red brome. The mesquite may also limit
competition form other less shade-tolerant native annuals that tend to germinate in the spring
when mesquite canopies are denser. Thus the under-canopy of the mesquite is a nursery for
red brome, providing (in the right circumstances) the ideal combinations of nutrients, moisture,
and shade to ensure the grasses success.
Slope and Aspect
Research supports the idea that B. rubens can thrive beneath a mesquite. But what explains the variability I observed
during my preliminary survey of the ridges around Biosphere 2. Why was the grass so much more abundant in some
circumstances (e.g. slopes with a Northern aspect) than in others? Even in adjacent areas, opposing sides of a ridgeline or
facing sides of a canyon, there was a notable difference in the presence of red brome. According to Hufstader (1974), foxtail
chess is a sub-dominant which prefers south facing slopes. That mesquite is a suitable micro-climate for red brome is
supported by the literature, but what are the determining factors in this facilitating relationship. Do slope and aspect of an
individual tree influence the percent cover of B. rubens growing beneath its canopy? And if so why? The relative abundance of
nitrogen and moisture under P. velutina should be consistent across the quadrants of the tree; i.e. if red brome should display
pattern in percent cover across the quadrants these factors alone would not be able to account for them.
Based on my observations I offer this hypothesis: that slope and aspect do in fact influence the growing patterns of B.
rubens because they are determining factors in the amount of light the grass receives during its growing period. In mesquite,
red brome has found a suitable microclimate for surviving in a more severe regional climate: mesquite provides the brome with
moisture, temperature stability, a steady nitrogen source, but perhaps most importantly a certain character of light.
Methods: Study Area
The study area included the hillsides and ridges on the B_2 Ranch near
Oracle, AZ (Study Area ). A climate features seasonal precipitation patterns,
with gentle consistent rains in the winter months and sporadic torrential
storms during the late summer. By the time this survey was conducted, late
June through early July 2001, the summer rain season had just begun.
Temperatures during this period were in the high 90’s to lower 100’s.
Site Selection
Applying a general
methodology offered in
Elzinga et al., a total of 12
sites near the Biosphere 2
Campus were sampled
(Map 1). For each cardinal
direction (N, S, E, W), or
aspect, 3 hillsides with
varying slopes were
chosen. These sites were
chosen subjectively and
were meant to create a
diversity of different
topographies to sample
from. Some of the sites
were adjacent to each
other—on opposite sides of
the same hill—, or opposite
each other-facing sides of
the same canyon. To
eliminate the
environmental factor of
grazing, other boundaries
were set by fence lines.
N
ASPECT
North
South
East
West
Biosphere 2
Oracle, AZ
Sampling
area
50m x 20m
At each of the sites, a
sampling transect was
established. A transect line of
50 was laid out parallel to the
contours of the hill. A position
on that line was selected
randomly. From that position a
sampling point no more than 20
downhill from the transect line
was chosen randomly, i.e., a
random number between 0-20
was generated and the
sampling point was set at that
perpendicular distance away
from the transect. The velvet
mesquite tree closest to the
sampling point was selected for
sampling, provided it was 1.)
alive, 2.) at least 1 meter in
diameter. This process was
repeated three times for each
site. Some sites had only 3
eligible trees.
Sampling Technique
i.
3
1
2
Data collection
ii.
NE
NW.
SE.
SW
Once trees were chosen, the following
characteristics were measured: Aspect. Sites were
chosen with their general slope and aspect in
mind. Where possible hillsides had aspects close to
the cardinal compass directions, but within these
sites, the aspects of individual trees could vary,
sometimes as much as +/- 20°. Aspect was
measured with a compass. A 12˚ declination was
added during to data analysis. Slope. Slope was
estimated 1 m above and below the base of the
tree. Landscape Position. The sampling area (50m
X 20m) position with respect to the entire hillside
was measured. Percent cover. The canopy area
was divided into four equal areas, or quadrants,
along the cardinal directions. Quadrants were
taped off to reduce error from overcounting.
Percent cover was estimated visually. Dimensions
of Prosopis. To account for the influence of the
size and shape of the tree in any patterns of
Bromus abundance, the height and average radius
of the canopy area was estimated. General
Observations were also made about the soil type,
the amount of litter, the plant species growing
beneath each tree, and the frequency of red
brome 1 meter or more beyond the canopy’s
shade edge.
Nomenclature based on Arizona flora (Kearney and
Peebles, 1951).
Data analysis
Data was tabulated and graphed in Excel. The dependent
variable, percent cover, was tested for statistical significance using both
a Kruskall-Wallis and Mann-Whitney tests in SPSS with respect to the
independent variables: aspect and slope.
As a secondary or subsampling question, the percent cover in
each quadrant of each tree were ranked (1,2,3,4) relative to each
other. The ranks for each tree were averaged across each the four
categories of sites (north-facing, south-facing, east-facing, west-facing
to examine the possibility of an abundance pattern beneath the
canopy.
Tree size and canopy size were also considered as potential
factors in the abundance of red brome.
In answer to the question: Do slope and
aspect of an individual Prosopis velutina
influence the percent cover of Bromus rubens
growing beneath its canopy? Percent cover
varies with aspect. B.rubens grows with
greater abundance beneath trees on hillsides and
embankments with northern and western aspects
(Figure 2). Percent cover varies with slope.
B.rubens grows with greater abundance beneath
trees either on steeper (>0.5) slopes (Figure 3).
Mean % cover
RESULTS
In answer to the question: Does the
abundance beneath a tree follow a pattern by
quadrant? The method of ranking its percent
cover by quadrant indicates that the growing
area beneath P. velutina for B. rubens lies
along a NW/SE axis (Figure 4). For each
aspect it grows more often in one of these two
quadrants than any other.
FIGURE 2: Mean % cover of Bromus rubens vs Aspect
A**
27.08
Mean % Cover
B*
C**
1.5
West
0.7
East
South
ASPECT
50
FIGURE 3: Mean % Cover Bromus Rubens vs. Slope
45
40
35
A**
37.75
Mean % Cover
30
25
20
15
B*
B*
10
Taken together these results give us a
fairly good predictor of where we will find red
brome in our biome.
B*
4.91
North
Mean % Cover
In answer to the question: Does the tree
height and canopy size effect the abundance of
red brome? The graphs did not support a strong
enough relationship between these factors to
merit statistical testing.
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
5.33
C
B**,C
5.55
5
2.67
2.52
Moderate
Moderately Steep
0
Shallow
Moderately Shallow
SLOPE
Steep
FIGURE 4: Rank of % Cover by Quadrant by Aspect
SE Quadrant
NW Quadrant
NE Quadrant
Rank
SW Quadrant
North
West
South
Aspect (˚)
East
DISCUSSION
Nothing occupies a privileged position
within the landscape. A matrix of
environmental functional factors like
nutrients, water, temperature, and light, all
effect the ways plants relate to each other,
not just as competitors but also, as in the
case of velvet mesquite with foxtail chess, as
facilitators. In his survey I establish a few of
the patterns characteristic of the frequency
and abundance of red brome: B. rubens
tends to grow 1.) along a NW/SE axis 2.)
beneath mesquites on north-facing hillsides
and embankments 3.) with steep slopes.
But what causes these patterns?
Nutrients, moisture, temperature, light all
contribute, but which of these defines the
relationships between percent cover of red
brome, and the slope and aspect of velvet
mesquite.
I believe in this case it is two principal
factors: photo-inhibition and human
disturbance. The first of these I was
cognizant of before I began my survey; the
other occurred to me as I tried to understand
my results.
Photoinhibition
Dr. Andrew Peterson and Karen Vitkay, who are conducting research on red brome at Biosphere 2,
shared with me their hypothesis that the pattern in red brome’s distribution with respect to aspect may have
something to do with photo-inhibition, or a plant’s tendency to actually be less productive in higher light
situations. Is it possible to evaluated the canopy shape and density and size as a predictor of the amount of PARi
light that passes through during the bromes germination period? Different canopy structures seem to offer the
B.rubens different advantages. Low hung branches gave the grass a dense shade canopy, but one that the grass
could grow through and reseed outside the limits of the canopy if the weather conditions were right. Higher
more bell-shaped canopies offered more space to spread, and indeed these larger open canopied trees denser
patches of the grass.
Bromus is a C3 grass which means that it is photosynthetic processes are better adapted to colder
weather and climates. Most desert grasses are use C4 photosynthetic pathway which allows them to sustain
themselves even in stressed environments with a great deal of solar radiation and high soil temperatures. Thus
B. rubens takes shelter beneath the shade of the mesquite. Mesquite is a summer-green tree. In the winter/dry
season it tends to lose its leaves, and when it does Br. Rubens takes advantage of the increased light levels to
germinate giving it a head start in its competition with native annuals for soil nutrients, moisture etc. The open
canopy of the mesquite, combined with the northwestern light provide advantageous growing conditions for
the brome.
It is important to note that to some extent the growth axis beneath the tree is repeated on a larger scale
in the distribution of red brome along the slope aspect gradient. Brome seems to do better with northern
exposure; in the desert it grows in the shade. B. rubens’ NW/SE growing track is by far the most consistent
trend among all samples. The only deviances occur when topographical features like slope of the hill or the
shade from a nearby shrub or rock, made another part of the canopy better suited (or equally well suited) to B.
rubens. The regularity of this pattern may have something to do with the regularity of its cause: the sun. Light
can actually be a limiting factor in the desert—too much of it may reduce seed quality and productivity in exotic
Mediterranean grasses like red brome which require a different temperature regime than desert plants.
Another, perhaps tangential, result of these findings is that they drive home the idea that even beneath
the canopy of a single species of tree, even within a single species sampled, there is a true difference in
distribution and in density across even very small gradations. The desert especially suited to microhabitats, and
the high degree of variability in topography and abiotic factors allow for a greater biodiversity and more
potential niches that species like red brome which very well can dominate and area, actually do not in these tiny
pools of perfect growing conditions C3 cycles: Part of the reason for this has to do with B. Rubens
photosynthetic cycle.
Human Disturbance
The other factor which shaped my results has to do with the
sites that were selected. I first noticed red brome growing in
abundance in arroyos and along roadsides on the Biosphere 2
campus. Andrew Peterson and Karen Vitkay directed me to a few
more sites, also by roadsides. I chose several of these sites to
measure percent cover, and in so doing inadvertently sample two
different brome populations: those close to roadsides and parking
lots and those that were not. Half (6) of the sites I chose were
places of human disturbance both on a daily basis, but also, given
the Biosphere’s recent genesis, over the past ten years as the
facilites here have been built. Is it possible that human disturbance
both disperses brome seeds and also limits the competition of other
species? Taken on its own, the data support this bypothesis.
AVG % cover for Disturbed vs. Undisturbed
10
9
AVG % cover
8
7
6
5
4
3
2
1
0
Disturbed
Undisturbed
A great deal of research has been done on how
invasibility is directly related to perturbation. “The single
most important factor influencing the invasibility of a
community,” writes Michael A. Huston, “is the degree to
which it has been disturbed” (1994). We can unwittingly
transport seeds hundreds of miles into completely new
environments. We also by our activities eliminate species,
change relative populations and make it difficult for native
species to bounce back. But what is the connection
between slope and aspect? It may be coincidental, but
many of the most well-covered sites were steep (almost
artificially so) hills and embankments near the man-made
structures at Biosphere 2: parking lots, RV parks, new
buildings and construction sites. Perhaps the differences
I noted had less to do with the slope and aspect of a site,
than with the proximity of the grass to disturbed
environments.
According to Newman, red brome is not
much of a competitor—other invasive grasses can
outperform it (Hutstadler 1976). In other words, red
brome may not be as fit for the desert: it germinates later
than some annual grasses, has a limited genetic variability
, and does not respond well to crowding (Newman). In
order to have been as successful as it has, it seems
brome must have found some type of niche with respect
to the velvet mesquite. The increased nutrient and
moisture levels working in consort with the shading effect
of the mesquite canopy seem to provide B. rubens with
the microclimate it needs to survive from year to year in
the Arizona Upland Desert. Red brome may be dispersed
by human activity; and human destruction of natural
environments means that like mesquite moving in in the
wake of overgrazing, humans are largely responsible for
the distribution/success of this species.
CONCLUSION
Foxtail chess (B. rubens) near the Cañada del Oro
tends to grow beneath the canopies of velvet mesquite
trees (P. velutina) growing at the bottom of steep Northfacing sandy washes in proximity to disturbed areas.
Beneath the trees themselves, it tends to grow along
NW/SE axis.
Once established the increased fertility beneath
the mesquite in terms of nitrogen and moisture levels, as
well as a relatively stable soil temperature may allow the
red brome to persist in a foreign environment that it’s own
lack of genetic variability make it vulnerable to. The
abundance of red brome in some of the survey sites may
have been a factor of their proximity to disturbed areas.
Grass in these areas represented in a higher percent cover
beneath the canopies of mesquite than grasses growing in
the canyons away from roads and human disturbance.
As for the tendency to grow along a NW/SE axis
may be as a result of the type of light available there
during the B rubens’ winter germination period. A
phenomenon know as photo inhibition. B. Rubens’
preference for this microhabitat beneath the mesquite
allows it to be more competitive with other ephemerals.
Thus one kind of invader helps another establish
inroads to the desert grasslands of the American
Southwest. B. rubens seems to have made the transition to
this climate with the help of two unlikely facilitators—velvet
mesquites and people.
REFERENCES
Burgess, Tony L. 1995. Desert Grassland, Mixed Shrub Savanna, Shrub Steppe, or Semidesert Scrub?: The
Dilemma of Coexisting Growth Forms. Pp. 31-67 in Mitchel P. McClaran and Thomas R. Van Devender
(eds.), The Desert Grassland. University of Arizona Press, Tucson.
Caldwell, Martyn M., Todd E. Dawson, and James Richards. 1998. Hydraulic lift: the consequences of
water efflux from the roots of plants. Oecologia, 113. 151-161.
Cox, Jerry R., Henry A. Schreiber, and Howard L. Morton. 1983. The initial growth of two range grasses
on nonfertilized and fertilized soils collected from creosotebush communities in the southwestern
United States. Journal of Range Management, 36, 6, 726-729.
Elzinga, Caryl L., Daniel W. Salzer and John W. Willoughby. 1998. Measuring and Monitoring Plant
Populations. Bureau of Land Management, Denver, CO.
Franco, A.C. and P.S. Nobel. 1989. The Effect of Nurse Plants on the Microhabitat and growth of cacti.
Journal of Ecology, 77, 870-886.
Garner, W. and Y. Steinberger. 1989. A proposed mechanism for the formation of ‘Fertile Islands’ in the
desert ecosystem. Journal of Arid Environments, 16, 257-262. Academic Press, London.
Halvorson, William L. and Duncan T. Patten. 1975. Productivity and flowering of winter ephemerals in
relation to Sonoran Desert shrubs. The American Midland Naturalist, 93, 2, 311-319.
Hufstader, R.W. 1978. Growth rates and phenology of some southern california grassland species. Journal
of Range Management, 31, 6, 465-466.
Hufstader, R.W. 1978. Precipitation, temperature, and standing crop of some southern California grassland
species. Journal of Range Management, 29, 5, 433-435.
Huston, Michael A. 1994. Biological Diversity: The coexistence of species on changing landscapes.
Cambridge University Press., Cambridge, England.
Huxman, Travis E. and Stanley D. Smith. 2001. Photosynthesis in an invasive grass and native forb at
elevated CO2 during an El Niño year in the Mojave desert. Springer-Verlag, New York. www.springerny.com
REFERENCES CON’T
Kearney, Thomas H. and Robert H. Peebles. 1951. Arizona flora. University of California Press.
Berkeley.
McPherson, Guy R. 1995. The Role of Fire in the Desert Grasslands. Pp. 130-149 in Mitchel P.
McClaran and Thomas R. Van Devender (eds.), The Desert Grassland. University of Arizona
Press, Tucson.
Muller, Cornelius. 1953. The association of desert annuals with shrubs. American Journal of
Botany, 40, 2, 53-60.
Newman, Dara. Unpublished. Element stewardship abstract for Bromus Rubens. The Nature
Conservancy, Arlington, VA
Rundel, P.W., E.T. Nilsen, M.R. Sharifi, R.A. Virginia, W.M. Jarrell, D.H. Kohl, and G.B. Shearer.
1982. Seasonal dynamics of nitrogen cycling for a Prosopis woodland in the Sonoran Desert.
Plant and Soil 67, 343-353.
Schlesinger, William H., Jane A Raikes, Anne E. Hartley, and Anne F. Cross. 1996. On the spatial
pattern of soil nutrients in desert ecosystems. Ecology 77,2, 364-374
Van Devender, Thomas R. and Mark A. Dimmitt. 2000. Desert grasses in Natural History of the
Sonoran Desert. University of California, Berkeley.
Valiente-Banuet, Alfonso and Exequiel Ezcurra. 1991. Shade as a cause of the association
between the cactus Neobuxbaumia Tetetzo and the nurse plant Mimosa Luisana in the
Tehuacán Valley, Mexico. Journal of Ecology, 79, 961-971.
Wright, R.A. and J. H. Honea. 1986. Aspects of desertification in southern New Mexico, U.S.A.:
soil properties of a mesquite duneland and former grassland. Journal of Arid Environments,
11, 139-145. Academic Press, London.
Acknowledgements
PROPS TO THE BOYS IN
THE BANANA SHIRTS.
Gift certificates to Barney’s NY are in the mail.
And hey erica, water which is too pure has
no fish.