SEED PRODUCTION BY THE NON-NATIVE BRASSICA TOURNEFORTII (SAHARA MUSTARD) ALONG DESERT ROADSIDES

2006·6 MB·English

by  Melissa R Trader

Madrono, Vol. 53, No. 4, pp. 313 320, 2006
SEED PRODUCTION BY THE NON-NATIVE BRASSICA TOURNEFORTII
(SAHARA MUSTARD) ALONG DESERT ROADSIDES
Melissa R. Trader, Matthew L. Brooks', and Julie V. Draper
U.S. Geological Survey, Western Ecological Research Center, Las Vegas Field Station,
NV
160 N. Stephanie St., Henderson, 89074
Abstract
Plant biomass and seed production were quantified for Brassica touruefortii Gouan, Brassicaceae
(Sahara mustard) from 3 sites spanningthe Mojave and Sonoran deserts, in the Southwestern United
States. We found strong linear relationships between plant biomass and seed production, with larger
plants producing more seeds per plant (R- = 0.93) and greater seed biomass per plant (R- = 0.94).
Both seed count (R- = 0.93) and seed biomass (R- = 0.90) were also greaterin 0.25 m- plots that had
higherplant biomass. These results and the law ofconstant final yield indicate that biomass and seed
production ofindividual Saharamustardplantscan behigherinplotswithlowerdensities. Thesedata
suggest that control efforts that do not remove all individuals may reduce densities but inadvertently
increase net seed production within treated areas.
Key Words: Brassicaceae, disturbance, exotic, invasive, Mojave Desert, Sonoran Desert, Brassica
touruefortii.
Brassica touruefortii Gouan (Brassicaceae [Sa- Europe, Sahara mustard has spread through the
hara mustard]) was first recognized in Coachella southwestern deserts of North America and is
Valley, California in the 1920s, and has since most common on roadsides, abandoned fields,
spread into cismontane California and through- and sand dunes (Minnich and Sanders 2000;
out the Mojave and Sonoran deserts of North Malusa et al. 2003). Roadsides offer increased
America (Minnich and Sanders 2000). It is soil moisture compared to the surrounding
regarded as one of the most invasive wildland landscape, as they collect surface runoff from
pest plants in California (CalEPPC 1999; Cal- adjacent wildland areas during rains (Johnson et
IPC 2005), and is beingconsidered foraddition to al. 1975; Vasek et al. 1975; Brooks and Lair
the Arizona noxious weed list (Ed Northam accepted). Another advantage Sahara mustard
personal communication) and the Nevada nox- has inestablishingalongroads is the formation of
ious weed list (Dawn Rafferty personal commu- a sticky gel coat on seeds when they become wet,
nication). Sahara mustard is also one of the which allows seeds to adhere to vehicles (Minnich
primary species targeted for control by land and Sanders 2000) and spread along roadsides.
managers participating in the Mojave Weed Sahara mustard poses several threats to native
Management Area in California and the Clark desert vegetation. First, it has an early phenology
County Cooperative Weed Management Area in giving it a head start over many native annual
southern Nevada. Other land management units plants in utilizing soil moisture and mineral
actively managing this species in the Mojave nutrients. The Mojave and Sonoran Deserts
Desert include Lake Mead National Recreation receive winter rain and annual plants typically
Area, Joshua Tree National Park, and the Las germinate in responseto rainfall events in fall and
Vegas Field Office of the Bureau of Land winterand bloom in early spring. Sahara mustard
Management. It is also an identified management plants can flower as early as December, set seed
concern in the Sonoran Desert at Organ Pipe as early as January, and most plants are fruiting
Cactus National Monument, Saguaro National or dead by April (Minnich and Sanders 2000; M.
Park (Sue Rutman personal communication) and Brooks and J. Draper personal observations).
t(hCeamCeoraochnelBlaarrVoawlsleayndNatTioodndalStWeifladnliicfepeRresfoungael bSeafhoarrea nmautsitveardplacnatns choamvpeleftuellyitsdelivfeelcoypcelde waenldl
catomGmurnaincdatiCoannsy)o,nanNdatoinonatlhe PCaorlkor(aMdaorPylaZtyelaou rBerporookdsucepedrs(Jo.naHloltobesteravla.tiuonnpsu)b.lisThheed dSaotna;orMa.n
personal communication).
Desert also receives summer monsoonal rainfall
Since its introduction from the semi-arid and which can stimulate germination of Sahara
arid deserts of North Africa, the Middle East, mustard (M. Brooks personal observations),
and the Mediterranean regions of southern
although its phenological timing in relationship
to native summer annuals has not been reported.
'Author for correspondence, email: matt_brooks@ Second, Sahara mustard has the potential to
usgs.gov create a continuous fuel load in areas where fuels
MADRONO
314 [Vol. 53
are otherwise scarce and fires are infrequent. and northeast into these deserts. The two criteria
Historically, fires in Mojave and Sonoran Desert in choosing sites were that Sahara mustard be
shrublands were infrequent due to low fuel loads present along a paved road and in the adjacent
between shrubs, preventing fires from spreading wildland area, and that there be minimal
beyond ignition points (Brooks and Pyke 2001). disturbance at the site with the exception of the
Theaddition ofextra fuel (Saharamustard) could road itself. The three sites were located in:
increase fire return intervals and many native Coachella Valley, California along Interstate
desert species are not adapted to fire and their Highway 10, which is near the initial point of
populations generally take much longer than colonization for this species in North America
non-native species to reestablish following fire (Minnich and Sanders 2000); Mohawk Dunes,
(Brooks and Minnich 2006). located offofInterstate Highway 8 east ofYuma, |
Desert annual plants are often referred to as Arizona; and along California State Highway 95
ephemerals because they grow in habitats with south ofNeedles, CA. Soils ranged from sandy at
unpredictable or infrequent resources such as the Mohawk Dunes site to loamy and rocky at
precipitation (Barbour et al. 1999). Sahara the Coachella Valley and Needles sites.
mustard populations survive periods of summer We sampled two microhabitats to include
drought as dormant seeds. During this life- variation in plant production at two extremes of
history stage individuals are susceptible to pre- an environmental gradient, and two habitats to
dation from granivores and mortality from representconditions typical ofroadsides. Thetwo
disease. The more seeds that can be produced, microhabitats were: 1) beneath canopy-under the
the safer the population is from depletion due to canopy of Larrea tridentata (DC.) Cov. (Zygo-
predation or disease before the next germination phyllaceae [Creosote Bush]) or Ambrosia dumosa
event. Seeds are also the singular unit of (A. Gray) Payne (Asteraceae [White Bursage])
propagule dispersal for Sahara mustard, either shrubs; and 2) interspace-the area between
when they are still within siliques attached to shrubs that receives no shade from shrubs. The
stem fragments and blown by wind, or after they two habitats were: 1) along the roadside berm;
are harvested by granivores (M. Brooks personal and 2) greater than 20 m from the berm. Two
observation). As a result, producing more seeds replicate 0.25 m^ plots were located within each
means the population has a greater potential for of the two microhabitats within each of the two
persistence and expansion. habitats at each ofthree sites, resulting in a total
Efforts to control invasive annual plants such as of 24 sampling plots.
Sahara mustard should ideally be designed to Plants were collected soon after the foliage
reduceinputsto theseedbank. Althoughremoving senesced in May 2003. Sahara mustard plants
individuals from a cohort may reduce their rooted within the boundaries ofeach 0.25 m^ plot
immediate effects on other plant species, the were carefully uprooted, the taproot was clipped 'i
competitive release that can follow thinnings e.g., directly below the rosette, and the samples were
(Brooks 2000) may benefit remaining Sahara stored in paper bags until processing.
mustard plants. Thus, control efforts that only Above-ground plant density and biomass, and
reduce density of Sahara mustard and do not seed density and biomass, of Sahara mustard
completelyremoveallreproductiveindividualsmay weremeasured foreach sample. Plant densitywas
inadvertently increase growth and reproductive defined as the number of plants rooted within
output ofany Sahara mustard plants that remain. each plot, and was reported to the nearest whole
Effective management of Sahara mustard number. Plant biomass was determined by
requires knowledge of seed production and seed weighing plants prior to seed removal (to count
bank dynamics. In this paper we report patterns the seeds) using an analytical balance (0.001 g
of seed production in Sahara mustard from the precision), and reported to the nearest 0.01 g. ^
Mojave and Sonoran deserts of southwestern Plants were individually weighed after they were '|
North America. We report seed production per air dried in a warehouse for 3 mo (May-July).
individual plant and per unit area, and specifi- Final dry biomass values were modified by \
cally evaluate the relationships ofseed count and subtracting the moisture mass, calculated using
biomass with plant density and biomass. We the average relative humidity ofthe storage space
focus on roadside populations, because this is during the last month of drying (32.39%). We ']
where most control efforts to stop the spread of acknowledge this is a non-standard technique for
this species occur. determining dry biomass values, but we did not ,
:
want to subject theseedsto ovenheatingsince the
Methods seeds were to be used in subsequent germination
and physiology studies. Seed density was de-
Sites were located in three areas in the Mojave termined by counting the number of seeds per |
and Sonoran deserts of California and Arizona, plant, and reported to the nearest whole number. j
near the area where Sahara mustard was first Seed biomass per plant was measured by
collected and in the paths ofits spread southeast collectively weighing all seeds removed from
2006] TRADER ET AL.: SEED PRODUCTION BY SAHARA MUSTARD 315
Table Density and Biomass of Sahara Larger plants produced more seeds and more
1.
Mustard Plantsand Seeds Reportedas Averages total seed biomass than smaller plants. Seed
(Range) for Individual Plantsand 0.25 m- Plots. count (Fig. la; seed count = 267(plant weight) -
90, = 0.95) and seed biomass (Fig. lb; seed
Per Plant (n = 135) Per Plot (n = 24) biomass = 0.32(plant weight) - 0.14, R- = 0.94)
Plant Density n.a. 6(1-23) were both strongly related with plant size across
Plant Biomass 4.07 (0.04-61.11) 22.92 (1.59-69.20) the range of plants sampled. The size of in-
(g) dividual seeds was also consistent across the
Seed Count 995.75 (0 16,554) 5,601 (344-16,760) range ofplant sizes analyzed, as indicated by the
Seed Biomass 1.17 (0.00 19.05) 6.57 (0.47 21.44) strong linear relationship between seed count and
(g) seed biomass per plant (seed count = 0.22(seed
biomass) - 0.15, R- = 0.94) (data not shown).
a plant, and reported to the nearest 0.01 g. Seeds Seed production was highest on plots with the
highest standing plant biomass. Seed count
wmearteurecouanntdedviaanblde.weViigabhieldityonlwyasifdtehteeyrmwienreed (RF-ig.=2a;0.s9e3e)d acnoduntse=ed26b8i(opmlaansts b(iFoimga.ss2b);- s5e3e0d,
braosuendd oanndsreuesdtschoalpoeredanwderceolcoorn;sisdeeedrsedtvhiaatblweeraes biomass = 0.33(plant biomass) - 1.00, R~ = 0.90)
were strongly related to biomass of Sahara
opposed to seeds that were flat or crushed and mustard plants per 0.25 m- plot. These results
black. These guidelines to identify viable seeds
follow from the positive linear relationships of
were substantiated by subsequent germination
seed count and seed biomass per plant with plant
tests (J. Holt et al. unpubHshed data).
size (Fig. la, b).
Preliminary analyses indicated that there were
Although not statistically significant, seed
no significant effects ofmicrohabitat, habitat, or count and biomass were lower in plots with the
their interactions on seed production (count and highest Sahara mustard densities (Fig. 3a, b).
biomass) at the individual or plot level. Seed
production did vary significantly among sites
(P>0.05), but the interactions of site with Discussion
microhabitat and habitat were not significant.
We therefore pooled the data into single analyses, ducSeodme> i1n6d,i0v0i0dusaeledSsaihnarthaismsutsutdya.rdHopwleavntesr,prtoh-e
using linear regression to evaluate relationships
between plant characteristics (density and bio- largest plants in our data set were only approx-
mass) and seed production. We log-transformed imately 50% the size of many of the largest
individuals found in the field during years of
the seed count data in order to normalize its especially high rainfall and thus the largest
distribution. individuals may produce substantially more seeds
(M. Brooks, personal observation). The Sahara
Results mustard plants used in this study were collected
A total of 135 Sahara mustard plants were i(nTa2b0l0e3,2)a.ye2a0r04thaatndrec2e0i0v5edwbeerleow-yeaavresraogfe rhaiignhfearl-l
collected within the 24 plots. Mean above-ground than-average rainfall and Sahara mustard plants
dry biomass (±1 SE) of individual Sahara in these years were noticeably larger than in 2003.
mustard plants was 4.07 ± 0.81 g, but ranged Other weedy plants occurring in the Mojave
widely from 0.04 to 61.11 g (Table 1). Above- Desert have been reported to produce widely
ground biomass perplot (0.25 m-) averaged 22.92 variable numbers of seeds. Two plants in the
± 3.90 g (916,800 kg ha '; 818,585 lbs acre '). Brassicaceae family, Brassicanigra (L.) Koch and
Density ofstanding plants per plot averaged 6 ± Sisymbrium altissimum L., reportedly produce
1 plants (225,200 ha~'), ranging from 1 13,400 and 80,400 seeds per plant, respectively
(40,000 ha ') to 23 (920,000 ha '). (Stevens 1932). Russian thistle or tumbleweed
Seed counts for individual Sahara mustard {Salsola tragus L.) can produce 24,700 seeds per
plants averaged 996 ± 222 seeds, ranging from plant (Stevens 1932). A non-native annual grass
0 to 16,554 seeds (Table 1). Seed counts per plot common in the Mojave Desert, Bromus madri-
averaged 5,601 ± 1079 seeds (224,043,200 ha '). teusis L. subsp. ruheiis (L.) Husnot, reportedly
Total seed biomass for individual plants averaged produces 446 to 575 seeds per plant (Huxman et
1.17 ± 0.27 g . Seed weights per plot averaged al. 1999). Clienopodium album L., a weedy annual
6.57 ± 1.36 g (262,800 kg ha '). Of 135 plants forb has been recorded as producing anywhere
analyzed, 11 weighed more than 10 grams and from 31,000 (Salisbury 1978) to 72,450 (Stevens
produced more than 2000 seeds, 46 plants 1932) seeds per plant
weighed between 1 and 10 grams and produced We know from the law of constant final yield
between 300 and 2000 seeds, and 78 plants (Kira et al. 1953; Hozumi et al. 1956) that as
weighed less than 1 gram and produced less than plant densities increase, plant size and seed
300 seeds per plant (Fig. la). production per plant decrease in response to
MADRONO
316 [Vol. 53
Sahara Mustard Seed Production vs. Plant Bionnass
CD
g
>
C
o
c
i
o
O
D
CD
CD
CO
10 20 30 40 50 60 70
Plant Wt (g)
25
y = 0.32x-0.14
p < 0.0001
(b)
r2 = 0.94
-i 20
CL
cu
g15
>
15
C
(/) 10 -
c/)
CD
E
o
GQ
D
CD
CD
(f)
—r-
10 20 30 40 50 60 70
Plant Wt (g)
Fig. 1. (a) Linearcorrelation ofseed count related to biomassperindividualplant, (b) Linearcorrelation ofseed
biomass related to biomass per individual plant.
2006] TRADER ET AL.: SEED PRODUCTION BY SAHARA MUSTARD 317
Fig. 2. (a) Linear correlation ofseed count related to plant biomass per 0.25 m- plot, (b) Linear correlation of
seed biomass related to plant biomass per 0.25 m^ plot.
MADRONO
318 [Vol. 53
Sahara Mustard Seed Production vs. Density
Plant Density (no.) / 0.25 plot
25
^ (b)
20 H
CM
E
in
C0NJ 15
3
CO
CO
1 10
g
GO
D
0
0) 5
CO
10 15 20 25
Plant Density (no.) / 0.25 plot
Fig. 3. (a) Scatterplot ofseed count values related to plant density within a 0.25 m- plot, (b) Scatterplot ofseed
biomass values related to plant density within a 0.25 m- plot.
2006] TRADER ET AL.: SEED PRODUCTION BY SAHARA MUSTARD 319
Table 2. Precipitationand Departurefrom Normal Precipitation (cm) atThree Weather Stations
NeartheNeedles, CoachellaValley, and Mohawk Dunes Study Sites (U.S. Dept.ofCommerce2003,
2004, 2005). * At time ofpreparation, only January-August 2005 precipitation data was available online.
2003 2004 2005*
Precipitation Departure Precipitation Departure Precipitation Departure
Weather station (cm) (cm) (cm) (cm) (cm) (cm)
Needles AP 11.58 -1.40 16.08 3.10 12.95 4.29
Palm Springs 12.42 missing 19.86 missing 18.54 8.81
Yuma Proving Ground 9.14 -0.51 18.44 8.79 13.41 7.39
limiting resources and increased competition. species, including Sahara mustard. The law of
Mean plant size and rates of emergence have constant final yield and the results of this study
been found to be negativelycorrelated with initial suggests that plots that are only thinned during
seed density in annual plant communities (Lortie control efforts for Sahara mustard, leaving some
and Turkington 2002). Species that have been individuals alive (e.g., hand-pulling or spraying
recorded as producing fewer seeds at increased a site once and not returning to look for
densities in the Mojave Desert include Bromus survivors), may not reduce seed production and
madritensis L. subsp. ruhens (L.) Husnot (Harper could inadvertently produce more standing bio-
1961; DeFalco et al. 2003), Vulpia octofJora mass and have higher seed production than plots
(Walter) Rydb., and Descurainiapinnata (Walter) that are left untreated. The net result may be
Britton (DeFalco et. al. 2003), Conyza canadensis a more plentiful seed bank than would have
(L.) Cronq. (Palmblad 1968), Pectocarya recur- existed ifthe smaller, less productive plants were
vata L M. Johnston, and Plantago patagonica left in place. Although this hypothesis remains to
Jacq. (Pantastico-Caldas and Venable 1993). be tested, it suggests the importance ofdestroying
Although not significant, the four plots in our all plants in a target area when control efforts are
study with the highest Sahara mustard densities employed. This may require resurveys and
had lower seed production than many plots with follow-up treatments during the remainder of
lower Sahara mustard densities. the growing season to locate and destroy plants
The preliminary results mentioned in the that were missed the first time and to remove
Methods section indicate that microhabitat (be- plants that have emerged since the initial treat-
neath shrub canopy and interspaces between ments. Second and even third cohorts of Sahara
shrubs) and habitat (roadside and off-road) did mustard have been observed within the same
not significantly affect seed production, but we growing season in the Mojave Desert (M. Brooks
believe this lack of significance to result from and M. Trader personal observations). Managers
dMioffhearewnkcesDuinnessoislisteahtasthseantdhryeseoisltsuwdhyicsihteSs.ahTahrea ssehvoeurladl gbeermaiwnaarteionthactohothritss pplearntyecaarnapnrdodtuhcaet
mustard prefers (Minnich and Sanders 2000), and additional treatments may be required to remove
can often dominate irregardless of other factors
such as localized conditions created by shrubs all cohorts, as well as those which survived
and roads (Brooks accepted, M. Brooks personal previous control treatments. It is therefore advis-
observations). The soils at Needles and Coachella able to focus control efforts on repeated treat-
Valley are loamy and rocky, which Sahara ment and monitoring ofsmaller areas, than single
mustard does generally not prefer, and where treatments of larger areas.
shrubs and roads can have a much stronger effect As stated in the introduction, Sahara mustard
on dominance of this species (Brooks accepted). poses threats to native desert vegetation. Cur-
Variation in microhabitat and habitat effects rently, little information about this plant exists in
among the three sites was one ofthe reasons for the literature and additional research is necessary
lack of statistical significance of these variables. to create effective management plans. Informa-
Additional sites, replicates, and stratification of tion related to seed banks is specifically needed to
soils are needed to strengthen these ideas about evaluate Sahara mustard seed bank dynamics,
Sahara mustard's habitat and microhabitat pref- seed longevity and persistence in soil, and the
erences. effects ofplant density, microhabitats, and roads
on seed production.
Management Implications
Acknowledgments
Seed production should be considered when
implementing control efforts and when creating We thank Mark Fain for collecting the Sahara
effective follow-up strategies for any annual plant mustard plants processed in this study and John C.
MADRONO
320 [Vol. 53
Hunter and two anonymous reviewers for constructive of competition. J of the Institute of Polytechnics,
comments on an earlier version ofthis manuscript. Osaka City University D7:15-34.
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