Comparing Phenotype and
Fitness of Native, Naturalized,
and Invasive Populations of
Downy Brome (Cheatgrass,
Bromus tectorum)
C. Lynn Kinter
Richard N. Mack
Abstract: The Eurasian grass downy brome (cheatgrass, Bromus
tectorum L.) was introduced into arid and semiarid bunchgrass and
shrub communities in New Zealand and North America over 100
years ago, but has strikingly different histories in these ranges. On
New Zealand’s South Island, it persists at low levels, while in
Western North America, it dominates vast areas. Inherent high
fitness in founder populations may have contributed to the invasive
character of North American populations, while low fitness may
have precluded a New Zealand invasion. In four common greenhouse environments, 62 populations from Western North America,
New Zealand, and the native range in Western Europe were
compared for 15 phenotypic and fitness traits (including height,
days to flowering, vegetative biomass). Significant differences among
source locations were evident for most traits. North American
populations were typically most vigorous, followed by European,
and lastly New Zealand populations. North American plants flowered earlier, and New Zealand plants later, than those from the
native range. Differences in levels of fitness between the sources of
founder populations have likely contributed to radically different
histories of downy brome in two of its new ranges. Our research has
important implications for screening and predicting invaders and
controlling invasive species.
Downy brome, or cheatgrass (Bromus tectorum), is a
cleistogamous, annual C3 grass native to Eurasia and northern Africa. It was introduced from Europe (Novak and Mack
2001) into both North America and New Zealand over 100
years ago (Kirk 1869; Mack 1981), but has radically different histories in the new ranges. In Western North America,
it dominates vast areas formerly dominated by shrubs and
bunchgrasses (Mack 1981). On these rangelands, it changes
the structure of the community, often comprising over 90
percent of the vegetative cover. It decreases forage values,
causes loss of native biodiversity, and increases fire frequency, erosion, and siltation. In contrast, in the central
part of New Zealand’s South Island, downy brome is naturalized at low levels in shrub and bunchgrass communities
(Connor 1964; Williams 1980). There, it persists primarily
on disturbed sites, such as roadsides or trampled areas, but
does not become dominant over large acreages.
These performance differences might typically be attributed to environmental differences between the two introduced ranges. However, in the region of New Zealand where
downy brome has persisted for decades, the physiognomies
of the native plant communities, climate, and recent fire and
grazing history are similar to those features in Western
North America where downy brome has invaded heavily
(Mack 1986; Wardle 1991). Even two species that commonly
attack downy brome in its native range—head smut fungus
(Ustilago bullata) and bird cherry-oak aphid (Rhopalosiphum
padi)—have been known for at least a century in both
introduced ranges (Lowe 1961; Pergande 1904). Consequently, we hypothesized that differences in performance of
downy brome in these new ranges may be explained by
differences in the founder genotypes, rather than differences in the environments in the new ranges.
We assessed the potential for different performance among
downy brome from three ranges: the portion of its native
range in Western Europe that served as the donor region for
both North America and New Zealand (Novak and Mack
2001), and the new ranges in Western North America and
the South Island of New Zealand. By growing all populations
in common greenhouse environments, we investigated genetically based differences in phenotype, vigor, and fitness
among these three ranges.
Methods _______________________
Sample Collection
In: Hild, Ann L.; Shaw, Nancy L.; Meyer, Susan E.; Booth, D. Terrance;
McArthur, E. Durant, comps. 2004. Seed and soil dynamics in shrubland
ecosystems: proceedings; 2002 August 12–16; Laramie, WY. Proceedings
RMRS-P-31. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Rocky Mountain Research Station.
C. Lynn Kinter is a Botanist and Ph.D. Candidate, and Richard N. Mack
is a Professor of Botany, School of Biological Sciences, Washington State
University, Pullman, WA 99164-4236. FAX: (509) 335-3184; phone: (208) 3734381; e-mail: lkinter@fs.fed.us
18
In the native range, we based our collecting efforts on
Novak and Mack’s (2001) allozyme research. They found
that the widespread genotypes in Western North America
were most closely related to those in Europe, and less closely
related to genotypes in Eastern North America, Asia, and
Northern Africa. They deduced that the cheatgrass genotypes, which were the early founders that spread widely
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Comparing Phenotype and Fitness of Native, Naturalized, and ...
across Western North America, were from Central and
Western Europe, probably around Slovakia, Czech Republic, or the eastern part of Germany. For this study, we
sampled heavily in that region, as well as north into Sweden
and west across The Netherlands, where Novak and Mack
did not sample but where many immigrants and shipments
left for the New World. We also included samples from Spain
and Italy. In the native range, downy brome is found at low
levels in native plant communities, such as stabilized dunes,
and on disturbed sites, such as railroad beds, farmsteads,
and cereal grain fields.
In Western North America, we sampled three populations
each in the vicinity of six founder locations identified by
Novak and Mack (2001)—from Nevada and Utah north to
British Columbia. In New Zealand, founder information is
not known, so we sampled three populations in the region
around Lake Pukaki and Bendigo (Canterbury and Otago
Counties) where herbarium specimens have been collected
since the 1930s. In total, we sampled 62 populations: 41 in
Europe, 18 in Western North America, and three in New
Zealand. From each population, we randomly collected 30
individuals, and ultimately chose six of those at random for
use in experiments.
Common Greenhouse Experiments
In December 2000, we planted seeds from six individuals
from each of 62 populations in four common greenhouse
environments: control, low water, low nutrients, and low
temperature. These seeds were one generation grown out
from the wild to minimize maternal effects (Roach and
Wulff 1987; Schaal 1984). No mortality occurred during the
grow-out phase, and all parent plants produced numerous
seeds for experiments. In the experimental phase, seeds
were sown in standard potting medium (60 percent peat, 20
percent pumice, 20 percent sand; pH 6.9–7.0) in individual
15-cm fiber pots on greenhouse benches under ambient
light. Temperatures were 24/16 ∞C (12h/12h) for 6 weeks
after sowing, followed by a 9-week cold period at 4/2 ∞C
(10h/14h) to ensure vernalization. Plants in the control
environment were watered daily to container capacity, and
fertilized weekly with Peters 20:20:20 delivered in line
during watering. Each of the remaining experimental environments differed from the control in only one factor.
Plants in the low-water environment were watered to
container capacity when the soil moisture content of 10
randomly chosen pots had fallen 50 percent. Plants in the
low-nutrients environment received only the nutrients
initially in the potting medium and trace minerals in tap
water. Three weeks after the seeds were sown, plants in the
low-temperature environment were placed in an adjacent
greenhouse room for a cold period of 4/2 ∞C (10h/14h) that
was 3 weeks longer than in the other environments. With
the exception of plants in the low-temperature environment during these 3 weeks, plants were arranged in a
completely randomized design, and rotated and re-randomized weekly to minimize block and bench effects. Water
and fertilizer treatments continued until all plants had
senesced. Each plant’s aboveground biomass was harvested when chlorophyll pigmentation in the panicles was
no longer visible—167 to 259 days after emergence—then
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Kinter and Mack
dried at room temperature for 16 weeks. Throughout the
study, any extraneous plant material or collection bags
were autoclaved before disposal to prevent escape of genetic material.
While the plants were actively growing, we recorded days
to emergence, height of the tallest leaf every 30 days following emergence, width of the tallest leaf at 60 and 120 days
after emergence, tiller number at 60 days after emergence,
and days from emergence to flowering. At senescence, we
recorded days from flowering to senescence, number of
panicles, and first internode length of the tallest panicle.
After harvest and drying, first internode diameter of the
tallest panicle, aboveground vegetative biomass, panicle
biomass, and presence of glume hairs were recorded. Three
traits were also recorded before seeds were sown: weight of
50 seeds, seed length, and awn length.
Statistical Analyses
To assess trait differences among ranges, we compared
responses using Multivariate Analysis of Variance
(MANOVA) and Analysis of Variance (ANOVA) in Proc GLM
(SAS 2000) with a two-stage nested design: population
nested within range, and six replicates nested within population. Both population and range were fixed effects. Data
from each environment were analyzed separately due to
small but significant range by environment interactions. We
conducted a multivariate analysis of seven of the response
variables (height, leaf width, and tiller number at 60 days;
culm diameter, vegetative biomass, panicle biomass, days
from emergence to flowering) using Canonical Discriminant
Function Analysis (SAS 2000). All European and New
Zealand populations were included, but only the geographically central population from each of the six North American
founder sites was included to facilitate interpretation of the
scatterplot by reducing the amount of overlap in symbols. In
all comparisons, differences were determined to be significant when P < 0.05.
Results and Discussion __________
We found significant differences among ranges for nearly
every trait assessed in each of the four environments. For
example, in all but the early cold treatments, plants from
North America were significantly taller, those from New
Zealand were shorter, and those from Europe were intermediate (fig. 1). Similarly, North American plants had wider
leaves, New Zealand plants had narrower leaves, and European plants were intermediate (fig. 2). For aboveground
vegetative biomass at senescence, North American plants
had higher values and New Zealand plants had lower values
than did the European plants in each treatment; however,
differences were not significant in every comparison (fig.
3). The small number of populations from New Zealand
(n = 3) reduced test power and significant differences in
some cases.
For most of the remaining phenotypic traits we found
significant differences among ranges in each experimental
environment. In each of the four environments, North American plants flowered earliest and New Zealand plants latest
(fig. 4)—147 days for North America, 151 for Europe, and
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Kinter and Mack
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Comparing Phenotype and Fitness of Native, Naturalized, and ...
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Figure 1—Mean height (cm) of downy brome
populations from North America, Europe, and
New Zealand at 30-day intervals after
emergence in each of four test environments.
Error bars indicate standard error.
20
150
60
120
Days after emergence
Figure 2—Mean leaf width (mm) of downy
brome populations from North America,
Europe, and New Zealand at 60-day intervals
after emergence in each of four test environments. Error bars indicate standard error.
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Comparing Phenotype and Fitness of Native, Naturalized, and ...
Kinter and Mack
Aboveground vegetative biomass (g)
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Figure 3—Aboveground vegetative biomass for
downy brome populations from North America
(black), Europe (gray), and New Zealand (white) in
each of four environments. Error bars indicate
standard error. Within environments, differing letters
indicate significant differences (P < 0.05) among
ranges.
Emergence to flowering time (days)
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Figure 4—Time from emergence to flowering for
downy brome populations from North America
(black), Europe (gray), and New Zealand (white) in
each of four environments. Error bars indicate
standard error. Within environments, differing letters
indicate significant differences (P < 0.05) among
ranges.
156 days for New Zealand—when averaged across the four
environments. Not only are these differences statistically
significant, they are likely to be ecologically significant,
because previous work (Mack and Pyke 1983, 1984; Rice and
others 1992) has shown that a few days difference in flowering time can allow seed filling in arid environments.
Though population-level variability is not the key focus of
this study, we note that for nearly every trait assessed, we
found significant differences among populations within a
single range. This parallels findings by other researchers
working on Western North American populations and measuring traits such as biomass and seed dormancy (Meyer
and Allen 1999; Rice and Mack 1991a,b). Additionally, we
USDA Forest Service Proceedings RMRS-P-31. 2004
found less phenotypic variability among North American
and New Zealand populations than European populations,
as would be expected with the restricted genetic variability
following founder events based on a small number of introduced genotypes (Barrett and Husband 1990).
In analyzing all phenotypic traits simultaneously using
Canonical Discriminant Function Analysis in SAS, we found
that the centroids for each range were significantly different
from each other at P < 0.0001 (fig. 5). In each of the four
environments, the introduced phenotypes are essentially a
subset of the native phenotypes. North American phenotypes most closely match those from Austria, Slovakia, and
Southeastern Germany, while New Zealand phenotypes
mostly closely match those from Northwestern Germany
and The Netherlands—areas that lie geographically to the
northwest of the North American matches.
While plants from the three ranges clearly differ in phenotype and vigor, fitness differences may occur as well. We
found differences in mortality during a single time period
when plants in the low water treatment became extra dry in
a sudden spell of very hot days. North American plants had
a significantly lower death rate, 8.33 ± 2.66 percent, compared to European and New Zealand plants at 22.95 ± 2.69
percent and 27.78 ± 10.60 percent, respectively. A set of
experimental stress tests under different environments may
show that North American plants typically have higher
survival. They may also be more water use efficient, and we
plan to compare water use efficiency among the ranges using
carbon isotope analysis.
To further assess fitness, we are conducting seed germination trials and cleaning seeds from panicles to get a measure
of seed biomass produced in the four treatments. We will use
these two measures to calculate relative fitness values for
each population. Although these data are still being collected, some insight may be found in a previous analysis we
conducted in a single environment with one North American
population from Smoot Hill, WA, and 10 populations from
Central and Western Europe. In this pilot study, the North
American plants ranked second for relative fitness between
populations from first-ranked Bratislava, Slovakia, and
third-ranked Vienna, Austria. Plants from these two European populations were among those that displayed phenotypes most similar to those from North America in the four
greenhouse environments, and which were most genetically
similar based on allozyme data (Novak and Mack 2001). If
the pattern found in the pilot study corresponds to the larger
test in four environments, we expect plants from Western
North America to rank among those with highest fitness
from the native range.
Conclusions ____________________
To summarize our findings, plants from the invasive
range in North America were typically largest and most
vigorous, followed by those from the native European range,
and lastly those from the naturalized New Zealand range.
These and other phenotypic differences we have documented
among ranges indicate that genetic differences, rather than
environment alone, explain many performance differences
between native and introduced populations. Our findings
highlight the importance of studying representative genotypes
21
Kinter and Mack
Comparing Phenotype and Fitness of Native, Naturalized, and ...
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Figure 5—Canonical Discriminant Function Analyses of seven phenotypic measurements
of downy brome in the control environment. Centroids of each range are indicated by large
letters: New Zealand (Z), Europe (e), and North America (A).
from across a species’ range in screening for invasive potential, testing for effectiveness of herbicides and biocontrol
agents, or similar studies. These results have ramifications
for quarantine regulations because, at present, an introduced species is often not considered to be a threat if it has
not been a problem in the past. Our study illustrates that the
outcome of an introduction can be very different based on
random genetic sampling in the native range. Serious ecological consequences may have occurred in New Zealand if
favorable habitats on the South Island had gained plants
from the populations that we unfortunately gained in Western North America.
22
Acknowledgments ______________
We thank J. R. Alldredge, M. A. Evans, and M. S. Minton for
statistical assistance; R. S. Bussa, C. A. Cody, M. A. Darbous,
M. Erneberg, S. K. Foster, G. K. Radamaker, B. D. Veley, and
K. D. Walker for technical assistance; and J. Baar, L. Barkan,
C. E. Hellquist, A. Klaudisova, N. L. Mack, H. C. Prentice,
R. R. Pattison, K. D. Rode, T. Sebastia-Alvarez, A. Tudela,
and L. J. van der Ent for seed collection assistance. We acknowledge helpful discussions with R. A. Black, L. D. Hufford,
M. Morgan, S. J. Novak, P. S. Soltis, and M. S. Webster. Funding sources include Betty W. Higinbotham and Noe
Higinbotham Research Awards, Hardman Foundation Award,
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Comparing Phenotype and Fitness of Native, Naturalized, and ...
Sigma Xi—Scientific Research Society Grant-in-Aid-of-Research, and Washington State University College of Sciences
and School of Biological Sciences.
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