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Population Biology |
Department of Ecology and Evolutionary Biology, University of California–Irvine, Irvine, California 92697 USA
Received for publication June 27, 2006. Accepted for publication February 8, 2006.
ABSTRACT
Invasiveness may result from genetic variation and adaptation or phenotypic plasticity, and genetic variation in fitness traits may be especially critical. Pennisetum setaceum (fountain grass, Poaceae) is highly invasive in Hawaii (HI), moderately invasive in Arizona (AZ), and less invasive in southern California (CA). In common garden experiments, we examined the relative importance of quantitative trait variation, precipitation, and phenotypic plasticity in invasiveness. In two very different environments, plants showed no differences by state of origin (HI, CA, AZ) in aboveground biomass, seeds/flower, and total seed number. Plants from different states were also similar within watering treatment. Plants with supplemental watering, relative to unwatered plants, had greater biomass, specific leaf area (SLA), and total seed number, but did not differ in seeds/flower. Progeny grown from seeds produced under different watering treatments showed no maternal effects in seed mass, germination, biomass or SLA. High phenotypic plasticity, rather than local adaptation is likely responsible for variation in invasiveness. Global change models indicate that temperature and precipitation patterns over the next several decades will change, although the direction of change is uncertain. Drier summers in southern California may retard further invasion, while wetter summers may favor the spread of fountain grass.
Key Words: California • environmental effects • fountain grass • invasiveness • Pennisetum setaceum • phenotypic plasticity • Poaceae • precipitation
Invasion success may result from both genetic and environmental factors that allow a species to survive, establish, and spread in a new habitat (Sakai et al., 2001
; Kolar and Lodge, 2001
; Lee, 2002
; Lambrinos, 2004
; Maron et al., 2004
; Hierro et al., 2005
). A number of studies have examined the relationship of molecular genetic diversity and invasiveness of species (reviewed in Bossdorf et al., 2005
), but in many species, variation of neutral genetic loci may be more limited than quantitative genetic trait variation (e.g., Merilä and Crnokrak, 2001
; Reed and Frankham, 2001
; McKay and Latta, 2002
; Streisfeld and Kohn, 2005
). Populations with low variation in molecular analyses, using presumably neutral markers, may still have extensive ability to adapt to novel environments, because of greater variation in traits related to fitness (McKay and Latta, 2002
). Traits related to reproduction, such as number of seeds, may be particularly important in determining the invasiveness of a species in a particular habitat (Levine, 2000
). In apomictic species, seeds are produced asexually and result in progeny that are genetically identical to the maternal plant, but some genetic variation may occur because few apomictic species are completely asexual (Richards, 1997
). The success of several apomictic species as invasive species has been presumed to be associated with high levels of phenotypic plasticity and a general-purpose genotype (Baker, 1965
). Even limited genetic variation in fitness-related traits, however, could explain differential invasiveness of apomictic species.
Pennisetum setaceum (Forsk.) Chiov. (fountain grass; Poaceae) is an apomictic, wind-dispersed, perennial, C4 bunch grass native to North Africa and the Middle East, primarily along arid coastal areas and in the Sahara (Williams et al., 1995
). It has also been reported as native to parts of tropical Africa (Eritrea, Ethiopia, Somalia, Sudan, Kenya, Tanzania, Zambia, and Zimbabwe; USDA-ARS, 1997
). Little has been written about its ecology in its native range, and the ecology of fountain grass in Hawaii is better known. Williams et al. (1995)
reported that fountain grass has a larger altitudinal distribution in Hawaii than in its native habitat, and fountain grass is now the dominant plant on dry lava flows and in dry shrubland on the arid side of the island of Hawaii (Williams and Black, 1994
). When fountain grass from Hawaii was grown in the greenhouse, different watering treatments resulted in significant differences in biomass (Williams and Black, 1994
). More complex interactions between nutrient levels, competition between species, and the effect of water on fountain grass biomass can also occur (Carino and Daehler, 2002
).
Fountain grass has successfully invaded a wide range of habitats worldwide and is present in several areas of the United States, including Hawaii, Arizona, and California (Lovich, 2000
; Williams et al., 1995
). Pennisetum setaceum was introduced as an ornamental into all three states within a relatively short time frame between 1917 and 1940 (Robbins, 1940
; van Devender et al., 1997
; Cabin et al., 2000
), but it is not equally invasive in these three states. In the Hawaiian Islands, P. setaceum is an extremely noxious weed because it is an early aggressive colonizer of lava fields and dry forests on the island of Hawaii, and this has led to the destruction of native communities by increasing fire frequency and by limiting germination, survival, and growth of native dry forest species (D'Antonio and Vitousek, 1992
; Williams et al., 1995
; Daehler and Carino, 1998
; Cabin et al., 2000
; Cabin et al., 2002
). In Hawaii Volcanoes National Park, 20 235–40 470 hectares are covered with fountain grass, including young lava flows (Obey, 2005
). In Arizona, it is prevalent and spreading quickly, with noticeable expansion of patches between 1998 and 2002 in Saguaro National Park, (D. Foster, Saguaro National Park, personal communication). In 2004, after concerted control efforts, about 109 acres in the park remained infested with fountain grass (National Park Service, 2004
). Pennisetum setaceum in southern California is usually confined to roadsides and ruderal areas and has not aggressively invaded natural habitats (J. Poulin, unpublished data). Thus, P. setaceum has a range in invasiveness across Hawaii (widespread noxious weed), Arizona (rapidly spreading), and southern California (limited in distribution).
Earlier molecular surveys using inter-simple sequence repeats (ISSRs) of fountain grass showed no genetic variation of molecular markers using 16 primers across 20 populations in these three states (Poulin et al., 2005
), but levels of variability in quantitative traits associated with life history and reproduction in these populations are unknown. If P. setaceum plants from these three areas grown in a common garden are also similar in quantitative traits, particularly in traits related to fitness, then variation in invasiveness among the three areas may result solely from phenotypic plasticity, with variation in fitness traits associated with environmental rather than genetic differences.
Differences in patterns of rainfall and temperature in general are critical factors in determining plant distribution and abundance (Stephenson, 1990
) and may affect fecundity and other traits affecting invasion in P. setaceum, leading to the differences in invasion success across the three states. In many Hawaiian habitats, temperatures are warm throughout the year, with more precipitation in the winter than summer, but with highly variable patterns of rainfall from month to month and year to year (Fig. 1). Pennisetum setaceum is apparently able to take advantage of rainfall with higher photosynthetic rates whenever there is enough precipitation (Williams and Black, 1994
). Seasonal rains in Arizona and southern California suggest different patterns (Fig. 1). In southern California, precipitation normally occurs only during the winter when temperatures may be too low for optimal growth and reproduction of this species. Arizona has light winter rains, but summer monsoons occur at the warmest time of year when both temperature and precipitation are greater than in coastal southern California.
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MATERIALS AND METHODS
Study system
Pennisetum setaceum spikelets each contain one flower that may produce a single caryopsis retained in the spikelet (hereafter, we refer to the caryopsis as the seed). Mature spikelets were collected from three natural populations in each of three states (Arizona, California, Hawaii; Table 1). Spikelets were identified by the maternal plant in the field for all but one population (seeds from Kaupulehu [HI-1] were mass-collected and used in only Experiment 2). Approximately 200 spikelets per pot were planted in 10-cm2 community pots in the UCI greenhouse, and pots were watered daily and not fertilized until after seedling emergence (see Poulin et al. [2005
] for detailed descriptions of growing conditions). Seedlings were transplanted to individual 5-cm2 pots 3–4 wk after planting; plants were watered as needed and fertilized monthly. A single seedling from each of 10 maternal field plants in each of the nine populations was chosen for the experiments when possible (Table 1). In September 2003, all plants were transplanted into 10-cm2 pots. Seeds from these plants were collected as the inflorescences matured for the second experiment. Plants were cloned for the first experiment in March 2004 after these seeds were collected.
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In the arboretum, plants in the fenced plot were in full sunlight, free from competition. Vegetation was removed from the experimental plot 3 weeks before planting by spraying with glyphosate (Monsanto, St. Louis, Missouri, USA), a systemic herbicide, and remaining weeds were pulled by hand. Each fountain grass plant was transplanted into the center of a 1-m2 plot on 25 March 2004. Root balls did not expand beyond a 0.5-m diameter during the experiment, thus limiting the effects of competition. Arboretum plants were watered daily for 2 wk and every other day for a third week; the plants did not have supplemental water or natural rainfall after this period. The plot was weeded weekly to limit competition. All plants in the arboretum survived the duration of the experiment.
In the greenhouse, plants were deliberately kept pot-bound and additional shading resulted in low light levels after the plants were divided. Plants were watered when dry, with minimal fertilization (20 : 20 : 20 complete fertilizer 1.5 t/3.79 L water every other mo; occasionally, once per mo) to encourage flowering (Goergen and Daehler, 2001
). Plants were rotated weekly to limit position effects. One of the 75 plants in the greenhouse died before flowering.
In the arboretum, inflorescences were removed before flowering for 5 wk to ensure that any spikelets collected had been produced in the experimental environment. Plants were allowed to flower beginning May 2004, and the first two inflorescences of each plant were allowed to mature to collect data on seed production. Subsequent inflorescences were counted and removed before seeds matured to prevent the spread of fountain grass. As a consequence, the number of inflorescences recorded is the number produced when plant resources were not being used for seed production, but this bias applied to all plants. In the arboretum, inflorescences were collected and counted from late May to late July (peak in mid June). In the greenhouse, flowering was delayed until fall 2004 because of poor growth conditions. Seeds were collected from two mature inflorescences per plant, and subsequent inflorescences were counted and removed before maturation from mid December–mid February 2005.
The number of flowers per inflorescence was determined by counting the number of spikelets on each of two inflorescences per plant, and the number of seeds per inflorescence was determined after removing the glume, palea, and lemma of each spikelet on these inflorescences. These measures allowed us to calculate the mean number of seeds per flower and the total number of seeds per plant.
Germination rates were determined for seeds from plants grown in the arboretum. Ten intact, presumably viable seeds (based on seed size, shape, and color) from each plant were planted individually in 5-cm2 pots in mid March 2005 and were misted regularly. The majority of seeds germinated in 6 d, and no germination was observed after 16 d. Germination rates were not calculated for greenhouse plants because many of these plants did not produce seeds.
Specific leaf area, aboveground biomass (excluding inflorescences) and belowground biomass were also measured to estimate performance of plants. Specific leaf area (SLA, mm2 one-sided leaf area/mg dry mass) is often a good estimate of relative growth rate and is a common estimator of maximum photosynthetic rate (Grotkopp et al., 2002
; Cornelissen et al., 2003
). To measure SLA, two young, fully expanded leaves were cut just below the ligule on each plant and were immediately placed on ice until they were weighed and scanned for leaf area (ADC Bioscientific Area Meter AM200, Hoddesdon, Herts, United Kingdom) within 1 h of removal. Scanned leaves were air-dried for 4 wk, oven-dried for 48 h at 60°C, and weighed. Greenhouse plants were not measured for SLA because of the poor growth conditions and lack of new leaves after production of inflorescences.
Aboveground biomass was harvested by cutting plants at the soil level (arboretum plants harvested 20–21 July 2004; greenhouse plants harvested 8 March 2005). Plants from both environments were air-dried for 3–6 wk, oven-dried for 48 h at 60°C, and weighed. Belowground biomass was measured by excavating complete root balls, including fine roots, from all arboretum plants. Roots were washed, air-dried for 8 wk, oven-dried for 48 h at 60°C, and weighed. Root mass was not measured for greenhouse plants because all plants were completely pot bound.
Experiment 2
Methods
Seeds were collected from each of the greenhouse plants grown from field-collected seeds and planted individually in 5-cm2 pots in the UCI greenhouse in late April 2004. Trays were rotated every other day to limit position effects. Seedlings were initially grown under optimal moisture conditions to promote germination and to provide enough plants for out-planting. Pots were misted regularly for 2 wk and then watered as needed. Seedlings were not fertilized. Three seedlings were selected from each of 73 maternal families (N = 25 maternal families each from Arizona [AZ] and California [CA]; N = 23 families from Hawaii [HI]). Four weeks after planting, seedlings were moved to an outdoor lath house where plants were placed under shade cloth and gradually acclimated to full sun.
Seedlings were planted into a fenced plot (17 x 28 m) at the UCI Arboretum at the end of May 2004 (seedlings were about 6 wk old). Plot preparation, watering, and spacing of plants were similar to procedures in the first experiment. In June 2004, three watering treatments were initiated: natural California summer rain (NC), simulated summer monsoons (M), and simulated light rain (LR). The NC treatment received no supplemental watering or rain after the establishment period; any additional moisture for these plants came from morning dew and fog. The M treatment was intended to simulate heavy rain similar to Arizona's summer monsoons. During an average summer, Arizona receives an average of 10 monsoon storms between late June and late September (14 wk), with an average of 1.4 cm of rain per storm (precipitation data from 1994–2003; National Oceanic and Atmospheric Administration, 2005
). Monsoon treatments consisted of 1.4 cm of rain every 10 d for a total of 10 storms during the 14 wk period. The LR treatment supplied the same total amount of water across the summer season as the monsoon treatment, but in smaller amounts (0.5 cm) at more frequent intervals (twice weekly). This treatment provided a contrast to the monsoon treatment to distinguish the relative importance of the evenness of rainfall on plant growth and reproduction.
Each plant receiving supplemental watering was encircled with irrigation tubing, and water from the irrigators remained in a 0.25-m2 circle around the plant both on surface soil and at depth, leaving a 0.5-m buffer zone between plants. Precipitation volumes were calculated based on volume per 1-m2 plot (1.4 cm rainfall = 13.7 L/m2; 4 emitters x 1.89L/h = 7.56L/hour precipitation rate per watered plant), and the amount of water per ‘storm' was controlled by the duration of irrigation.
The first inflorescences formed in early July, and most spikelets matured and were ready to disperse in early August (14–15 wk after germination). After two inflorescences were harvested for measurements, additional inflorescences were counted and removed before seeds matured to prevent dispersal of P. setaceum into surrounding natural areas. Seeds per flower (proportion of flowers setting seed), seeds per inflorescence, total number of seeds per plant, SLA, aboveground biomass (N = 219 plants for these traits), and belowground biomass (N = 81) were determined as in Experiment 1. Aboveground biomass and belowground biomass were harvested in early October 2004.
To examine possible maternal effects resulting from watering treatment, a random subset of intact seeds from each plant in the three watering treatments was weighed individually (N = 1308 seeds) and planted in a common greenhouse environment in June 2005. Germination rates, SLA after 7 mo, and aboveground biomass after 9 mo were recorded for each plant.
Analysis
Experiment 1
In initial analyses, for each environment, a one-way, nested ANOVA (PROC MIXED, SAS Institute 2002–2003
; version 9.1, Cary, North Carolina, USA) was used to compare measures of plants from all states. In each environment, state of origin was a fixed effect, and population (nested within state of origin) was a random effect. Variance components of random effects were estimated using restricted maximum likelihood. Because the population effect was not significant for any of the traits, population was dropped from the model and a one-way ANOVA with state of origin as a fixed effect was used for all traits (Proc GLM; SAS). All data were normally distributed. Significance levels were adjusted using Bonferroni corrections (adjusted alpha = 0.007 for seven comparisons in the arboretum; 0.0125 in the greenhouse for four comparisons).
Performance measures of plants from both the greenhouse and the arboretum were compared using paired samples t tests (df = 73 for all comparisons except shoot mass, where df = 70).
Analysis
Experiment 2
In initial analyses using a mixed-effects nested ANOVA in PROC MIXED (SAS) with watering treatment and state of origin as fixed effects and population (nested within state of origin) as a random effect, factors involving population (population nested within state and interaction of population within state by treatment) were not significant for any traits. As a result, population was dropped from the model, and a two-way ANOVA with watering treatment and state of origin as fixed effects was used (PROC GLM, SAS). Most traits were normally distributed, with the exception of SLA, root mass, and shoot mass. These data were also analyzed using ANOVA despite the lack of normality, because data transformation accentuated non-normality, and because ANOVA is reasonably robust to departures from normality (Sokal and Rohlf, 1995
). Posthoc comparisons between watering treatments and between state of origin were used. Bonferroni corrections were applied (adjusted alpha of 0.003 with six traits). As a multivariate way of describing the differences between the states or between watering treatments, these differences were also investigated using MANOVA and canonical discriminant analysis. To avoid related traits (number of seeds per flower, number of seeds per inflorescence, and total number of seeds per plant), we used the total number of seeds per plant as well as SLA, aboveground biomass, and root biomass in these analyses.
Maternal effects were examined in the progeny of the plants grown in the three watering treatments. The mean progeny values for each maternal plant in the watering treatment were used for traits (seed mass, percentage germination, number of inflorescences, SLA, and aboveground vegetative biomass) to avoid pseudoreplication. A two-way ANOVA was used, with state of origin and watering treatment as fixed effects (Proc GLM), after initial analyses again indicated that population was not significant for any trait. Posthoc comparisons between the watering treatments of the mothers were used.
RESULTS
In general, plants from different states did not differ from each other within environment for the quantitative traits measured. The results of these experiments also indicated a large range in phenotypic plasticity in the greenhouse, arboretum, and in different watering treatments, with the second experiment in general demonstrating increased growth and reproduction with increased precipitation during warmer summer months. The differences resulting from watering treatments did not carry over as maternal effects into the next generation.
Experiment 1
Plants from different states were similar to each other in all traits measured in both the arboretum (Arb) and greenhouse (GH; Fig. 2; in both Arb and GH: seeds per flower, number of seeds per inflorescence, total number of seeds per plant, and aboveground biomass; in Arb: specific leaf area, belowground biomass, and percentage germination). As anticipated, values for all traits differed greatly between the two environments (Fig. 2, paired t tests not shown).
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). Using the adjusted alphas, 1524 and 1701 plants would be needed for the greenhouse and arboretum, respectively.
Experiment 2
There were no significant interactions between state of origin and watering treatment for the traits measured (seeds per flower, seeds per inflorescence, total number of seeds, SLA, or aboveground and belowground biomass; Table 2), with one exception. Aboveground biomass showed a significant interaction between state and watering treatment (Table 2), where unwatered plants from Hawaii had relatively greater aboveground biomass than plants from other states.
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).
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DISCUSSION
Phenotypic plasticity associated with environmental differences, rather than adaptations through genetic differences reflected by the state or population of origin, appears to be the most significant factor in determining the invasiveness of fountain grass in California, Arizona, and Hawaii. With one exception, we found no evidence for potential genetic differentiation of plants from different states in phenotypic traits related to performance.
The similarity among plants from different states in traits related to fitness is consistent with earlier work on these populations showing a lack of variation among states in molecular markers (Poulin et al., 2005
). Although traits related to fitness may have greater variation than the variation found in neutral molecular markers (McKay and Latta, 2002
), this does not appear to be the general case in fountain grass. Differences in performance of plants in the greenhouse, arboretum, and under different watering treatments were related to phenotypic plasticity. There was a significant interaction of state and watering treatment in only one case, with the Hawaiian plants having relatively greater aboveground biomass in the treatment without supplemental watering. This interaction suggests that the Hawaiian plants may differ genetically from plants from other states, but expression of this ability to produce greater aboveground biomass is only evident in the driest summer environment. The extremely strong response to differences in watering treatment and the lack of difference in aboveground biomass by state suggests that further studies may be needed to understand the significance of this interaction.
Fountain grass showed an extremely strong response to greater water availability during the warm summer season, whether the precipitation occurred regularly as light rains (LR treatment) or infrequently as heavy monsoonal rains (M treatment). In water-limited systems, pulses of precipitation such as the monsoon treatment may trigger biological activity in ways different from more evenly distributed precipitation (Schwinning and Ehleringer, 2001
). In this experiment, fountain grass responded to greater availability of moisture, independent of the timing of its occurrence. These results showing a strong response to precipitation are consistent with earlier studies comparing P. setaceum in Hawaii to a native bunch grass (Heteropogon contortus) and showing that fecundity and invasiveness are strongly affected by the amount of precipitation (Goergen and Daehler, 2001
, 2002
).
The response of fountain grass to increased water availability during the summer in southern California suggests a possible mechanism controlling variable invasiveness in these three regions. In southern California, the absence of natural rainfall during the warm summer months may limit seed production and growth of this species. Both southern California and Hawaii have similar high average summer temperatures at these study sites (Fig. 1), but in southern California rainfall during the summer is extremely rare, while in Hawaii precipitation occurs throughout the year. Arizona receives the majority of its annual precipitation during the summer through monsoonal rains. Although southern California has greater total precipitation than Arizona (Fig. 1), rainy season temperatures (both highs and lows) are much cooler at the study sites in southern California (see Fig. 1 for high temperatures; lows in California are 12°C cooler than Hawaii, 17°C cooler than Arizona; National Oceanic and Atmospheric Administration, 2005
). In southern California, the cooler temperatures during the wetter months and lack of rain during the warmer summer months may put this C4 grass at a distinct disadvantage, possibly explaining why fountain grass is less invasive in this region. In general, summer rainfall favors C4 grasses such as P. setaceum (Knoop and Walker, 1985
; Ehleringer et al., 1991
). Supplemental watering treatments during the summer resulted in dramatic increases for both growth and seed production, variables often related to invasiveness (Maillet and Lopez-Garcia, 2000
; Grotkopp et al., 2002
). Plants in the supplemental water treatments produced more than twice as many seeds as plants without additional watering. Seeds and seedlings produced by plants in different watering treatments did not differ in seed mass, germination, specific leaf area, or aboveground vegetative biomass, suggesting that there were few effects of maternal environment on the quality of seeds produced by those plants, despite the large differences in the quantity of seeds produced.
Phenotypic plasticity can allow invasive species that have little or no genetic variation to spread across broad environmental gradients (Parker et al., 2003
). Pennisetum setaceum from Hawaii has highly plastic responses to a variety of environmental factors (Williams and Black, 1993
, 1996
; Williams et al., 1995
). Our common garden studies are consistent with results of Williams and Black (1996)
showing that plants from populations across a coastal to subalpine altitudinal gradient responded to nutrient supplementation with greater aboveground biomass, height, and number of inflorescences. Reciprocal transplants across this same altitudinal gradient showed large effects of site with no evidence of local adaptation, and plants from all populations were similar in performance traits related to fitness (Williams et al., 1995
). The ability to survive across such a large altitudinal gradient with variable moisture and nutrient regimes seems wholly based on phenotypic plasticity and broad ecological tolerance in P. setaceum (Williams and Black, 1993
). At a much larger geographic scale, our results are very similar to those of Williams and Black (1993)
and are also consistent with a lack of molecular differences among fountain grass from different states (Poulin et al., 2005
). Such broad ecological tolerance and tolerance of suboptimal conditions are likely to be important factors promoting invasiveness in this species. These long-lived perennials are able to withstand extreme environmental stress and are able to quickly take advantage of more optimal conditions (Williams and Black, 1993
; Goergen and Daehler, 2002
). In our study, all plants in the unwatered treatment survived through the entire experiment and most were also able to produce viable seeds, albeit in smaller numbers.
Global climate change also may affect invasion by P. setaceum in Arizona, Hawaii, and particularly in southern California where P. setaceum is currently less invasive. Global change models indicate that temperature and precipitation patterns over the next several decades will change, but the direction and the amount of change depend on many factors (Hayhoe et al., 2004
). Drier summers in southern California may retard further invasion, while wetter summers may favor the spread of fountain grass.
FOOTNOTES
1 The authors thank S. Cordell for seeds from Hawaii and use of unpublished data; D. Foster for providing access to Arizona field sites and information on fountain grass invasion in Arizona; T. Arakelian, M. Behbehani, E. Hayes, N. Nguyen, M. Poulin, M. Sargious, M. Subelsky, and E. Surya for assistance with greenhouse and experimental garden work; F. Boonstra, T. Ng, T. Dang, P. Dang, S. Hareesh, J. Kutaka, C. Lucas, P. Ngo, and Q.-P. Tran for help weighing seeds and harvesting plants; R. Basile for irrigation advice and installation; W. Yang and R. Basile for greenhouse care; D. Campbell, B. Hawkins, and K. Suding for advice on experimental design; Trader Joe's for supplies; and the UCI Arboretum for providing the experimental garden site. They gratefully acknowledge support to J.P. from Sigma Xi and GAANN (U.S. Dept. of Education, Graduate Assistance in Areas of National Need) and to T.N. from the UCI Undergraduate Research Opportunities Program. 
2 Author for correspondence (e-mail: aksakai{at}uci.edu
)
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