Aboveground plant community and seed bank composition along an invasion gradient

Invasive plants can reduce plant diversity and abundance in native grassland communities; however, the effect on the native seed bank is less clear. The objective of this study was to assess the effects of invasion by the exotic grass old world bluestem (OWB; Bothriochloa spp.) on native aboveground plant species composition and seed bank diversity and abundance (i.e., cover, density). In this central Great Plains grassland, OWB invasion had differential effects on native diversity and abundance of both aboveground and seed bank plant communities. Native plant species diversity and cover showed a steep decline as OWB cover increased. No change in native seed density or richness was observed in response to OWB invasion, however, OWB seed density increased with increasing invasion, thus increasing total seed density. Our results indicate that as OWB invasion increases, the native plant community decreases in diversity and abundance. Although, no effect on native seed bank diversity and density was observed in this study, as native seeds are lost through a loss of native species in the plant communities, native seed bank diversity and density is expected to decline.     

Feeding preferences of Melanoplus femurrubrum grasshoppers on native and exotic grasses: behavioral and molecular approaches

Generalist insect herbivores, such as grasshoppers, may either avoid feeding on exotic plants, potentially enabling these plants to become invasive in the introduced range, or insects may incorporate exotic plants into their diet, contributing to the biotic resistance of native communities and potentially preventing plant invasions. Accurate determination of insect diet preferences with regard to native and exotic plants can be challenging, but this information is critical for understanding the interaction between native herbivores and exotic plants, and ultimately the mechanisms underlying plant invasions. To address this, we combined behavioral and molecular approaches to accurately compare food consumption of the polyphagous red‐legged grasshopper, Melanoplus femurrubrum (De Geer) (Orthoptera: Acrididae), on native [Andropogon gerardii Vitman and Bouteloua curtipendula (Michx.) Torr.] and exotic, potentially invasive grasses [Miscanthus sinensis Andersson and Bothriochloa ischaemum (L.) Keng] (all Poaceae). We found that M. femurrubrum grasshoppers demonstrated strong feeding preferences toward exotic grasses in experiments with intact plants under both field and greenhouse conditions, but they showed no preference in experiments with clipped leaves. Additionally, we sampled the gut contents of M. femurrubrum collected in the field and identified the ingested plant species based on DNA sequences for the non‐coding region of the chloroplast trnL (UAA) gene. We found that exotic plants were prevalent in the gut contents of grasshoppers collected at study sites in Ohio and Maryland, USA. These results suggest that the generalist herbivore M. femurrubrum does not avoid feeding on exotic grasses with which they do not share coevolutionary history. In addition, by demonstrating greater food consumption of exotic plants, these grasshoppers potentially provide biotic resistance should these grasses escape cultivation and become invasive in the introduced range.     

Changes in growth of two C4 grasses (Schizachyrium scoparium and Paspalum plicatulum) in monoculture and mixture: influence of soil depth

The growth of Schizachyrium scoparium (Michx.) Nash (little bluestem) and Paspalum plicatulum Michx. (brownseed paspalum), two C₄ grasses, was examined in monoculture and in mixed culture in three soil depths. Effects of soil depth and competition were both significant. Growth of both species increased as soil depth increased from 30 to 180 cm in pot experiments. However, P. plicatulum dry mass increased approximately four times more than S. scoparium as soil depth increased. Schizachyrium scoparium dry mass was always greater in monoculture while P. plicatulum dry mass was always greater in mixture. Significant interspecific competition was not measured in the shallow soil treatment (30 cm) suggesting equal competition. Significant interspecific competition was measured in the deeper soil treatments (90 and 180 cm), and P. plicatulum dry mass was clearly greater in mixtures in the two deeper soils. Paspalum plicatulum does not appear to prevent the growth of S. scoparium roots in the deeper soil, but seems able to exploit deeper soil resources by proliferating roots at depths where S. scoparium roots do not seem capable of growth.     

Invasive warm-season grasses reduce mycorrhizal root colonization and biomass production of native prairie grasses

Soil organisms play important roles in regulating ecosystem-level processes and the association of arbuscular mycorrhizal (AM) fungi with a plant species can be a central force shaping plant species' ecology. Understanding how mycorrhizal associations are affected by plant invasions may be a critical aspect of the conservation and restoration of native ecosystems. We examined the competitive ability of old world bluestem, a non-native grass (Caucasian bluestem [Bothriochloa bladhii]), and the influence of B. bladhii competition on AM root colonization of native warm-season prairie grasses (Andropogon gerardii or Schizachyrium scoparium), using a substitutive design greenhouse competition experiment. Competition by the non-native resulted in significantly reduced biomass production and AM colonization of the native grasses. To assess plant-soil feedbacks of B. bladhii and Bothriochloa ischaemum, we conducted a second greenhouse study which examined soil alterations indirectly by assessing biomass production and AM colonization of native warm-season grasses planted into soil collected beneath Bothriochloa spp. This study was conducted using soil from four replicate prairie sites throughout Kansas and Oklahoma, USA. Our results indicate that a major mechanism in plant growth suppression following invasion by Bothriochloa spp. is the alteration in soil microbial communities. Plant growth was tightly correlated with AM root colonization demonstrating that mycorrhizae play an important role in the invasion of these systems by Bothriochloa spp. and indicating that the restoration of native AM fungal communities may be a fundamental consideration for the successful establishment of native grasses into invaded sites.     

Effects of Phenology at Burn Time on Post‐Fire Recovery in an Invasive C4 Grass

The spread of non‐indigenous, C₄ grasses threatens global conservation of savannas and subtropical grasslands. Identifying control methods to selectively target these invasives has proven difficult. Here, we tested the hypothesis that the effectiveness of prescribed burns for control is determined, in part, by the phenology of the target species at burn time. We conducted two experiments in a subhumid, C₄ grassland in central Texas. The focal invasive was the C₄, perennial bunchgrass Bothriochloa ischaemum (L.) Keng (KR bluestem). Burns were conducted in early and late fall when plants were in different phenological states. In addition, we attempted to manipulate phenological state through temporary rainout shelters to expedite maturation. The two experiments differed in the timing of the rainout shelter application (experiment 1: May to July, experiment 2: August and September), but otherwise had the same complete factorial design (burn time × shelter). Across experiments, when at least 50% of all tillers were pre‐reproductive at burn time, either due to shelter treatment or time of year, spring tiller densities were significantly lower than when plants were burned in a more advanced reproductive state. Trends in fall biomass generally followed trends in spring tiller densities, with one exception where plants in no‐shelter plots burned in October had lower biomass than expected based on tiller densities. Treatment responses for the native C₄ grass B. laguroides were consistent with those of B. ischaemum. These findings suggest that strategic burns can be used to reduce the subsequent recovery of invasive C₄ grasses while not disadvantaging native grasses.     

Productivity and Subordinate Species Response to Dominant Grass Species and Seed Source during Restoration

Grasses can be important regulators of species diversity and ecosystem processes in prairie systems. Although C₄ grasses are usually assumed to be ecologically similar because they are in the same functional group, there may be important differences among species or between seed sources that could impact restorations. I tested whether C₄ grass species identity, seed source, or grass species richness scales to influence aboveground net primary productivity (ANPP), resistance to weed invasion, or establishment of subordinate prairie species during restoration. Plots in western Iowa, United States, were planted with equal‐sized transplants of one of five common grass species (Panicum virgatum L., Sorghastrum nutans (L.) Nash, Andropogon gerardii Vitman, Schizachyrium scoparium (Michx.) Nash, and Bouteloua curtipendula (Michx.) Torrey) either from local seed or from cultivar seed sources. These plots were compared to plots containing all five species in mixture and to nonplanted plots. Differences in ANPP were found among species but not between cultivars and noncultivars or between monocultures and mixtures. Panicum virgatum, S. nutans, and S. scoparium were more productive than A. gerardii and B. curtipendula. Weed invasion was much higher when plots were not planted with grasses. Schizachyrium scoparium allowed greater establishment of subordinant prairie species than all other focal grass species. There were two separate mechanisms by which grasses suppressed prairie species establishment either (1) by growing tall and capturing light or (2) by quickly filling in bare space by spreading horizontally through rhizome growth in short species. These results suggest that high ANPP can be found with noncultivar plantings during the first 2 years after planting and that subordinate species establishment is most likely when shorter bunchgrasses such as S. scoparium are dominant.     

Biomass production and relative competitiveness of a C3 legume and a C4 grass co-dominant in the semiarid Loess Plateau of China

Bothriochloa ischaemum L. and Lespedeza davurica (Laxm.) Schindl. are two co-dominant species of great importance in reducing soil and water loss and maintaining the distinctive natural scenery of the semiarid Loess Plateau of China. Our aim was to determine the growth and interspecific competition between these species under water stress to facilitate the prediction of community succession and guide the selection of appropriate methods of conservation and use in the area. A pot experiment was designed to investigate the effects of water stress and competition on biomass production and allocation, relative competitive ability and water use efficiency of the two species. Bothriochloa ischaemum (a C₄ perennial herbaceous grass) was planted in the same pot with L. davurica (a C₃ perennial leguminous subshrub) at density ratios of 12:0, 10:2, 8:4, 6:6, 4:8, 2:10, and 0:12. The response of the species to their mutual presence at the different ratios was evaluated at three levels of soil moisture (80%?±?5% field capacity, FC (HW), 60%?±?5% FC (MW) and 40%?±?5% FC (LW)). Indices of aggressivity (A), competitive ratio (CR) and relative yield totals (RYTs) were calculated from the dry shoot, root and total weight data. Water stress decreased the biomass production by both species whether in monoculture or mixture, but B. ischaemum was more sensitive to water deficit. Across moisture levels, the growth of L. davurica was more adversely affected by mixed planting. Bothriochloa ischaemum had significantly (P?<?0.05) smaller root:shoot ratios than L. davurica and the root mass of both species tended to increase relative to shoot mass as soil water deficit increased. The aggressivity (A), competitive ratio (CR) and relative yield totals (RYTs) of B. ischaemum were positive across treatments. Bothriochloa ischaemum had much higher CR under each water treatment, but water stress considerably reduced its relative CR while increasing that of L. davurica. The RYT values of the two species indicated some degree of resource complimentarity under both water sufficient and deficit conditions. Our results suggest that it is advantageous to grow the two species together to maximize biomass production. We recommend a mixture ratio of 8:4 of B. ischaemum to L. davurica because it gave significantly higher RYT and transpiration water use efficiency under deficit water conditions.     

Traits of an invasive grass conferring an early growth advantage over native grasses

Aims Invasive species often have higher relative growth rates (RGR) than their native counterparts. Nutrient use efficiency, total leaf area and specific leaf area (SLA) are traits that may confer RGR differences between natives and invasives, but trait differences are less prominent when the invasive species belongs to the same plant functional type as the dominant native species. Here, we test if traits displayed soon after germination confer an early size advantage. Specifically, we predicted that invasive species seedlings grow faster than the natives because they lack trade-offs that more strongly constrain the growth of native species.Methods We quantified plant morphological and physiological traits and RGR during early seedling growth at high and low nutrient levels in three dominant perennial native C₄ grasses: Panicum virgatum L. (switchgrass), Schizachyrium scoparium (Michx.) Nash (little bluestem) and Andropogon gerardii Vitman (big bluestem); and a perennial C₄ exotic invasive grass, Sorghum halepense (L.) Pers. (Johnsongrass).Important findings After 2 weeks of growth, Johnsongrass seedlings had greater biomass, SLA and photosynthetic nitrogen use efficiency, but lower leaf N concentrations (% leaf N) and root:shoot ratio than natives. As growth continued, Johnsongrass more quickly produced larger and thicker leaves than the natives, which dampened the growth advantage past the first 2 to 3 weeks of growth. Investment in carbon gain appears to be the best explanation for the early growth advantage of Johnsongrass. In natives, growth was constrained by an apparent trade-off between allocation to root biomass, which reduced SLA, and production of leaves with high N content, which increased carbon gain. In Johnsongrass, root:shoot ratio did not interact with other traits, and % leaf N was decoupled from RGR as a result of a trade-off between the positive indirect association of % leaf N with RGR and the negative direct association of % leaf N with RGR.     

Leaf gas exchange responses to abrupt changes in light intensity for two invasive and two non-invasive C4 grass species

Transient and steady state responses of leaf gas exchange (photosynthesis (A) and stomatal conductance to water vapor (g_s)) to marked changes in photosynthetic photon flux density (PPFD) were studied for two invasive [Cynodon dactylon (L.) Pers. and Sorghum halepense (L.) Pers.] and two non-invasive, native [Bothriochloa ischaemum (L.) Keng and Chrysopogon gryllus (Torn.) Trin.] perennial C₄ grass species from semiarid temperate grasslands or croplands. Following an abrupt drop in PPFD from 1300 to 270 μmol photon m⁻² s⁻¹, the two invasive species reduced g_s to a greater extent than A, resulting in higher intrinsic photosynthetic water use efficiency (PWUE = A/g_s) at low, compared to high-light conditions. For non-invasives, a comparable drop in g_s and A led to invariant PWUE, which was lower than that for the invasive group under low light. The duration and speed of stomatal closure was similar for the four species. However, unlike the other grasses, the noxious weed S. halepense exhibited a negligible net loss in PWUE during the high-to-low light transition. Responses of the native B. ischaemum were mostly intermediate between those of the two invasive species and the non-invasive C. gryllus, which is in agreement with the species’ ecological intermediacy: non-invasive but often reaches local dominance following a disturbance. With a sudden reverse change in PPFD photosynthetic light induction was not faster for invasives than for non-invasives. These results indicate more efficient water use under variable light for invasive compared to non-invasive perennial C₄ grasses which may contribute to their success in semiarid temperate habitats with a heterogeneous light regime. Yet, rapid photosynthetic light induction appears to be of less importance in such environments.     

Leaf gas exchange responses to abrupt changes in light intensity for two invasive and two non-invasive C4 grass species

Transient and steady state responses of leaf gas exchange (photosynthesis (A) and stomatal conductance to water vapor (g_s)) to marked changes in photosynthetic photon flux density (PPFD) were studied for two invasive [Cynodon dactylon (L.) Pers. and Sorghum halepense (L.) Pers.] and two non-invasive, native [Bothriochloa ischaemum (L.) Keng and Chrysopogon gryllus (Torn.) Trin.] perennial C₄ grass species from semiarid temperate grasslands or croplands. Following an abrupt drop in PPFD from 1300 to 270 μmol photon m^?2 s^?1, the two invasive species reduced g_s to a greater extent than A, resulting in higher intrinsic photosynthetic water use efficiency (PWUE = A/g_s) at low, compared to high-light conditions. For non-invasives, a comparable drop in g_s and A led to invariant PWUE, which was lower than that for the invasive group under low light. The duration and speed of stomatal closure was similar for the four species. However, unlike the other grasses, the noxious weed S. halepense exhibited a negligible net loss in PWUE during the high-to-low light transition. Responses of the native B. ischaemum were mostly intermediate between those of the two invasive species and the non-invasive C. gryllus, which is in agreement with the species’ ecological intermediacy: non-invasive but often reaches local dominance following a disturbance. With a sudden reverse change in PPFD photosynthetic light induction was not faster for invasives than for non-invasives. These results indicate more efficient water use under variable light for invasive compared to non-invasive perennial C₄ grasses which may contribute to their success in semiarid temperate habitats with a heterogeneous light regime. Yet, rapid photosynthetic light induction appears to be of less importance in such environments.     

Selective and Non-Selective Control of Invasive Plants: The Short-Term Effects of Growing-Season Prescribed Fire, Herbicide, and Mowing in Two Texas Prairies

Conservation of North American grasslands is hampered by the impact of invasive herbaceous species. Selective control of these plants, although desirable, is complicated by the shared physiology and phenology of the invader and the native components of the invaded plant community. Fortunately, there is evidence that some management practices, such as prescribed fire, herbicide, and mowing, can cause differential responses in native and invasive grassland species. However, timing of treatment is critical, and fire has been shown to increase rates of invasion when implemented during the dormant season. Bothriochloa ischaemum, an introduced C4 Eurasian grass is an increasing problem in grasslands, particularly in southern and central regions of North America. To date, there has been little success in effective selective control. Two invaded grassland sites representative of Blackland Prairie and Edwards Plateau ecoregions were subjected to two growing‐season prescribed fire treatments, single and double herbicide applications, and single and double mowing treatments. Mowing had no effect on either B. ischaemum or other dominant species at either site one‐year posttreatment. However, growing‐season fire and herbicide were both effective at reducing the abundance of B. ischaemum, with other codominant species responding either negatively to herbicide or neutrally or positively to fire. The vulnerability of B. ischaemum to growing‐season fire may be associated with the ecology of its native range. The negative growth response to growing‐season fire, combined with its lower implementation costs, indicates that this method warrants further investigation as a selective management tool for other problematic species in invaded grasslands.     

Effects of auxin concentration on induction and growth of embryogenic callus from young inflorescence explants of Old World bluestem (Bothriochloa spp.) and bermuda (Cynodon spp.) grasses

Objectives of this research were to test the effects of plant genotypes and auxin 2,4-D (2,4-dichlorophenoxyacetic acid) medium concentrations on embryogenic (E) callus production of two grass species. Two Old World bluestem,Bothriochloa ischaemum, accessions (A-8793 and A-8911c) and three bermudagrass,Cynodon dactylon (L.) Pers., accessions (A-10978b, A12164, and Brazos) supplied the explant material. Immature inflorescences 9 mm in length were placed on modified Murashige-Skoog (MS) agar medium containing 0, 1, 3, or 5 mg L^-1 of 2,4-D. Explants of all genotypes produced callus by the end of a 4-week dark incubation period at 25°C. When subcultured onto fresh media and maintained at 25°C with a 16 hr photoperiod, calli became embryogenic within 8 weeks of inoculation. Three mg L^-1 of 2,4-D in the media maximized E callus production in both bluestem genotypes and in A-10978b and A-12164 bermudagrass genotypes. Maximum E callus production from Brazos bermudagrass resulted from the 1 mg L^-1 treatment. Somatic embryos developed after subculture under light. Embryos showed scutellum-like structures and coleoptile-coleorhiza bipolar organization. Plantlets were regenerated from all genotypes except Brazos, whose embryoids failed to germinate. All callus from Brazos eventually senesced. Light and scanning electron microscopy confirmed regeneration through somatic embryogenesis.     

Effects of phosphorus on growth and competitive interactions of native and introduced species found in White Box woodlands

Abstract White Box (Eucalyptus albens Benth.) woodlands are among Australia's most endangered ecosystems and are threatened by exotic species invasion. There is evidence from other Australian communities that phosphorus enrichment can facilitate invasion, and differential growth of native and exotic species under increased phosphorus is a possible mechanism. Two glasshouse experiments were designed to test the following three questions relating to species responses to phosphorus: (i) do exotic and native species have different patterns of growth along a gradient of increasing phosphorus?; (ii) do exotic species have a greater competitive effect on native species than do conspecifics?; (iii) does phosphorus enrichment compound the competitive effect of exotic species on native species? Four native perennial species (Themeda australis (R. Br.) Staph., Bothriochloa macra (Steud.) S. T. Blake, Austrodanthonia racemosa (R. Br.) H. P. Linder and Eucalyptus albens) and two exotic annual species (Vulpia bromoides (L) Gray and Echium plantagineum L) were used. In the first experiment, plants were grown individually under six levels of soil phosphorus ranging from 0 to 60 p.p.m. In the second experiment, individuals of Eucalyptus albens and B. macra were grown alone, with a conspecific competitor, or with an exotic (V. bromoides or Echium plantagineum) competitor under low (10 p.p.m.) and high (100 p.p.m.) phosphorus. Both exotic species showed a greater positive response to increased phosphorus than the native species in experiment 1, and Eucalyptus albens seedlings grown with Echium plantagineum were significantly smaller than individuals grown alone or with Eucalyptus albens in experiment 2. There was no evidence that high phosphorus increased the competitive effect of the exotic species, but the combination of a strong positive response to phosphorus and a strong effect on growth of a native species indicates that phosphorus enrichment could favour exotic species in woodland remnants and that field studies testing the effect of phosphorus in a broader context would be appropriate.     

Competitive abilities of introduced and native grasses

Differencesin competitive ability may explain the maintenance of existing plantpopulationsand the invasion of new areas by plant species. We used field experiments toexamine the competitive responses of Agropyron cristatum(L.) Gaertn., an introduced C₃ grass, and Boutelouagracilis (HBK.) Lag., a native C₄ grass, and thecompetitive effects of Agropyron-dominated vegetation andsuccessional prairie. We also tested whether the outcome of competitiveinteractions varied with water availability. In each vegetation type,transplants of each species were grown under two levels of competition(presenceor absence of neighboring vegetation) and three levels of water availability(high, medium, or low). Transplant survival, growth, and biomass allocationpatterns were measured. Water availability had no effect on the measuredvariables, suggesting that both species were limited by another resource.Growthrates were affected more by competition, while survival and root: shoot ratiowere affected more by transplant species identity. In the successional prairie,neighboring vegetation suppressed the growth of Agropyrontransplants less than that of Bouteloua transplants,suggesting that Agropyron has a stronger ability to resistcompetitive suppression in that vegetation type. The spread ofAgropyron into surrounding vegetation may relate to itsability to resist competitive suppression. In theAgropyron-dominated vegetation, neighboring vegetationsuppressed the growth of both species by the same extent. However, competitionaccounted for more variation in transplant growth inAgropyron-dominated vegetation than in successionalprairie, suggesting that Agropyron has strong competitiveeffects which hinder plant growth and prevent other species from establishinginAgropyron fields.     

Growth, gas exchange, leaf nitrogen and carbohydrate concentrations in NAD‐ME and NADP‐ME C4 grasses grown in elevated CO2

Plants with the C₄ photosynthetic pathway have predominantly one of three decarboxylation enzymes in their bundle sheath cells. Within the grass family (Poaceae) bundle sheath leakiness to CO₂ is purported to be lowest in the nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME, EC 1.1.1.40) group, highest in the NAD-ME (EC 1.1.1.39) group and intermediate in the phosphoenolpyruvate carboxykinase (PCK, EC 4.1.1.32) group. We investigated the hypothesis that growth and photosynthesis of NAD-ME C₄ grasses would respond more to elevated CO₂ treatment than NADP-ME grasses. Plants were grown in 8-1 pots in growth chambers with ample water and fertilizer for 39 days at a continuous CO₂ concentration of either 350 or 700 µl l^?1. NAD-ME species included Bouteloua gracilis Lag. ex Steud (Blue grama), Buchloe dactyloides (Nutt.) Engelm. (Buffalo grass) and Panicum virgatum L. (Switchgrass) and the NADP-ME species were Andropogon gerardii Vittman (Big bluestem), Schizachyrium scoparium (Michx.) Nash (Little bluestem), and Sorghastrum nutans (L.) Nash (Indian grass). Contrary to our hypothesis, growth of the NADP-ME grasses was generally greater under elevated CO₂ (significant for A. gerardii and S. nutans), while none of the NAD-ME grasses had a significant growth response. Increased leaf total non-structural carbohydrate (TNC) was associated with greater growth responses of NADP-ME grasses. Decreased leaf nitrogen in NADP-ME species grown at elevated CO₂ was found to be an artifact of TNC dilution. Assimilation (A) vs intercellular CO₂ (C_i) curves revealed that leaf photosynthesis was not saturated at 350 µl l^?1 CO₂ in any of these C₄ grasses. Assimilation of elevated CO₂-grown A. gerardii was higher than in plants grown in ambient CO₂. In contrast, B. gracilis grown in elevated CO₂ displayed lower A, a trait more commonly reported in C₃ plants. Photosynthetic acclimation in B. gracilis was not related to leaf TNC or nitrogen concentrations, but A:C_i curves suggest a reduction in activity of both phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39). Some adaptation of stomatal functioning was also seen in B. gracilis and A. gerardii leaves grown in elevated CO₂. Our study shows that C₄ grasses have the capacity for increased growth and photosynthesis under elevated CO₂ even when water and nutrients are non-limiting. While it was the NADP-ME species which had significant responses in the present study, we have previously reported significant growth increases in elevated CO₂ for B. gracilis.     

Germination of grass species in soil affected by crude oil contamination

Many grass species exist in the oil exploration areas of North Dakota. The objective of this study was to evaluate seed germination of 65 grass species affected by crude oil. Germination of all species was reduced by crude oil, ranging from 4.3 to 100%. Twenty-eight species were tolerant, 29 moderately tolerant, 6 moderately sensitive, and 2 sensitive. Based on the tolerance levels, the following were used to further test the dose response to crude oil: strong creeping red fescue (Festuca rubra L. ssp. rubra), perennial ryegrass (Lolium perenne L.), orchardgrass (Dactylis glomerata L.), buffalograss [Buchloe dactyloides (Nutt.) Engelm.], little bluestem [Schizachyrium scoparium (Michx.) Nash], witchgrass (Panicum capillare L.), sand dropseed [Sporobolus cryptandrus (Torr.) Gray], Johnsongrass [Sorghum halepense (L.) Pers.], and smooth crabgrass [Digitaria ischaemum (Schreb.) Schreb. ex Muhl.]. The EC₅₀ of germination and biomass was effective in ranking the 9 species. Buffalograss, sand dropseed, and orchardgrass were ranked as the most tolerant species with EC₅₀ values of 0.1, > highest concentration tested, 0.05 m³ m^?3 (P < 0.05), respectively. Smooth crabgrass and little bluestem were ranked as most sensitive with EC₅₀ values of 0.03 and 0.04 m³ m^?3 (P < 0.05), respectively. Buffalograss showed the lowest germination (10.4%) and biomass reduction (25%) (P < 0.05).     

Ecological impacts of the invasive grass Sorghum halepense on native tallgrass prairie

Invasive plants frequently have competitive advantages over native species. These advantages have been characterized in systems in which the invading species has already become well established. Surprisingly, invader impacts on native communities currently undergoing invasion are lacking from most ecological studies. In this work we document and quantify shifting patterns in plant community structure in a native ecosystem (remnant tallgrass prairie) undergoing invasion by the invasive exotic Sorghum halepense (Johnsongrass). Further, we use manipulative field and greenhouse studies to quantify impacts of potential allelochemicals contained in whole-plant S. halepense leachates on growth of the dominant native grass, Schizachyrium scoparium (Little Bluestem), and tested the inhibitory effects of the potential soil legacy of S. halepense on the native grass in the greenhouse. Plant diversity indices revealed three distinct plant communities within the remnant prairie: a native community, a densely S. halepense invaded area, and a transitional zone between the two. Dominance of the native grass, determined by relative percent cover, significantly declined with increased S. halepense invasion via rhizomatous growth. Annual global positioning system monitoring of the S. halepense invasion front was used to quantify advancement into native prairie, documented at an average rate of 0.45 m year^?1. In the manipulative field and greenhouse studies, native S. scoparium treated with invasive S. halepense leachate had significantly less biomass and fewer inflorescences than control plants. These findings indicate the prolific clonal growth in conjunction with the plant chemistry of S. halepense play a significant role in displacement of the native grass.     

California native and exotic perennial grasses differ in their response to soil nitrogen,exotic annual grass density,and order of emergence

Early emergence of plant seedlings can offer strong competitive advantages over later-germinating neighbors through the preemption of limiting resources. This phenomenon may have contributed to the persistent dominance of European annual grasses over native perennial grasses in California grasslands, since the former species typically germinate earlier in the growing season than the latter and grow rapidly after establishing. Recently, European perennial grasses have been spreading into both non-native annual and native perennial coastal grass stands in California. These exotic perennials appear to be less affected by the priority effects arising from earlier germination by European annual grasses. In addition, these species interactions in California grasslands may be mediated by increasing anthropogenic or natural soil nitrogen inputs. We conducted a greenhouse experiment to test the effects of order of emergence and annual grass seedling density on native and exotic perennial grass seedling performance across different levels of nitrogen availability. We manipulated the order of emergence and density of an exotic annual grass (Bromus diandrus) grown with either Nassella pulchra (native perennial grass), Festuca rubra (native perennial grass), or Holcus lanatus (exotic perennial grass), with and without added nitrogen. Earlier B. diandrus emergence and higher B. diandrus density resulted in greater reduction in the aboveground productivity of the perennial grasses. However, B. diandrus suppressed both native perennials to a greater extent than it did H. lanatus. Nitrogen addition had no effect on the productivity of native perennials, but greatly increased the growth of the exotic perennial H. lanatus, grown with B. diandrus. These results suggest that the order of emergence of exotic annual versus native perennial grass seedlings could play an important role in the continued dominance of exotic annual grasses in California. The expansion of the exotic perennial grass H. lanatus in coastal California may be linked to its higher tolerance of earlier-emerging annual grasses and its ability to access soil resources amidst high densities of annual grasses.     

Relative herbivory tolerance and competitive ability in two dominant : subordinate pairs of perennial grasses in a native grassland

We evaluated herbivory tolerance and competitive ability within twodominant : subordinate pairs of C₄, perennial grasses at each of twosites to determine the contribution of these processes to herbivore-inducedspecies replacement. Herbivory tolerance was assessed by cumulative regrowthfrom defoliated plants of each species and competitive ability was evaluated byrelative uptake of a ¹⁵N isotope placed into the soil between pairedspecies in the field. Herbivory tolerance was similar for the dominant andsubordinate species in both plant pairs and defoliation intensity had a greaterinfluence on herbivory tolerance than did defoliation pattern. Both specieswithin the Sorghastrum nutans : Schizachyriumscoparium pairs exhibited comparable nitrogen acquisition from a¹⁵N enriched pulse with or without defoliation. In contrast,S. scoparium acquired more ¹⁵N than did itssubordinate neighbor, Bothriochloa laguroides when thisspecies pair was undefoliated. Uniform defoliation of this species pair at adefoliation intensity removing 70% of the shoot mass accentuated this responsefurther demonstrating the greater competitive ability of the dominant comparedto the subordinate species. Although the 90% defoliation intensity reducednitrogen acquisition by the dominant relative to the subordinate species,B. laguroides, it did not reduce nitrogen acquisition bythe dominant below that of the subordinate neighbor. The occurrence of similarherbivory tolerance among dominant and subordinate species indicates thatselective herbivory suppressed the greater competitive ability, rather than thegreater herbivory tolerance, of the dominant grasses in this experimentaldesign. These data suggest that interspecific competitive ability may be ofequal or greater importance than herbivory tolerance in mediatingherbivore-induced species replacement in mesic grasslands and savannas.