Native Grass Populations Research Articles

Implications of plant functional traits and drought survival strategies for ecological restoration

Abstract Restoration of degraded grasslands through active revegetation often involves re‐establishing populations of native grasses, which must withstand increasing drought stress to persist beyond initial establishment. In perennial species, superior dehydration tolerance is expected to result in more conservative growth, but this trade‐off has seldom been studied among populations of herbaceous species. We measured seasonal growth and foliar and root functional traits under non‐limiting water conditions, followed by recovery after severe drought in four populations of Elymus glaucus, a perennial grass from California's Mediterranean‐climate region. We hypothesized that populations from harsher sites would be more dehydration tolerant, summer dormant and resource‐conservative. Dehydration tolerance and summer dormancy were associated with a more conservative strategy (lower productivity, lower specific leaf area and specific root length), as well as earlier reproductive phenology. Multivariate trait variation was associated with water availability in both the growing season and summer, while greater dehydration tolerance and summer dormancy were associated with increasing summer climatic water deficit. Synthesis and applications. Our study provides evidence of an intraspecific trade‐off between dehydration tolerance and rapid resource acquisition. We discuss the implication that restored populations with superior drought survival may therefore be less competitive, and recommend further investigation to inform plant materials selection protocols and management practices.

Effects of fenced seed production areas and restoration treatments on the size and composition of the native grass seedbanks in moderately degraded rangelands in semiarid Australia

Seed production areas containing populations of native grasses were fenced on topographically high areas at two study sites in the semiarid rangelands of north-west New South Wales, Australia. The surrounding slopes were initially almost bare of pastorally preferred grasses. Three restoration treatments (control, mechanical pitting, and contour-aligned branch piles) were established on these surrounding slopes and the size and composition of the native grass seedbanks determined. A positive influence of the seed production areas mostly occurred within 15 m of the fence and was probably insignificant beyond 33 m at both sites. On a hard-setting red earth site, the size of the native grass seedbank in all three treatments increased over 2 years with the greatest increase under the piles of branches and smaller increases in the pitting and control treatments. The piles of branches preferentially incorporated seed of the pastorally preferred species Monachather paradoxus Steud into the seedbank whereas pits preferentially incorporated seed of the pastorally unpreferred species Aristida jerichoensis (Domin) Henrad. Evidence generally pointed to incoming seed rain rather than the seedbank as the main source of new seedlings. On a medium-textured lithosol site neither the pits nor the branches treatment was effective in enhancing either the size or composition of the seedbank; in fact the piles of branches resulted in a decrease in its size.

Revegetation with Australian native grasses – a reassessment of the importance of using local provenances

Many restoration guidelines strongly recommend the use of local sources of seed in native plant revegetation projects. These recommendations are based on assumptions that the species used for revegetation are cross-pollinated and woody, as they were developed for overstorey vegetation from the northern hemisphere. Their validity is challenged with respect to replacing or enhancing the native Australian grass component of degraded ecosystems. First, far from being the dominant pollination mechanism, obligatory cross-pollination has not been recorded in any Australian native grasses except for a few dioecious species. Indeed, the common Australian native grasses so far studied have revealed complicated breeding systems that provide the evolutionary resilience necessary for coping with the variable Australian climate as well as with future climate change. It is suggested that a key feature of this flexibility is polyploidy and its implications for sourcing seed are discussed. Second, it is argued that the genetic dissimilarity among populations of a species is not proportional to the distance between them but is more related to the environmental stresses that have been placed on those populations in the past. This is illustrated by different scales of ecotypic variation that are often trait-dependent. Evidence for this can be found in several translocation experiments, where populations of native grasses from a great distance away survive and often perform better than local populations. It is concluded that there is little justification for the recommendation that only local sources of seed of Australian native grasses should be used for revegetation projects, and particularly in large-scale programs. Instead, it is argued that studies on Australian native grasses provide clear evidence that distinct adaptive advantages may be gained by sourcing non-local provenance seed, which is matched to the environment of the revegetation site, and which contains appropriate stress tolerance genes, or by mixing populations from several locations to increase the genetic diversity of seed sources. Some general guidelines are provided for deciding whether or not to use local provenances depending on the purpose of the revegetation, the degree of environmental modification of the site and the characteristics of the species of choice.

Can Invasive Species Enhance Competitive Ability and Restoration Potential in Native Grass Populations?

Native plant individuals often persist within communities dominated by exotics but the influence of this exposure on native populations is poorly understood. Selection for traits contributing to competitive ability may lead to native plant populations that are more tolerant of the presence of exotic invaders. In this way, long‐term coexistence with an exotic may confer competitive advantages to remnant (experienced) native populations and be potentially beneficial to restoration. In past studies we have documented genetic differentiation within native grass populations exposed to the exotic invader Russian knapweed (Acroptilon repens). Here, we examine populations of a cool‐season grass, needle‐and‐thread (Hesperostipa comata [Trin. & Rupr.]) and a warm season, alkali sacaton (Sporobolus airoides [Torr.]) collected from Russian knapweed‐invaded sites and adjacent noninvaded sites to assess their relative competitive ability against a novel exotic neighbor, Canada thistle (Cirsium arvense). Experienced S. airoides (from within A. repens invasions) appear to better tolerate (accumulate biomass, leaf nitrogen content, and to initiate new tillers) the presence of a novel competitor (C. arvense). Experienced and inexperienced H. comata genets differ in their response to the presence of C. arvense. Relative neighbor effects of native grasses on C. arvense were generally greater from experienced grasses. The ability to compete with novel neighbors may be driven by general competitive traits rather than species‐specific coevolutionary trajectories. Irrespective of competitive mechanisms, the conservation of native species populations within weed invasions may provide an important restoration tool by retaining unique components of native gene pools selected by competitive interactions with exotics.

Native Perennial Grasses Show Evolutionary Response to Bromus tectorum (Cheatgrass) Invasion

Invasive species can change selective pressures on native plants by altering biotic and abiotic conditions in invaded habitats. Although invasions can lead to native species extirpation, they may also induce rapid evolutionary changes in remnant native plants. We investigated whether adult plants of five native perennial grasses exhibited trait shifts consistent with evolution in response to invasion by the introduced annual grass Bromus tectorum L. (cheatgrass), and asked how much variation there was among species and populations in the ability to grow successfully with the invader. Three hundred and twenty adult plants were collected from invaded and uninvaded communities from four locations near Reno, Nevada, USA. Each plant was divided in two and transplanted into the greenhouse. One clone was grown with B. tectorum while the other was grown alone, and we measured tolerance (ability to maintain size) and the ability to reduce size of B. tectorum for each plant. Plants from invaded populations consistently had earlier phenology than those from uninvaded populations, and in two out of four sites, invaded populations were more tolerant of B. tectorum competition than uninvaded populations. Poa secunda and one population of E. multisetus had the strongest suppressive effect on B. tectorum, and these two species were the only ones that flowered in competition with B. tectorum. Our study indicates that response to B. tectorum is a function of both location and species identity, with some, but not all, populations of native grasses showing trait shifts consistent with evolution in response to B. tectorum invasion within the Great Basin.

Inherited microbial symbionts increase herbivore abundances and alter arthropod diversity on a native grass

Some microbial symbionts of plants are maternally inherited and thus functionally increase genetic and phenotypic variation within plant populations. This variation, coupled with that of the host plant and environment, may alter abundances, diversity, and trophic structure of associated plant and animal communities. Fungal endophytes in the genus Neotyphodium are vertically transmitted, asexual microbial symbionts of grasses that remain asymptomatic and rely upon their hosts for resources and transmission via seeds, often providing benefits to their hosts, including protection against herbivores. Endophyte infections may influence associated arthropod communities in agronomic grasses, but the long-term effects of endophytes and variation in host genotype and resource availability on arthropod communities in native grass populations are unknown. We conducted a long-term field experiment with four maternal genotypes of an infected (E+) native grass (Festuca arizonica) from whence the endophyte was experimentally removed (E-) and water availability was controlled, to test the effects of infection, plant genotype, and resources on abundances, biomass, diversity (richness and evenness), and trophic structure of the arthropod community. Generally, E+ grasses harbored more arthropods, including more herbivores, predators, and detritivores, suggesting that the effects of endophytes cascaded upward through trophic levels in terms of abundances, at least in early ontogeny of the host. That E+ plants harbored more herbivorous insects than E- plants suggests that infection does not increase but instead decreases resistance to herbivores, contrary to prevailing concepts of endophytes as defensive mutualists. Infection did not alter overall species richness of the arthropod community or richness of herbivores but reduced natural enemy richness, especially that of parasites, and increased richness of detritivores. Reduced richness and shifts in evenness of natural enemies on E+ plants suggest that endophytes may disproportionately affect diversity at higher trophic levels and may partially explain increases in abundances of herbivorous insects on E+ plants. Biomass of predators, detritivores, and omnivores increased on plants with supplemented water, and arthropod and herbivore biomass varied by plant genotype. Symbiont-mediated phenotypic variation interacts with variation from plant genotype and environmental factors to alter arthropod abundances and diversity, and these effects shift with ontogeny of the host.

Asexual Endophytes in a Native Grass: Tradeoffs in Mortality, Growth, Reproduction, and Alkaloid Production

Neotyphodium endophytes are asexual, seed-borne fungal symbionts that are thought to interact mutualistically with their grass hosts. Benefits include increased growth, reproduction, and resistance to herbivores via endophytic alkaloids. Although these benefits are well established in infected introduced, agronomic grasses, little is known about the cost and benefits of endophyte infection in native grass populations. These populations exist as mosaics of uninfected and infected plants, with the latter often comprised of plants that vary widely in alkaloid content. We tested the costs and benefits of endophyte infections with varying alkaloids in the native grass Achnatherum robustum (sleepygrass). We conducted a 4-year field experiment, where herbivory and water availability were controlled and survival, growth, and reproduction of three maternal plant genotypes [uninfected plants (E-), infected plants with high levels of ergot alkaloids (E+A+), and infected plants with no alkaloids (E+A-)] were monitored over three growing seasons. Generally, E+A+ plants had reduced growth over the three growing seasons and lower seed production than E- or E+A- plants, suggesting a cost of alkaloid production. The reduction in vegetative biomass in E+A+ plants was most pronounced under supplemented water, contrary to the prediction that additional resources would offset the cost of alkaloid production. Also, E+A+ plants showed no advantage in growth, seed production, or reproductive effort under full herbivory relative to E- or E+A- grasses, contrary to the predictions of the defensive mutualism hypothesis. However, E+A+ plants had higher overwintering survival than E+A- plants in early plant ontogeny, suggesting that alkaloids associated with infection may protect against below ground herbivory or harsh winter conditions. Our results suggest that the mosaic of E-, E+A+, and E+A- plants observed in nature may result from varying biotic and abiotic selective factors that maintain the presence of uninfected plants and infected plants that vary in alkaloid production.

HERBIVORY MEDIATES GRASS–ENDOPHYTE RELATIONSHIPS

Endophytic fungi are plant symbionts living asymptomatically within plant tissues. Neotyphodium spp., which are asexual vertically transmitted systemic fungal endophytes of cool-season grasses, are predicted to be plant mutualists. These endophytes increase host plant resistance to environmental stresses and/or increase the production of alkaloid-based herbivore deterrents. The ubiquity of this defense mutualism is unclear, and a variety of alternative mechanisms may explain the observed variation in infection rates, levels of deterrence, and the maintenance of asexual endophytes in grass populations. We found that grass-endophyte interactions are variable and ordered along an herbivory gradient in an undisturbed subarctic alpine ecosystem. Native grass populations in grazed sites had significantly greater frequency of Neotyphodium infection compared to ungrazed sites. Tillers from grazed sites had significantly higher hyphal densities compared to ungrazed sites. The ability of grass-Neotyphodium constituents to deter vertebrate herbivory in natural systems is thought to be rare. In grazed meadows, we showed that endophyte infection resulted in the deterrence of grazing by native vertebrate herbivores. However, the same herbivores did not distinguish between infected and uninfected grass harvested from ungrazed areas. These results demonstrate that the relationship between vertically transmitted endophytes and grasses in the alpine tundra vary greatly within populations. This may be based in part on defense mutualism and is consistent, under varying levels of herbivory, with the predictions of optimal defense theory.

Invasion by Non‐Native Annual Grasses: The Importance of Species Biomass, Composition, and Time Among California Native Grasses of the Central Valley

Abstract The Central Valley of California is noted for its dearth of remnant native grass populations and for low native grass seedling establishment within grasslands now dominated by non‐native annual species. In contrast, remnant populations are common along the coast, and studies have shown an ability for seedlings and adults to compete with non‐native annual grasses. The invasibility of well‐established populations of native grasses in the Central Valley remains unclear. The objectives of this study were to compare the invasibility of native grasses differing in density and species composition and, given the species in this study, to assess the ability of mixes with greater species richness to resist invasion relative to their abilities in monoculture. In the Sacramento Valley of California, six species of native grasses were planted at three densities in monospecific and mixed‐species plots. Percent cover of native perennial and non‐native annual grasses was measured in years 2 and 3, and biomass was sampled in year 5. Native grass biomass and, to a lesser extent, species composition were important in explaining variation in non‐native grass invasibility in the fifth year. Species‐rich treatments did not experience less invasion than would be expected by the proportional invasibility of each species in monoculture. However, invasibility of plots consisting of slower growing, shorter statured species decreased over time, suggesting a successional benefit to diverse communities. This study demonstrates that established stands of native grasses in the Sacramento Valley can resist invasion by non‐native annual grasses and that stand biomass is a particularly important factor in determining invasibility.

Potential selection in native grass populations by exotic invasion

Ecological impacts of invasive plant species are well documented, but the genetic response of native species to invasive dominance has been often overlooked. Invasive plants can drastically alter site conditions where they reach dominance, potentially exerting novel selective pressures on persistent native plant populations. Do native plant populations in old exotic invasions show evidence of selection when compared to conspecific populations in adjacent, noninvaded areas? We employ amplified fragment length polymorphism (AFLP) analysis to screen a large number of loci from two native grass species (Hesperostipa comata (Trin. & Rupr.) Barkworth and Sporobolus airoides Torr.) that occur in old infestations of the invasive forb Acroptilon repens. We then compare observed locus by locus F(ST) values with distributions of F(ST) estimated from simulation models under expectation of neutrality. We also compare the proportion of loci possibly linked to selection and those not linked to selection which exhibit parallel trends in divergence between two community types (invaded, noninvaded). Few loci (H. comata, 2.6%; S. airoides, 8.7%) in the two native grasses may be linked to genes under the influence of selection. Also, loci linked to selection showed a greater portion of parallel trends in divergence than neutral loci. Genetic similarities between community types were less than genetic similarity within community types suggesting differentiation in response to community alteration. These results indicate that a small portion of scored AFLP loci may be linked to genes undergoing selection tied to community dominance by an invasive species. We propose that native plants in communities dominated by exotic invasives may be undergoing natural selection.

Detecting Genetic Changes over Two Generations of Seed Increase in an Awned Slender Wheatgrass Population Using AFLP Markers

Diverse native grass populations are being developed for revegetation and land reclamation purposes, but little is known about the maintenance of the genetic diversity of these developed populations during the process of seed increase. The objectives of this study were to assess the genetic shift over two generations of seed increase in a multisite composite population (AC Pintail) of the self‐pollinating awned slender wheatgrass [Elymus trachycaulus subsp. subsecundus (Link) Gould] and to compare its genetic variation with the released cultivar AEC Hillcrest. AC Pintail was formed by bulking seed of 200 plants collected from 60 sites across the prairie of western Canada. The amplified fragment length polymorphism (AFLP) technique was applied to assay 50 plants from each of four populations (AC Pintail G0, G1, and G2 and AEC Hillcrest breeder seed). For each sample, seven AFLP primer pairs were applied and 194 polymorphic bands were scored. AC Pintail revealed more polymorphic bands (74%) than AEC Hillcrest (47%), and most of the scored bands for AEC Hillcrest had occurrence frequencies approaching 1 or 0. The largest within‐population AFLP variation observed resided within AC Pintail G0 (30.3), followed by G1 (29.7), G2 (27.9), and AEC Hillcrest (10.4). Significant differences were found among these seed sources and >95% of the total AFLP variation resided within the three AC Pintail populations. Fifty‐three bands displayed significant changes from G0 to G1 and 76 from G0 to G2 of AC Pintail. These results indicate that AC Pintail harbored more genetic variation than AEC Hillcrest but could lose up to 8% of the original diversity in the first two generations of seed increase.

Long-term grazing effects on genetic variation in Idaho fescue

The effect of cattle grazing on the genetic structure of native grass populations has received little attention. We investigated the effect of cattle grazing on genetic variation in Idaho fescue (Festuca idahoensis Elmer) using ISSR (inter-simple sequence repeat) DNA markers. The ISSR markers are hypervariable and are generally interpreted as being selectively neutral. Idaho fescue tillers were sampled from inside (N = 31) and outside (N = 34) a 64-year-old cattle exclosure in southeastern Oregon. We extracted DNA and used 2 ISSR primers to determine the genotypes for grazed and ungrazed plants at 60 variable loci. No statistically significant differences were observed between grazed and ungrazed samples for percent polymorphic loci (grazed = 85%; ungrazed = 80%), mean expected heterozygosity (grazed = 0.1393; ungrazed = 0.1365), or for a measure of loci dissimilarity (grazed = 0.506; ungrazed = 0.536). We also found that the ungrazed individuals sampled inside the exclosure were not significantly genetically differentiated from the grazed individuals sampled outside the exclosure (G st = 0.0008 averaged across all loci). Our results differ from past studies that found demographic and physiological differences between Idaho fescue inside and outside of grazing exclosures at the same site. Our results mirror those of other researchers who have also failed to detect genetic differences at marker loci in response to grazing. We propose that either the mechanisms that must be present to cause changes in neutral genetic variation are not affected by cattle grazing for Idaho fescue at this site, or that any effects of grazing on neutral genetic variation were overwhelmed by gene flow between the grazed and ungrazed samples.

Long-term grazing effects on genetic variation in Idaho fescue

The effect of cattle grazing on the genetic structure of native grass populations has received little attention. We investigated the effect of cattle grazing on genetic variation in Idaho fescue (Festuca idahoensis Elmer) using ISSR (inter-simple sequence repeat) DNA markers. The ISSR markers are hypervariable and are generally interpreted as being selectively neutral. Idaho fescue tillers were sampled from inside (N = 31) and outside (N = 34) a 64-year-old cattle exclosure in southeastern Oregon. We extracted DNA and used 2 ISSR primers to determine the genotypes for grazed and ungrazed plants at 60 variable loci. No statistically significant differences were observed between grazed and ungrazed samples for percent polymorphic loci (grazed = 85%; ungrazed = 80%), mean expected heterozygosity (grazed = 0.1393; ungrazed = 0.1365), or for a measure of loci dissimilarity (grazed = 0.506; ungrazed = 0.536). We also found that the ungrazed individuals sampled inside the exclosure were not significantly genetically differentiated from the grazed individuals sampled outside the exclosure (Gst = 0.0008 averaged across all loci). Our results differ from past studies that found demographic and physiological differences between Idaho fescue inside and outside of grazing exclosures at the same site. Our results mirror those of other researchers who have also failed to detect genetic differences at marker loci in response to grazing. We propose that either the mechanisms that must be present to cause changes in neutral genetic variation are not affected by cattle grazing for Idaho fescue at this site, or that any effects of grazing on neutral genetic variation were overwhelmed by gene flow between the grazed and ungrazed samples.

Endophyte-grass-herbivore interactions: the case of Neotyphodium endophytes in Arizona fescue populations.

Neotyphodium endophytes in introduced agronomic grasses are well known to increase resistance to herbivores, but little is known of interactions between Neotyphodium endophytes and herbivores in native grass populations. We investigated whether endophytes mediate plant-herbivore interactions in a native grass species, Festuca arizonica in the southwestern United States, in two ways. First, to test the prediction that the presence and frequency of endophyte-infected (E+) plants should increase with increasing herbivory, we determined endophyte frequencies over a 4-year period in six natural Arizona fescue populations. We compared Neotyphodium frequency among plants growing inside and outside long-term vertebrate grazing exclosures. Second, we experimentally tested the effects of Neotyphodium infection, plant clone, and soil nutrients on plant resistance to the native grasshopper Xanthippus corallipes. Contrary to predictions based upon the hypothesis that endophytes increase herbivore resistance, levels of infection did not increase in plants subjected to grazing outside of exclosures relative to ungrazed plants within exclosures. Instead, endophyte frequencies tended to be greater inside the exclosures, where long-term vertebrate grazing was reduced. The grasshopper bioassay experiment corroborated these long-term patterns. Survival of grasshoppers did not differ between infected (E+) and uninfected (E-) plants. Instead, mean relative growth rate of grasshoppers was higher on E+ grasses than on E- ones. Growth performance of newly hatched grasshopper nymphs varied among host plant clones, although two of six clones accounted for most of this variation. Our results suggest that Neotyphodium-grass-herbivore interactions may be much more variable in natural communities than predicted by studies of agronomically important Neotyphodium-grass associations, and herbivory is not always the driving selective force in endophyte-grass ecology and evolution. Thus, alternative hypotheses are necessary to explain the wide distribution and variable frequencies of endophytes in natural plant populations.

Genetic Structure and Gene Flow in Elymus glaucus (blue wildrye): Implications for Native Grassland Restoration

AbstractInterest in using native grass species for restoration is increasing, yet little is known about the ecology and genetics of native grass populations or the spatial scales over which seed can be transferred and successfully grown. The purpose of this study was to investigate the genetic structure within and among populations of Elymus glaucus in order to make some preliminary recommendations for the transfer and use of this species in revegetation and restoration projects. Twenty populations from California, Oregon, and Washington were analyzed for allozyme genotype at 20 loci, and patterns of variation within and among populations were determined. Allozyme variation at the species level was high, with 80% of the loci polymorphic and an average expected heterozygosity (an index of genetic diversity) of 0.194. All but two of the populations showed some level of polymorphism. A high degree of population differentiation was found, with 54.9% of the variation at allozyme loci partitioned among populations (Fst= 0.549). A lesser degree of genetic differentiation among closely spaced subpopulations within one of the populations was also demonstrated (Fst= 0.124). Self‐pollination and the patchy natural distribution of the species both likely contribute to the low level of gene flow (Nm= 0.205) that was estimated. Zones developed for the transfer of seed of commercial conifer species may be inappropriate for transfer of E. glaucus germplasm because conifer species are characterized by high levels of gene flow. Limited gene flow in E. glaucus can facilitate the divergence of populations over relatively small spatial scales. This genetic differentiation can be due to random genetic drift, localized selective pressures, or both. In order to minimize the chances of planting poorly adapted germplasm, seed of E. glaucus may need to be collected in close proximity to the proposed restoration site.