Grass Establishment Research Articles

Influence of resource pulses and perennial neighbors on the establishment of an invasive annual grass in the Mojave Desert

Invasion by exotic annual grasses is one of the most significant threats to arid ecosystems in the western USA. Current theories of invasibility predict plant communities become more susceptible to invasion whenever there is an increase in the amount of unused resources. The objective of this field study was to examine how resource pulses and temporal variation in resource demand by the native shrub vegetation influences establishment of the invasive annual grass Schismus arabicus. Water and nitrogen were applied as pulses in early spring, mid-spring, or continuously throughout the growing season to plots containing either Atriplex confertifolia or Atriplex parryi shrubs. The effect of resource pulses on Schismus density and biomass was highly dependent on the seasonal timing of the resource pulses and the identity of the neighbor shrub. When resource pulses coincided with high rates of resource capture and growth of the native vegetation, density and biomass of Schismus was reduced. Schismus establishment was greater under continuous resource supply compared to pulsed resource supply, likely because more soil resources were available at a shallow depth when resources were supplied at a continuous low rate. These results suggest that the establishment of invasive annual grasses in arid systems can be influenced by the magnitude and spatial distribution of resource pulses in addition to the seasonal timing of resource pulses.

Advantages in Water Relations Contribute to Greater Photosynthesis in Centaurea maculosa Compared with Established Grasses

Semiarid steppe communities in North America appear particularly vulnerable to persistent infestations by exotic, taprooted forbs, such as European spotted knapweed (Centaurea maculosa). We determined whether species differences in ecophysiological response to water availability could help link traits of Centaurea with invasibility of steppe communities. Plant‐soil water relations and photosynthesis were measured under three water levels in a greenhouse and at two sites over two years in the field for Centaurea and dominant rangeland species of southwestern Montana: Pseudoregneria spicata, Pascopyron smithii, and Bromus inermis. Centaurea had greater and more seasonally persistent photosynthesis than the other species under field conditions but not in the greenhouse, where water availability was similar for the species. Centaurea had no greater water use efficiency, except under unusually dry conditions, but maintained greater water potentials despite greater transpiration than the grasses. Changes in soi...

Virus infection and grazing exert counteracting influences on survivorship of native bunchgrass seedlings competing with invasive exotics.

Summary Invasive annual grasses introduced by European settlers have largely displaced native grassland vegetation in California and now form dense stands that constrain the establishment of native perennial bunchgrass seedlings. Bunchgrass seedlings face additional pressures from both livestock grazing and barley and cereal yellow dwarf viruses (B/CYDVs), which infect both young and established grasses throughout the state. Previous work suggested that B/CYDVs could mediate apparent competition between invasive exotic grasses and native bunchgrasses in California. To investigate the potential significance of virus-mediated mortality for early survivorship of bunchgrass seedlings, we compared the separate and combined effects of virus infection, competition and simulated grazing in a field experiment. We infected two species of young bunchgrasses that show different sensitivity to B/CYDV infection, subjected them to competition with three different densities of exotic annuals crossed with two clipping treatments, and monitored their growth and first-year survivorship. Although virus infection alone did not reduce first-year survivorship, it halved the survivorship of bunchgrasses competing with exotics. Within an environment in which competition strongly reduces seedling survivorship (as in natural grasslands), virus infection therefore has the power to cause additional seedling mortality and alter patterns of establishment. Surprisingly, clipping did not reduce bunchgrass survivorship further, but rather doubled it and disproportionately increased survivorship of infected bunchgrasses. Together with previous work, these findings show that B/CYDVs can be potentially powerful elements influencing species interactions in natural grasslands. More generally, our findings demonstrate the potential significance of multitrophic interactions in virus ecology. Although sometimes treated collectively as plant ‘predators’, viruses and herbivores may exert influences that are distinctly different, even counteracting.

Soil Erosion Thresholds and Alternative States in Northeastern Patagonian Rangelands

In semiarid rangelands, continuous grazing may decrease vegetation cover, accelerating soil erosion and eventually causing a transition to an alternative, degraded state. State-and-transition models invoke process-based explanations of alternative states, but there are few examples that use empirical data on key factors and processes. We used rainfall simulation to determine 1) the relationships between soil surface characteristics and interrill erosion in 3 spatially related plant communities: stable grass with scattered shrubs (GS), degraded grass with scattered shrubs (DGS), and degraded shrub steppes (DSS), and 2) the site conservation threshold (SCT) of this rangeland. We also analyzed the effect of past erosion on soil and vegetation characteristics. In the GS, sediment production and sediment concentration were significantly lower (P<0.05) than in the DGS and the DSS.The main soil protection factors in the GS and in the DGS were perennial grass and litter cover, while in the DSS, gravel cover became the main soil protection factor. The SCT, the point at which the rate of soil erosion increases markedly, corresponded to a plant-and-litter cover close to 90% and occurred within the DGS. Although this plant community may reverse back to the conserved GS, long-term accelerated erosion may result in enough soil loss to trigger irreversible changes and prompt the transition to the DSS. The threshold underlying this transition would be reached when the A horizon is severely reduced by erosion. Under these conditions, the soil hydrological properties are affected irreversibly, preventing perennial grass establishment. While the GS represents a resource conserving plant community, desirable for both forage production and soilprotection, the DSS represents a dysfunctional state with a minimum forage value. The DGS represents an unstable and transitional community that, without management intervention to halt soil erosion, will likely change into the DSS.

Nitrogen Use Efficiency and Herbage Production of an Established Grass Sward in Relation to Moisture and Nitrogen Fertilization

ABSTRACT Herbage growth and nitrogen (N) use efficiency in grazed or mown grasslands are generally low, associated mostly with poor response to fertilizer N. The aim of the present investigation was to examine the short-term response of grass to fertilizer N with respect to herbage yield and nitrogen use efficiency (NUE) in order to provide a better basis for improving the efficient use of fertilizer N in grassland ecosystems. Both NO3 − and NH4 + sources of N were applied to an established grass sward with three moisture levels, i.e., natural conditions (63% water-filled pore space, WFPS), near field capacity level (71% WFPS), and slightly wetter than field capacity (84% WFPS). Herbage yield, i.e., dry matter (DM), N uptake, N recovery efficiency, yield efficiency, and physiological efficiency were determined over a 7–28 d period. Addition of N fertilizers significantly increased the herbage yield and N uptake of grass sward over that of the control. In the plots where NO3 −−N was added as the N source, DM yield was between 1760–1870 kg ha−1, N recovery efficiency was between 24%–43%, and yield and physiological efficiency were in the range of 2.1–3.2 and 6.4–8.8 kg DM kg− 1 N, respectively. In NH4 +−N added plots, the DM yield was between 3190–3700 kg ha− 1, N recovery efficiency was between 39%–48% while yield and physiological efficiency were in the range of 3.5–5.6 and 9.0–11.6 kg DM kg− 1 N, respectively. Results indicated that total DM yield, N uptake, and NUE depend on the source of N and the level of moisture in the field. Assimilation of N is also affected by the stage of plant development after N fertilization. About 50%–54% of applied N was recovered in the initial 14 and 21 d after fertilizer application and thereafter translocation of N slowed. A fall in herbage production and minimal response to N fertilizer has been observed at 84% WFPS, while the maximum herbage yield and N recovery efficiency was recorded in soil near or below field capacity. The grass sward with added NH4 +−N produced a larger yield and had higher NUE relative to the sward with NO3 −−N. Results confirm that applied N was not utilized efficiently by grass sward and a decrease in N uptake and its utilization seem to be the key factors responsible for the poor herbage productivity often observed in pastoral agriculture. These results suggest that both moisture and N source have a substantial effect on herbage yield and N utilization by plants and therefore should be considered for efficient management of N fertilization and recommendations for grass sward.

The effect of land management on plant community composition, species diversity, and productivity of alpine Kobersia steppe meadow

AbstractGrassland degradation is widespread and severe on the Tibet Plateau. To explore management approaches for sustainable development of degraded and restored ecosystems, we studied the effect of land degradation on species composition, species diversity, and vegetation productivity, and examined the relative influence of various rehabilitation practices (two seeding treatments and a non‐seeded natural recovery treatment) on community structure and vegetation productivity in early secondary succession. The results showed: (1) All sedge and grass species of the natural steppe meadow had disappeared from the severely degraded land. The above‐ground and root biomass of severely degraded land were only 38 and 14.7%, respectively, of those of the control. So, the original ecosystem has been dramatically altered by land degradation on alpine steppe meadow. (2) Seeding measures may promote above‐ground biomass, particularly grass biomass, and ground cover. Except for the grasses seeded, however, other grass and sedge species did not occur after seeding treatments in the sixth year of seeding. Establishment of grasses during natural recovery treatment progressed slowly compared with during seeding treatments. Many annual forbs invaded and established during the 6 years of natural recovery. In addition, there was greater diversity after natural recovery treatment than after seeding treatments. (3) The above‐ground biomass after seeding treatment and natural recovery treatment were 114 and 55%, respectively, of that of the control. No significant differences in root biomass occurred among the natural recovery and seeded treatments. Root biomass after rehabilitation treatment was 23–31% that of the control.

Effects of Sowing Treatment and Landscape Position on Establishment of the Perennial Tussock Grass Themeda triandra (Poaceae) in Degraded Eucalyptus Woodlands in Southeastern Australia

Abstract Grassy woodlands dominated by Eucalyptus species such as E. albens Benth., E. melliodora A. Cunn. ex Schauer, and E. microcarpa Maiden are poorly conserved in Australia, owing to widespread agricultural development. Understorey restoration is vitally important to enhance degraded remnants but no reliable techniques are available to restore herbaceous understorey species over large areas. Reestablishment of dominant native grasses such as Kangaroo grass (Themeda triandra Forssk.) is particularly important. This study compared Themeda establishment using a variety of sowing techniques across a range of landscape positions in degraded woodlands in central New South Wales. Four localities were sampled at three landscape positions (upper, mid‐, and lower slopes). Existing vegetative cover was sprayed with herbicide and removed by mowing. A randomized block experiment was established at each site with five replicates of five seedbed treatments: all four factorial combinations of soil disturbance (disturbed, nondisturbed) and weed control (atrazine, nonatrazine), plus topsoil scalping followed by soil disturbance. After 117 days, all plots were subject to a common herbicide (atrazine) treatment to control weeds. Themeda establishment was not significantly affected by landscape position, despite significant differences in the cover of exotic species among landscape positions. The scalped/disturbed treatment resulted in significantly better establishment at 400 days (18% or 3.9 plants/m2) than the control (8% or 1.8 plants/m2). All other treatments did not differ significantly from the control suggesting that seedbed treatments are not critical to successful Themeda establishment. These results suggest that Themeda swards can be reestablished in degraded woodlands relatively simply. The similarity in outcomes across all landscape positions suggests that general recommendations for “best‐bet” establishment methods may prove robust under a wide range of environmental and seasonal conditions.

Erosion risk assessment of controlled burning of grasses established on steep slopes

It is a standard practice to establish grasses on steep slopes (batters) of embankments and cuttings to minimise erosion problems. However, the increase in grass density (high biomass) on the steep slopes poses a greater risk of fire. Controlled burning is a common fuel hazard reduction program employed to minimise the fire risks. The increased risk of erosion on the steep slopes after controlled burning has received little attention if any. This paper assesses the erosion risks associated with controlled burning of grasses established on steep slopes. Grasses, with and without the aid of waste ballast rock mulch, were established on 10 m wide railway embankment batter experiment plots. Two-and-a-half years after the grass establishment, selected plots were controlled burned. Runoff and soil loss from the experimental plots were monitored throughout the 3½-year period of the experiment. After one year the grass cover on the burned plots has hardly exceeded 60%, far below the average pre-burn levels of about 80%. All treatments achieved an incredible soil loss reduction of over 95% (compared with the bare scenario) without controlled burning at the end of the 3½-year period. This percentage value was decreased numerically by 14 where controlled burning was implemented. Compared with the 100% grass cover treatment, runoff rates tripled while erosion rates increased by nine-fold for the waste ballast treatment, and 17-fold for the non-waste ballast treatment, during the first year following controlled burning.

PHYSICAL FACTORS VS. BIOTIC RESISTANCE IN CONTROLLING THE INVASION OF AN ESTUARINE MARSH GRASS

We examined the relative roles of biotic resistance and abiotic factors in controlling the establishment and early survival of an invasive marine grass, Spartina anglica, in the Pacific Northwest. S. anglica can invade diverse estuarine habitats, including established salt marshes, mudflats, and cobble beaches; these vary in native species assemblages and in abiotic conditions, especially sediment salinity, water content, and composition. In four habitat types in northern Puget Sound, we conducted a seed addition experiment to quantify germination and early growth. In vegetated habitats we also manipulated plant neighbors. Seed germination and early growth varied with habitat and zone, but physical factors played a more important role than species interactions. Seedlings in low‐salinity marshes and mudflats grew almost 10 times larger than those in coarse‐grained or high‐salinity habitats. Patterns of seedling persistence among habitats matched those of natural seedling abundances and mirrored the degree of invasion in different habitat types in the region. The lack of biotic resistance may be due to the stressful nature of saline systems or the lack of species that can compete with or consume the invader. In such systems, managers should focus on how physical vulnerabilities of the habitat can provide invasion “windows of opportunity.”

Establishment and Persistence of Field Sown North American Prairie Grasses in Southern England in Response to Mulching and Extensive Weed Management

Abstract Establishment and management of North American prairie grasses by field sowing was investigated at the Royal Horticultural Society Garden at Wisley, Surrey, in Southern England. Untreated seed of little bluestem (Schizachyrium scoparium (Michx.) Nash); indian grass (Sorghastrum nutans (L.) Nash); and prairie dropseed (Sporobolus heterolepis (A.Gray) A.Gray.) was sown in May 1997 at 550 seeds/m2 onto topsoil plots (control), topsoil with charcoal mulch, and topsoil covered with transparent, perforated crop polyethylene post sowing. Highest percentage emergence was recorded in the clear polyethylene mulch followed by the charcoal mulch. Weed competition was managed by cutting to 75 mm (≅ 3 in) above the soil surface. Dry weight of prairie grass seedlings in October 1997 was strongly negatively correlated with percentage weed cover in June 1997. Despite the weed competition, on plots where sufficient seedlings survived, Schizachyrium and Sorghastrum were eventually able to dominate Agrostis stolonifera, the most abundant weed, when burnt annually in April. No regeneration of prairie grasses from self-sown seed was observed within the experimental treatments over an 8 year period. The application of this research to more sustainable horticultural practice in urban landscapes is discussed.

Establishing Native Grasses in a Big Sagebrush–Dominated Site: An Intermediate Restoration Step

AbstractMany semiarid rangelands in the Great Basin, U.S.A., are shifting dominance to woody species as a consequence of land degradation including intense livestock grazing and fire suppression. Whereas past rehabilitation efforts in Big sagebrush (Artemisia tridentata) steppes removed the shrub and added introduced forage grasses to successfully shift communities from shrublands to grasslands, current consensus is that native species should be included in restoration projects and that retention of some woody plants is desirable. We examined the potential for interseeding grasses into dense shrub communities as a precursor to thinning shrubs and releasing grasses from shrub interference. We compared seedling establishment of the native grass, Bluebunch wheatgrass (Pseudoroegneria spicata), with that of the Eurasia grass, Crested wheatgrass (Agropyron desertorum), in denseAr. tridentatastands. Shrubs may play an important role as nurse plants for seedling establishment (reduced solar radiation, “island of fertility” effect) but result in highly contrasting light environments and root interference for seedlings. In experimental plots, we examined effects ofAr. tridentatashade levels (0, 40, 70, and 90% reduction of solar radiation) and initial root exclusion (present/absent) on the establishment and growth ofP. spicataandAg. desertorumseedlings. With this design we evaluated the interference effects ofAr. tridentataon the two grasses and identified the most beneficial microsites for grass restoration inAr. tridentata–dominated communities. We predicted seedling survival and growth to be greater under moderate shade (40% reduction) and limited root competition than under no or strong shade conditions (0 and 90%) and unrestricted root interactions. Fifty to 85% of theP. spicataandAg. desertorumseedlings survived the dry summer months of 1995 and 1996 and the intervening winter. Neither shading nor root exclusion fromAr. tridentataaffected final seedling survival of either species. Seedling biomass of both grass species was negatively affected by initial root interactions withAr. tridentata.However, the analysis of seedling biomass variability (coefficient of variation) indicated that in all shade and root‐exclusion treatments, some seedlings of both species developed to large individuals to survive inAr. tridentata–dominated rangelands. Thus, the use of interseeding techniques shows promise for restoring herbaceous species in denseAr. tridentatastands and should be given further consideration when shrub retention is an important consideration.

Light limitation creates patchy distribution of an invasive grass in eastern deciduous forests

Species interactions and their indirect effects on the availability and distribution of resources have been considered strong determinants of community structure in many different ecological systems. In deciduous forests, the presence of overstory trees and shrubs creates a shifting mosaic of resources for understory plants, with implications for their distribution and abundance. Determination of the ultimate resource constraints on understory vegetation may aid management of these systems that have become increasingly susceptible to invasions by non-native plants. Microstegium vimineum (Japanese grass) is an invasive annual grass that has spread rapidly throughout the understory of forests across the eastern United States since it was first observed in Tennessee in 1919. M. vimineum occurs as extensive, dense patches in the understory of eastern deciduous forests, yet these patches often exhibit sharp boundaries and distinct gaps in cover. One example of this distributional pattern was observed relative to the native midstory tree Asimina triloba (pawpaw), whereby dense M. vimineum cover stopped abruptly at the drip line of the A. triloba patch and was absent beneath the A. triloba canopy. We conducted field and greenhouse experiments to test several hypotheses regarding the causes of this observed pattern of M. vimineum distribution, including allelopathy, seed dispersal, light limitations, and soil moisture, texture, and nutrient content. We concluded that light reduction by the A. triloba canopy was the environmental constraint that prevented establishment of M. vimineum beneath this tree. Whereas overstory tree canopy apparently facilitates the establishment of this shade-tolerant grass, the interaction of overstory canopy with midstory canopy interferes with M. vimineum by reducing the availability of sunflecks at the ground layer. It is likely that other midstory species influence the distribution and abundance of other herb-layer species, with implications for management of understory invasive plant species.

How resilient are southwestern ponderosa pine forests after crown fires?

The exclusion of low-severity surface fire from ponderosa pine (Pinus ponderosa P. & C. Lawson) forests of the Southwest has changed ecosystem structure and function such that severe crown fires are increasingly causing extensive stand mortality. This altered fire regime has resulted from the intersection of natural drought cycles with human activities that have suppressed natural fires for over a century. What is the trajectory of forest recovery after such fires? This study explores the regeneration response of ponderosa pine and other species to crown fires that occurred in the region from the late 1940s to the mid-1970s. We address two main questions: (1) What is the success of ponderosa regeneration and establishment, and (2) Can these sites, burned in stand-destroying fires, be "captured" by other species on the scale of decades? Two main trajectories of recovery were found: (1) establishment of unnaturally dense ponderosa pine stands vulnerable to further crown fire and (2) establishment of nonforested grass or shrub communities.

Errata for “Optimum Use of Erosion Control Blankets and Waste Ballast (Rock) Mulch to Aid Grass Establishment on Steep Slopes” by Yeboah Gyasi-Agyei

Errata for “Optimum Use of Erosion Control Blankets and Waste Ballast (Rock) Mulch to Aid Grass Establishment on Steep Slopes” by Yeboah Gyasi-Agyei

Effects of Canopy Cover and Ground Disturbance on Establishment of an Invasive Grass in an Australia Savanna1

ABSTRACT Andropogon gayanus (gamba grass) is an introduced pasture grass that threatens Australia's tropical savannas by modifying fire regimes and species composition. To understand the establishment requirements of A. gayanus, we undertook a field experiment to determine the effect of canopy cover and ground layer disturbance on seedling emergence and survival. Seed was sown under three canopy treatments (undisturbed, artificial canopy gap, and natural canopy gap) and under three ground layer treatments (Control, Vegetation disturbed, and Soil disturbed). Results have shown that A. gayanus can establish and survive regardless of canopy cover or ground disturbance, although such site disturbances will increase establishment success. Disturbance of both the overstorey canopy and the ground layer increased A. gayanus emergence, whereas seedling survival to 12 mo after seed sowing was affected by ground layer disturbance alone. Disturbance of the canopy increased light transmission, which may have promoted germination. Ground layer disturbance may also have increased light transmission and suitable sites for establishment, and reduced competition for resources, such as water and nutrients. The ability of A. gayanus to spread along disturbed areas, establish in relatively undisturbed savannas, and resprout after fire within 6 mo after seedling emergence suggests that this species will become increasingly widespread in Australia's tropical savannas. Its control is urgently required.

Establishment of native and exotic grasses on mine overburden and topsoil in the Hunter Valley, New South Wales

Native grasses have an important role to play in mine rehabilitation throughout Australia, but there have been few scientifically designed studies of field establishment of native grasses from sown seed in this country. Current recommendations for rehabilitation of open-cut coal mines in the Hunter Valley involve the sowing of exotic pasture species to reinstate mined land to Class IV and V under the Rural Land Capability System. Despite the importance of native grasses in the pre-mined landscape, they are currently not widely included in mine rehabilitation. To address this issue a project was conducted between 1994 and 2000 to research the use of native grasses for rehabilitation of open-cut coal mines in the Hunter Valley. This paper reports on 2 mine site experiments that aimed to assess establishment and persistence of a broad range of native and exotic grass species from an autumn sowing in both topsoil and raw spoil over a period of 61 months. The most promising natives in terms of early establishment, persistence and spread over time, included six C3 accessions (five Austrodanthonia spp. and Austrostipa bigeniculata) and one C4 accession (Cynodon dactylon). Persistence of these accessions was better in raw spoil than topsoil, despite initial low numbers, due to a lack of weed competition and their ability to spread by self-seeding. In topsoil, and in the absence of any biomass reduction, native species were mostly out-competed by vigorous exotic perennial grasses which were sown in these experiments and from seed influx from adjacent rehabilitation areas or from the soil seed bank. The effects of climatic conditions and differences in soil physical, chemical and seed bank characteristics at the 2 mine sites are also discussed.

Emergence of Dallisgrass as Affected by Soil Water Availability

Water supply affects seed germination and seedling establishment of shallow-rooted warm-season grasses. This may explain the difficulty of incorporating Dallisgrass (Paspalum dilatatum Poir.) into humid temperate grasslands through interseeding. We studied the effects of water availability on seed germination and seedling growth under controlled conditions to determine which step of the establishment process was most affected. In a laboratory experiment, seeds were germinated at 0, –0.25, –0.5, –0.75, and –1 MPa water availability generated with solutions of polyethylene glycol. Although both maximum rate and total germination (P < 0.05) significantly decreased with increased water stress, the speed of germination was even more sensitive. In a greenhouse experiment, variations in seedling emergence and morphological characteristics were measured in relation to water availability. Pregerminated and dry seeds were sown in pots that were irrigated at 1-, 2- , 4-, or 7-day intervals. This species showed high sensitivity to water stress during germination and early emergence. High emergence was obtained from the daily irrigation treatment. In all other treatments, where watering frequency was extended, emergence was decreased. Results suggest that rapid germination and early adventitious root growth can be obtained only with reliable availability of water. These conditions, combined with the high temperatures required for breaking seed dormancy, occur infrequently, explaining the difficulty of achieving successful establishment of Dallisgrass. Water availability during seed germination and seedling emergence is a critical factor for survival of this species.

Emergence of Dallisgrass as Affected by Soil Water Availability

Water supply affects seed germination and seedling establishment of shallow-rooted warm-season grasses. This may explain the difficulty of incorporating Dallisgrass (Paspalum dilatatum Poir.) into humid temperate grasslands through interseeding. We studied the effects of water availability on seed germination and seedling growth under controlled conditions to determine which step of the establishment process was most affected. In a laboratory experiment, seeds were germinated at 0, -0.25, -0.5, -0.75, and -1 MPa water availability generated with solutions of polyethylene glycol. Although both maximum rate and total germination (P &lt; 0.05) significantly decreased with increased water stress, the speed of germination was even more sensitive. In a greenhouse experiment, variations in seedling emergence and morphological characteristics were measured in relation to water availability. Pregerminated and dry seeds were sown in pots that were irrigated at 1-, 2- , 4-, or 7-day intervals. This species showed high sensitivity to water stress during germination and early emergence. High emergence was obtained from the daily irrigation treatment. In all other treatments, where watering frequency was extended, emergence was decreased. Results suggest that rapid germination and early adventitious root growth can be obtained only with reliable availability of water. These conditions, combined with the high temperatures required for breaking seed dormancy, occur infrequently, explaining the difficulty of achieving successful establishment of Dallisgrass. Water availability during seed germination and seedling emergence is a critical factor for survival of this species.

Identification of phytotoxic substances in soils following winter injury of grasses as estimated by a bioassay

Under northern conditions, winter survival of grass species for hay production is quite uncertain because of winter stresses to plants. Damage to plants may be caused by variability in snow cover, low temperatures, ice encasement and pathogens. Compared to renovation by ploughing, direct drilling without ploughing has some beneficial aspects and may be an alternative method for renovating hay fields. However, successful establishment of grasses without ploughing has been difficult to achieve both in scientific studies and under practical conditions. The objective of this investigation was to study whether a dead grass sod, killed by different winter injuries, may leak phytotoxins into the soil, thus causing the poor results observed by direct drilling in hayfields. Experiments, including soil water extraction and the use of bioassays, were conducted in growth chambers, to study the effects of the winter stresses frost, ice encasement, and snow mould on the accumulation of potentially phytotoxic biochemical compounds in the soil. Snow mould did not kill the grasses in the experiments and no phytotoxicity was measured. Although both ice encasement and low temperatures killed the grasses, only ice encasement caused phytotoxicity. The present investigation shows that the occurrence of phytotoxic substances, especially butyrate, after ice encasement may be a cause of poor establishment of direct-drilled grass plants after winter injuries. In serious cases of ice encasement this may justify a 1-2 wk delay of sowing, which in turn may result in reduced soil water for germination and may cause increased competition from other species. Key words: Hay fields, grassland, winter injuries, phytotoxic substances, allelochemicals

Perennial Grass Establishment Integrated with Clopyralid Treatment for Yellow Starthistle Management on Annual Range1

Yellow starthistle is an aggressive annual forb that has invaded millions of hectares of California's annual range. Control efforts such as burning and herbicides have been effective for short-term management. However, recruitment from the seedbank or reinvasion of the annual grassland system results in a rapid return to yellow starthistle dominance. Establishing perennial grasses would be ideal for suppression of yellow starthistle. However, a lack of effective weed control options in California during a seeding program has limited perennial grass establishment. Clopyralid was used to control yellow starthistle annually for 1, 2, or 3 yr to provide a window of reduced competition for pubescent wheatgrass establishment. Total plant cover, yellow starthistle density, biomass, and seedhead number were quantified for 6 yr. Clopyralid treatment significantly reduced yellow starthistle and allowed pubescent wheatgrass establishment with a single treatment. Both clopyralid treatment and pubescent wheatgrass est...