Grass Competition Research Articles

CONSTRAINTS ON TREE SEEDLING ESTABLISHMENT IN MONTANE GRASSLANDS OF THE VALLES CALDERA, NEW MEXICO

Montane and subalpine grasslands are prominent, but poorly understood, features of the Rocky Mountains. These communities frequently occur below reversed tree lines on valley floors, where nightly cold air accumulation is spatially coupled with fine soil texture. We used field experiments to assess the roles of minimum temperature, soil texture, grass competition, and ungulate browsing on the growth, photosynthetic performance, and survival of transplanted ponderosa pine (Pinus ponderosa) seedlings at 32 sites straddling such reversed tree lines in the Valles Caldera National Preserve (VCNP) of the Jemez Mountains, New Mexico (USA). Seedling growth increased most strongly with increasing nighttime minimum temperatures away from the valley bottoms; seedlings experiencing the coldest temperatures on the caldera floor exhibited stunted needles and often no measurable height growth. Based on the chlorophyll fluorescence ratios PhiPSII and Fv/Fm, we found that low minimum temperatures, low soil moisture, and fine soil texture all contributed to photoinhibition. Neighboring herbs had only minor negative effects on seedlings. We found no effect of ungulates, but golden-mantled ground squirrels (Spermophilus lateralis) caused substantial seedling mortality. Second-year seedling survival was highest on sandy soils, and third-year survival was highest at sites with higher minimum temperatures. We conclude that differential tree seedling establishment driven by low minimum temperatures in the valley bottoms is the primary factor maintaining montane grasslands of the VCNP, although this process probably operated historically in combination with frequent surface fire to set the position of the tree line ecotone. As at alpine tree lines, reversed tree lines bordering montane and subalpine grasslands can represent temperature-sensitive boundaries of the tree life form.

Effects of elevated CO2 and N on tree–grass interactions: an experimental test using Fraxinus excelsior and Dactylis glomerata

1 The invasion of grasslands by woody species is often associated with changes in environmental conditions, but few studies have addressed the impact of climate change on the competitive interactions between tree seedlings and herbaceous vegetation. We examined patterns of growth and morphology in Fraxinus excelsior seedlings germinating in the presence or absence of grass competition (Dactylis glomerata) at either low (380 p.p.m.) or high (645 p.p.m.) atmospheric carbon dioxide (CO2), and at two levels of nitrogen (N) nutrition. 2 Elevated CO2 had a positive effect on Fraxinus dry mass irrespective of N treatment, but the magnitude of growth response was small. In contrast, Dactylis dry mass showed a significant CO2 × N interaction: no apparent response to elevated CO2 in the low N treatment compared with a 25% dry mass increase in the high N, high CO2 treatment. 3 Dactylis and Fraxinus showed greater responses to N compared with CO2 in terms of dry mass, morphology and biomass allocation. The direction of Fraxinus responses to N varied depending on the trait examined and the grass competition treatment. 4 Both the competitive intensity (Cint) and the importance of the competition (Cimp) experienced by Fraxinus seedlings increased with an increase in N availability. Contrary to expectations, elevated CO2 had no significant effect on either Cint or Cimp. 5 Plant plasticity may have important implications for the long-term success of tree seedlings in grasslands. Our results suggest that the combination of both grass and tree seedling responses to CO2 may have indirect benefits for the persistence of woody invaders in high-nutrient grasslands under future atmospheric conditions.

Influence of grass suppression and sowing rate on the establishment and persistence of forb dominated urban meadows

In practice in the UK, grasses and ruderal forbs recruited from the urban soil seed bank often have a detrimental impact on establishment and development of sown meadows. This study investigated the effect of graminicide, and sowing rate on establishment, survival and longer term development of sown meadow forbs. A seed mix containing 19 forb species was sown at two sowing rates in a randomized block experiment in Sheffield, UK. The two most abundant grasses recruited, Arrhenatherum elatius, and Holcus lanatus, were highly damaging to survival and development of sown forbs. Cutting the meadow to 50 mm during the first year did not alleviate grass competition. Weedy ruderal forbs had a lesser effect on sown forbs than the perennial grasses. Where a graminicide was used, sown forb density and biomass were significantly higher in the second and third year of the study, and forb richness significantly greater in all 3 years. Sown forb density was higher at the high sowing rate in all three years, and forb richness in years 1 and 2. Sowing rate did not significantly increase forb biomass in any year. In general, suppressing grass growth had a more beneficial effect on sown forb establishment and development than increasing sowing rate.

Responses of Medicago sativa and M. falcata type alfalfas to different defoliation times and grass competition

Incorporating alfalfa into rangelands can enhance the quantity and quality of forage production. We evaluated the impact of clipping timing and selective clipping on two M. falcata (Anik and Yellowhead) and one M. sativa type alfalfas (Vernal) near Mandan, North Dakota, USA. Cultivars were space-planted into an existing mixed grass prairie and clipped at the mid-bud, flower or flower and subsequent vegetative stages. In each clipping treatment, half of the plots had only the alfalfa clipped and half had both the alfalfa and the associated native vegetation clipped. Plots without alfalfa were also included to evaluate the impact of alfalfa on grass and forb biomass. Including Yellowhead increased total productivity by 38 to 185% without lowering the productivity of the grass or forb biomass components. Plots with Yellowhead produced 17 to 26% more total biomass than the next highest entry every year. In 2003 and 2005, alfalfa biomass was increased 1.5 to 2.7 times by clipping only alfalfa in the flower and regrowth stages compared with a mid-bud clipping of only alfalfa. Vernal and Yellowhead generally produced more total biomass when clipping was deferred to the flower stage, but clipping Anik in the mid-bud stage produced as much or more total biomass than did the later clipping treatments. Selective clipping of the alfalfa did not have a consistent impact on yield. Yellowhead appeared to be a good choice for incorporating into rangelands. Producers with Yellowhead or Vernal should consider delaying defoliation until flowering to maximize productivity. Key words: Grazing-type alfalfa, hay-type alfalfa, defoliation timing, biomass productivity

Studies of competition between Nassella trichotoma (Nees) Hack. ex Arechav. (serrated tussock) and native pastures. 2. Seedling responses

Native perennial grass competition can substantially reduce the invasion of Nassella trichotoma (serrated tussock), a major perennial grass weed problem in south-eastern Australia. This paper reports on a field experiment that investigated the recruitment of N. trichotoma seedlings, and determined what level of native grass competition was needed to prevent establishment in the central-west of NSW. Grasslands that maintained >2 t dry matter (DM)/ha and 100% ground cover (measured in spring) prevented N. trichotoma seedling recruitment. Relatively small amounts of perennial grass (>0.5 t DM/ha measured in spring) resulted in mortality of N. trichotoma seedlings that had recruited earlier in the year, through the next summer. Flupropanate also markedly reduced native perennial grasses and substantially increased N. trichotoma seedling establishment 12 months after application. Rotational grazing to maintain adequate levels of DM was an important management tactic that prevented N. trichotoma establishment and survival.

Interseeding Warm-season Grasses Followed by High Intensity Grazing Enhances Pasture Productivity

Pasture renovation by interseeding can be an efficient way to introduce native warm-season grasses into low-producing cool-season pastures, provided that the existing sod can be managed to favor establishment and stand persistence. The objectives were to test interseeding warm-season grasses into cool-season pasture using grazing or herbicide sod suppression treatments and the effects of prescribed grazing events on stand development. The grazing treatment was applied using sheep at a high stocking rate to achieve a residual grazing height of < 2.5 cm in autumn and in the spring prior to seedling emergence. The herbicide treatment was an application of glyphosate in early October. Native warm-season grasses were seeded in mid-March (2004 and 2005). Warm-season grass biomass harvested the year after establishment was greater (P < 0.01) for the herbicide sod suppression treatment (5700 kg/ha) than the grazing treatment (20 kg/ha). Three years of prescribed grazing significantly (P < 0.01) enhanced the warmseason grass composition from 56 to 70% and from 1 to 12% in the herbicide and grazed sod suppression plots, respectively. Herbicides are integral to successfully interseeding warm-season grasses into cool-season pastures. Shortduration, high-density grazing can be appropriately timed to increase the competitiveness of warm-season grasses and decrease the cool-season grasses of interseeded stands.

Hydrologic Impacts of Mechanical Seeding Treatments on Sagebrush Rangelands

In and around the Great Basin, United States, restoration of shrub steppe vegetation is needed where rangelands are transitioning to annual grasslands. Mechanical seedbed preparation can aid native species recovery by reducing annual grass competition. This study was designed to investigate the nature and persistence of hydrologic and erosion impacts caused by different mechanical rangeland seeding treatments and to identify interactions between such impacts and related soil and vegetation properties. A cheatgrass (Bromus tectorum L.)–dominated site was burned and seeded with native grasses and shrubs in the fall of the year. An Amazon-drill and a disk-chain seeder were used to provide varying levels of surface soil disturbance. An undisturbed broadcast seeding was used as a control. Simulated rainfall was applied to 6 large (32.5-m2) plots per treatment over 3 growing seasons at a rate of 63.5 mm · h-1. Rainfall was applied for 60 minutes under dry antecedent moisture conditions and for 30 minutes, 24 hours later under wet antecedent moisture conditions. The disk-chain created the largest reduction in infiltration and increase in sediment yield, which lasted for 3 growing seasons posttreatment. The Amazon-drill had a lesser impact, which was insignificant after the second growing season posttreatment. Surface soil properties showed little correlation with treatment-induced hydrologic and erosion impacts. Hydrologic recovery was strongly correlated with litter dynamics. The seeding treatments were unsuccessful at establishing seeded plant species, and the site once again became dominated by cheatgrass. A continuous upward trend in biomass production and surface litter cover was observed for all treatments between the beginning and end of the study because of cheatgrass invasion. Although the initial goal of using mechanical seeding treatments to enhance recovery of native grass species failed, cheatgrass production provided sufficient biomass to rapidly replenish surface litter cover necessary for rapid hydrologic stability of the site. Adentro y alrededor de la Gran Cuenca, Estados Unidos, se necesita la restauración de la vegetación de estepa arbustiva, donde los pastizales son una transición a zacatales anuales. La preparación mecánica de la cama de siembra puede ayudar a la recuperación de las especies nativas al reducir la competencia de zacates anuales. Este estudio se diseño para investigar la naturaleza y persistencia de los impactos hidrológicos y de erosión causados por diferentes tratamientos mecánicos de siembra de pastizales y para identificar interacciones entre tales impactos y las propiedades relacionadas del suelo y vegetación. Un sitio dominado por “Cheatgrass” (Bromus tectorum L.) fue quemado y sembrado con zacates nativos y arbustos en el otoño. Se usaron una sembradora amazona y una de cadena y discos para crear diferentes niveles de disturbio en la superficie del suelo. Una siembra al voleo en un sitio sin disturbio se usó como control. Se aplicó lluvia simulada, a una tasa de 63.5 mm · h-1, en seis parcelas grandes por tratamiento a lo largo de tres estaciones de crecimiento. La lluvia se aplicó por 60 minutos bajo condiciones secas del suelo y por 30 minutos bajo condiciones húmedas del suelo. La sembradora de cadena y disco produjo la mayor reducción en la infiltración y aumentó la producción de sedimentos, efecto que duró las tres estaciones de crecimiento posteriores a la aplicación de los tratamientos. La sembradora amazona tuvo un impacto menor, el cual fue insignificante después de la segunda estación de crecimiento posterior a la aplicación del tratamiento. Las propiedades de la superficie del suelo mostraron una baja correlación con los impactos hidrológicos y de erosión inducidos por los tratamientos. La recuperación hidrológica estuvo altamente correlacionada con la dinámica del mantillo. Los tratamientos de siembra no fueron exitosos para el establecimiento de plantas de las especies usadas, y el sitio fue dominado nuevamente por el “Cheatgrass.” Se observó una tendencia ascendente continua en la producción de biomasas y la cobertura de mantillo de la superficie en todos los tratamientos entre el inicio y fin del estudio, a consecuencia de la invasión del “Cheatgrass.” Mientras que la meta inicial de usar tratamientos de siembra mecánica para mejorar la recuperación de las especies nativas fracasó, la producción del “Cheatgrass” suministró suficiente biomasa para reponer rápidamente la cobertura de mantillo de la superficie del suelo necesaria para una rápida estabilidad hidrológica del sitio.

Post-fire succession on savanna habitats in the Okavango Delta wetland, Botswana

Drying floodplains that were not inundated for more than 10 y were investigated to relate plant species and vegetation structure to time-since-fire for the southern Okavango Delta wetland in Botswana. The aims of the research were to study post-fire succession on savanna habitats in the Okavango Delta and to investigate the mechanisms behind vegetation changes with time-since-fire. We hypothesize that vegetation structure rather than species occurrence is affected by the fire event and that the driving forces of post-fire succession change from fire-driven to competition-driven with increasing time-since-fire. A total number of 153 study sites were investigated, representing areas of 2–12 y without a fire. Both woody and herbaceous plant species were recorded and cover/abundance values were measured. The vegetation changes with time-since-fire could be assigned to three different successional stages: (1) the post-fire conditions shortly after fire, supporting high species numbers due to open space and little competition; (2) the dominance of competitive perennial grass species (e.g. Urochloa mosambicensis) after 6–8 y; and (3) the enhanced establishment of woody species after about 10 y, creating new habitats in their vicinity. High species numbers were found after short and long fire-free periods, and different species occurred shortly after the fire event and at late stages of the post-fire succession. A variety of different inter-fire intervals is therefore expected to be responsible for sustaining plant species richness and habitat diversity in the study area. Post-fire succession could be described as not only being driven by the life history of the species, but also by seedbank dynamics and plant mobility in early successional stages and by competition and niche differentiation in late-successional stages.

After grazing exclusion, is there any modification of strategy for two guerrilla species: Elymus repens (L.) Gould and Agrostis stolonifera (L.)?

Elymus repens (L.) Gould and Agrostis stolonifera (L.), are competitive grasses with guerrilla strategy that invade grasslands with a low stocking rate. In this work, we tested the hypotheses that grazing exclusion facilitates vegetative development of rhizomes and stolons of these clonal grasses and that such change is a key mechanism for their abundance in set-aside grasslands. The competitive capacities of these two guerrilla species were characterised by samples in plant community (species richness and biomass) and on the level of individual species (morphometric measurements on stolons and rhizomes) during a growing season. Compared to grasslands where grazing was excluded for three years, species richness was higher in grazed site and the plant community structure differed. Indeed, with grazing exclusion, a shift from annual species with a diversified growth-form to perennial species with a tall tussock and graminoid growth-form was monitored. In ungrazed situation, Elymus repens and Agrostis stolonifera were the dominant grasses, and the standing biomass for the lowland community showed a significant increase compared to the grazed site. Vegetative development increased competitive capacities of these two guerrilla species and led by phenomenon of competitive exclusion to the disappearance of annuals species. With grazing cessation, Elymus repens was found to increase the size of aerial traits (shoot length and the number of leaves per shoot) and this may both be propitious for achieving dominance within plant communities and also maintaining its competitive local advantage. By contrast, Agrostis stolonifera showed an increase in a root trait, i.e. rhizome length, in the fenced site, which provide good ability for spatial propagation and then to explore adjacent patches. We concluded that Elymus repens presented a morphological capacity to change its colonising strategy from a guerrilla strategy to a phalanx strategy, by morphological variability of aerial organs, when it was submitted to competitive stress and environment modifications. Agrostis stolonifera showed a capacity to escape aerial competition resulting from grazing cessation, than to increase underground propagation capacity. The present study highlighted the capacities of Elymus repens to respond in an adaptative way to competitive pressure.

LONG-TERM EFFECTS OF SEEDED COVER-CROP ON VEGETATIVE CHARACTERISTICS, YIELD AND GRAPE AND WINE COMPOSITION OF 'GRECHETTO' GRAPEVINES IN CENTRAL ITALY

The long-term effects of inter-row cover-crop (Festuca arundinacea and Lolium perenne), on canopy characteristics, yield and grape and wine composition were studied in Grechetto G109 grapevines grown in Central Italy under non-irrigated conditions. In a six-year trial (1999-2004), cover-crop as compared to tillage constantly reduced vine vigour and yield, without altering must sugar concentration. Festuca arundinacea and Lolium perenne cover-crop lead to a decrease in vineyard efficiency ranging from 9% (rainy seasons) up to 40% (dry season). In dry seasons berry polyphenols and colour were increased by cover-crop, which fostered ripening, thus advancing harvest and reducing risks of fungi disease onset. On the other hand, grass competition for nitrogen lead to a reduction of assimilable nitrogen concentration in the must. In dry seasons, wine from cover-crop plots showed a decrease in alcohol concentration and a marked increase in total polyphenols and colour. Competitive cover-crop can be used to lower vine productivity, without additional costs, whenever yield limitation is required (i.e. high soil fertility, high vigorous rootstocks and cultivar combination, high bud-load, high rainfall during the season, etc.). Yield reduction depends on the season and can be due to a reduction in bud fertility in the seasons following hot and dry years or to a decrease in berry and cluster weights in dry seasons.

Nitrogen and Water Availabilities and Competitiveness of Bluejoint: Spruce Growth and Foliar Carbon‐13 and Nitrogen‐15 Abundance

How resource availabilities affect the competitiveness of Canada bluejoint grass [Calamagrostis canadensis (Michx.) P. Beauv., hereafter referred to as bluejoint] is poorly understood. Bluejoint is a widespread grass species in boreal forests and competes with tree species such as white spruce [Picea glauca (Moench) Voss] for belowground resources (e.g., soil N and water) when their supply is limited. In this greenhouse‐based study, we tested the following hypotheses: (i) bluejoint competition reduces white spruce growth when belowground resource availabilities are limited; (ii) greater N and water availabilities may increase bluejoint competition and its adverse effects on white spruce growth; and (iii) white spruce foliar δ13C and δ15N are affected by soil N and water availabilities and bluejoint competition. A 2 × 2 × 2 (competition × N availability × water availability) factorial experiment was conducted using pots of planted white spruce seedlings with or without bluejoint. Bluejoint competition reduced the volume index (diameter2 × height) of white spruce by 50%. The competitiveness of bluejoint appeared to be independent of resource availabilities, but bluejoint had greater growth response to increased N availability than white spruce. Bluejoint competition depleted white spruce foliar δ13C and δ15N by 1.2 and 1.2‰, respectively, even under adequate water supply, indicating that N deficiency caused by bluejoint competition had a dominant effect (increasing 13C discrimination during photosynthesis) compared with the potential effect of drought stress on foliar δ13C, and that strong NH4 uptake by bluejoint may have prevented significant soil N losses and 15N enrichment through nitrification and subsequent denitrification.

Grass competition induces N2 fixation in some species of African Acacia

1. Indigenous species of Acacia are common in African savannas that have N-rich soils. This raises doubt regarding the extent of plant dependence on N 2 -fixation. Why would Acacia spp. enjoy an advantage over other tree species on N-replete soils? 2. We tested the hypothesis that competition by grass for nutrients would induce increased nodulation of Acacia karroo, A. nilotica, A. tortilis and A. nigrescens seedlings that would enable them to survive better than a non-nodulating congeneric species (A. ataxacantha). A glasshouse pot experiment was conducted to determine the capacity of the Acacia spp. to nodulate. The Acacia spp. were also grown in a randomized field plot experiment in Hluhluwe-iMfolozi Park with and without grass coexistence, and biomass accumulation and δ 15 N values were determined. We also sampled a range of legume and non-legume saplings from the Hluhluwe-iMfolozi Park for δ 15 N values. 3. In the pot experiment all species, except A. ataxacantha, which is a forest margin species, nodulated and consequently had lower δ 15 N isotope values than A. ataxacantha. In the field experiment the δ 15 N values of the plants grown with grass were significantly lower (δ 15 N = 0.77± 0.08‰) than those grown without grass (δ 15 N = 5.0 ± 0.16%o) for all species, except A. ataxacantha. The δ 15 N isotope abundances of field-collected leaves of legume saplings were found to be significantly lower than those of non-legume species (legume δ 15 N = 0.98 ± 0.32%o, non-legume δ 15 N = 2.15 ± 0.32%o). 4. These data confirm that A. karroo, A. nilotica, A. tortilis and A. nigrescens seedlings are capable of nodulating, and do so in their native habitat. The decreased δ 15 N in plants grown with grass indicates that N 2 fixation was strongly enhanced by competition with grass for N. 5. N 2 fixation may thus be an important attribute allowing legume tree seedlings to survive competition with grass through a critical period when tree roots and grass roots must compete for nutrients.

The effect of roots and litter of Calamagrostis canadensis on root sucker regeneration of Populus tremuloides

Summary Marsh reed grass ( Calamagrostis canadensis (Michx.) Beauv.) is a common, highly competitive grass native to the boreal mixedwood forest. This grass increases in abundance after clear-cut logging but little is known about its effects on trembling aspen ( Populus tremuloides Michx.) sucker regeneration. The effects of Calamagrostis sod and its litter on aspen regeneration were studied in two separate greenhouse studies. Calamagrostis sod did not affect the initiation of suckers, but resulted in 30 per cent fewer suckers emerging above the soil that were smaller and had 40 per cent less leaf area. Calamagrostis litter had little effect on the initiation and number of emerged suckers; however, it delayed emergence by 10 days. The physical barrier by roots and litter of Calamagrostis reduced or delayed the expansion of suckers and therefore prolonged their dependence on root reserves. By the time the suckers reached the surface, they had to compete for light with Calamagrostis shoots that had emerged a week earlier. This, coupled with low soil temperatures associated with Calamagrostis in other experiments, will signifi cantly reduce the number and growth of suckers. Any reduction and delay in sucker emergence will decrease aspen regeneration and productivity since the growing season in the boreal forest region is short.

Grass competition induces N2 fixation in some species of African Acacia

Grass competition induces N2 fixation in some species of African Acacia

Efficacy ofSclerotinia minorfor dandelion control: effect of dandelion accession, age and grass competition

Control of Taraxacum officinale (common dandelion) and other broad-leaved weeds in temperate turfgrass has been readily achieved with phenoxy herbicides. The herbicide option has been revoked through municipal and provincial legislation in many regions of Canada, necessitating alternative approaches. We examined the effects of dandelion accessions, age and grass competition on the performance of Sclerotinia minor (IMI 344141) as a biological control for dandelion in turfgrass. Disease symptoms were identical on all 14 different accessions of dandelion and the above- and below-ground biomass were reduced by 94% and 96%, respectively, with no difference among accessions. Foliar damage and dandelion mortality caused by S. minor was affected by plant age and the presence of grass competition. Dandelions of all ages were more severely affected by S. minor in the presence of grass competition. Grass competition had greater impact on foliar biomass, whereas the fungus had a greater impact on root biomass of newly established dandelions. In addition to competition for resources, we were hypothesised that the grass sward provides a microenvironment favouring the success of S. minor as a biological control agent of dandelion. Thus, proper management of the turfgrass environment may be complementary to the efficacy of S. minor as a biocontrol for dandelion.

Biotic constraints on the invasion of diffuse knapweed (Centaurea diffusa) in North American grasslands

Knapweeds (Centaurea spp.) are among the most invasive of non-indigenous plant species that have colonized western North America over the last century. We conducted a 4-year experiment in a reconstructed grassland to test hypotheses related to the ability of grasslands to resist the invasion of diffuse knapweed (C. diffusa). We experimentally invaded C. diffusa and three native species into areas where we manipulated soil nitrogen (N) and phosphorus (P) availability and removed extant grasses to reduce competition. We evaluated the growth response of these species to these resources and competitive manipulations. Of the native species that were experimentally added, only one species, Ratibida pinnata (prairie coneflower), established in any numbers. Establishment values in intact vegetation were low for both species, but establishment by C. diffusa (0.02%) clearly outperformed that of R. pinnata (0.001%). Under reduced grass competition, establishment was enhanced, but the values for C. diffusa (0.68%) were not statistically different from those of R. pinnata (0.57%). Neither species performed better under higher soil nutrients in the presence of competing grasses. In plots with both species, biomass of the two planted species was positively correlated, but the biomass of both species was negatively correlated with non-added weedy species. Subsequent harvests of C. diffusa indicated that establishment was enhanced in treatments with higher soil nutrients but that the biomass of these plants could only be enhanced when plant competition was also reduced. These results indicate that C. diffusa can establish in intact grasslands at rates higher than natives, but opportunism rather than competitive ability best describes the invasiveness of C. diffusa. Thus, the mechanisms contributing to the establishment of this knapweed species are different from factors identified as contributing to the dominance of this invader.

Reply to Short Comment on: 'Nitrogen mineralisation in relation to previous crops and pastures'

In a recent paper we reported rapid nitrogen mineralisation in soil where lucerne and a mixture of the perennial grasses Phalaris aquatica and Dactylis glomerata had been growing (Angus et al. 2006). We were surprised that pastures containing perennial grasses led to higher mineralisation rates than pure lucerne pastures, because we expected that pure lucerne would fix more N and its residue would release mineral N more rapidly than a lucerne+ perennial grass mixture. In their comment on this paper, Luxhoi and HauggaardNielsen (2007, this issue) found the results to be unsurprising and offered 2 explanations. They speculated that pastures containing perennial grass had recovered fertiliser N, some of which was later mineralised. They also speculated that the presence of grass may have reduced the level of soil mineral N and, hence, stimulated symbiotic N2-fixation by lucerne. The first speculation is inappropriate as no N fertiliser was applied to the pastures in our experiments, reflecting normal practice for pastures growing on mixed cropping–livestock farms in Australia. The only fertiliser applied was phosphatic. The second speculation is probably unjustified; N2-fixation by the different pastures was not compared in the season when mineralisation was measured. However, in a previous season at the same experiment, N2-fixation by pure lucerne was substantially greater than by the lucerne+ perennial grass mixture (Peoples et al. 1998).While itwas reasonable forLuxhoi andHauggaard-Nielsen (2007) to presume that the proportion of lucerne N obtained from biological fixation might be increased by grass competition, this was not reflected in the earlier measurements of Peoples et al. (1998), and any gain would be more than offset by the reduction in lucerne growth. Luxhoi and Hauggaard-Nielsen (2007) also point out that similar results to ours were reported from an experiment in Denmark by Hauggaard-Nielsen et al. (1998). We accept that the results were similar, but the nature of the experiments was so different that any similarity may have been coincidental. They reported high rates of net mineralisation in soil after ungrazed mixed grass–clover leys that had received no N fertiliser, but which contained 87 kg/ha of above-ground N at the time of incorporation in spring. Our experimental system consisted of grazed pastures, all with negligible above-ground biomass in autumn when they were cultivated before sowing. Their experiment may have received significant input of mineral

Influence of Herbivory and Competition on Invasive Weed Fitness: Observed Effects of &lt;I&gt;Cyphocleonus achates&lt;/I&gt; (Coleoptera: Curculionidae) and Grass-Seeding Treatments on Spotted Knapweed Performance

The root-feeding weevil Cyphocleonus achates (Fahraeus) is a promising biological control agent for managing the exotic, invasive weed spotted knapweed. The objective of this study was to compare the relative and potentially interactive effects of competition and specialized herbivory on spotted knapweed fitness. Competition was assessed through three grass seeding treatments: a nonseeded control and seeding to long-lived wheatgrasses originating either from North America, Pseudoroegneria spicata (Pursh) A. Love ssp. spicata, or Europe, Thinopyrum intermedium (Host) Barkworth and D.R. Dewey. Comparisons were made of above-ground biomass, stem length, number of flowers, and life stage densities for spotted knapweed plants either infested or free of C. achates and growing under one of the grass seeding treatments. Infested adult spotted knapweed plants had lower shoot weight, stem length, and flowers/plant. The density of adult spotted knapweed plants was lower in grass-seeded plots than in nonseeded control plots. Results indicate C. achates herbivory is correlated with a reduction in measures of fitness and reproductive potential in field populations of spotted knapweed in North America. Furthermore, these results suggest that this agent might play a significant role in reducing spotted knapweed populations when competitive grasses are also present.

Interactions between influences of genotype and grass competition on growth and wood stiffness of juvenile radiata pine in a summer-dry environment

Clones of radiata pine (Pinus radiata D. Don) were established in two experiments on two contrasting pasture sites in Canterbury, New Zealand, with four levels of competition from grass as main plots within each experiment. Control was extended over the first 5 years at one site, whereas grass at the other site was controlled by a single herbicidal spray immediately after planting. At the latter site, 1 m diameter spots cleared with herbicide were compared with the same size spot created using a woven mat. Grass competition added up to 3 years to a rotation, and the gain from control was correlated with spot size. Competition control was most critical during the first year after planting. Trees grown within spots cleared by mats grew more slowly than those in weed-free spots cleared with herbicide. Green dynamic modulus of elasticity was related to genotype and increased with both slenderness and higher levels of pruning. Significant interactions between genotype and level of competition were observed for growth, indicating that choice of genotype can be a useful way of coping with pasture competition if managers wish to reduce the use of herbicides.

Canopy Growth and Density of Wyoming Big Sagebrush Sown with Cool-Season Perennial Grasses

Post-mining revegetation efforts often require grass seeding and mulch applications to stabilize the soils at the same time as shrub seeding, creating intraspecific competition between seeded shrubs and grasses that is not well understood. Artemisia tridentata Nutt. ssp. wyomingensis (Beetle and Young) (Wyoming big sagebrush) is the dominant premining shrub on many Wyoming mines. The Wyoming Department of Environmental Quality, Land Quality Division requires reestablishment of 1 shrub m−2on 20% of post-mined lands in Wyoming. Reclamationists seldom document the impacts of grass competition on shrub canopy size after reclamation plantings become established even though shrub canopy development is important to vegetative structural diversity. In 1999, we initiated a study at the Belle Ayr Coal Mine near Gillette, Wyoming, to evaluate the influence of grass competition on establishment and growth of Wyoming big sagebrush. Combinations of three sagebrush seeding rates (1, 2, and 4 kg pls ha−1) and seven cool-season perennial grass mixture seeding rates (0, 2, 4, 6, 8, 10, and 14 kg pls ha−1) were seeded during winter 1998–1999. Shrub density and grass cover were assessed from 1999 to 2004. We monitored sagebrush canopy size in 2001, 2002, and 2004. All sagebrush seeding rates provided shrub densities ≥1 shrub m−2 after six growing seasons. Grass production ≥75 g m−2 was achieved by seeding grasses at 6 to 8 kg pls ha−1. Canopy growth of individual sagebrush plants was least in the heaviest grass seeding rate. Reduced grass seeding rates can aid in achieving Wyoming big sagebrush density standards and enhance shrub canopy growth.