Carduus Nutans Research Articles

Seasonal life‐history models for the integrated management of the invasive weed nodding thistle Carduus nutans in Australia

Summary It is widely accepted that combining several control options into integrated pest management strategies is the most effective way to provide long‐term suppression of pest populations. However, full factorial field trials of all single and integrated control options for a pest species would be prohibitively expensive and time consuming. Methods to allow triage of the huge array of management options would be of great value in streamlining the decision‐making process. We present a seasonally structured, individual‐based model, specifically designed to compare and rank detailed management strategies for a noxious pasture weed. The model structure is determined in part by the demographic data available, and in part by the management options under consideration. The case study is for nodding thistle Carduus nutans in Australia. Eight years of demographic data, for more than 8000 mapped plants, were used to parameterize the model, which is age‐, size‐ and density‐dependent and incorporates individual variation. Management options for this plant include three biocontrol agents, as well as conventional herbicide and grazing management strategies, which can be used alone or in a variety of combinations. Data on management impacts were drawn from multiple studies. The model predicts that the root‐crown weevil Trichosirocalus mortadelo will more effectively suppress weed populations than either of the two flowerhead‐feeding insect agents Urophora solstitialis and Rhinocyllus conicus. Crash grazing (up to four times the regular grazing pressure) in any single season, or when most effectively applied across spring and summer, is less effective than T. mortadelo, while combinations of crash grazing and biocontrol agents strongly decrease weed population persistence. However, lethal herbicide is the best single strategy, while spring spray–grazing (a combination of non‐lethal herbicide and grazing) is the best integrated weed management strategy. Synthesis and applications. The model is structured by, and serves to integrate, available information on demography and management from multiple sources. The subset of strategies that performed well forms the focus for fewer, more thorough, field trials. The decision‐making approach illustrated here is also applicable to any species and any array of management options.

Estimating Biophysical Characteristics of Musk Thistle (Carduus nutans) With Three Remote Sensing Instruments

Identifying the dynamics and extent of noxious weeds in a spatial and temporal context improves monitoring, planning, and management practices. Musk thistle (Carduus nutans L.), a noxious weed, is a good candidate for detection by remote sensing platforms because it may produce a unique spectral signature due to a large, purple-red flower head. Therefore, 3 remote sensing instruments—a ground hyperspectral spectrometer, a multispectral ground radiometer, and an airborne hyperspectral imaging spectrometer—were used to establish regression models between reflected data and the biophysical parameters (density, height, flower head density, and percent ground cover) of musk thistle. The coefficients of determination (R2) obtained from simple regression models for vegetation indices and musk thistle biophysical variables ranged from 0.46 to 0.77. Multiple regression models with up to 3 variables increased R2 by an average of 0.07. This study indicated that normalized difference and simple ratio indices can be used for specific applications such as detection of musk thistle biophysical variables in rangelands. Once applied to the image, these results will produce a map of parameters that can be used to determine the size of infestation and the reduction in rangeland productivity.

Estimating Biophysical Characteristics of Musk Thistle (Carduus nutans) With Three Remote Sensing Instruments

Identifying the dynamics and extent of noxious weeds in a spatial and temporal context improves monitoring, planning, and management practices. Musk thistle (Carduus nutans L.), a noxious weed, is a good candidate for detection by remote sensing platforms because it may produce a unique spectral signature due to a large, purple-red flower head. Therefore, 3 remote sensing instruments—a ground hyperspectral spectrometer, a multispectral ground radiometer, and an airborne hyperspectral imaging spectrometer—were used to establish regression models between reflected data and the biophysical parameters (density, height, flower head density, and percent ground cover) of musk thistle. The coefficients of determination (R2) obtained from simple regression models for vegetation indices and musk thistle biophysical variables ranged from 0.46 to 0.77. Multiple regression models with up to 3 variables increased R2 by an average of 0.07. This study indicated that normalized difference and simple ratio indices can be used for specific applications such as detection of musk thistle biophysical variables in rangelands. Once applied to the image, these results will produce a map of parameters that can be used to determine the size of infestation and the reduction in rangeland productivity.

Predicting and mitigating weed invasions to restore natural post-fire succession in Mesa Verde National Park, Colorado, USA

Six large wildfires have burned in Mesa Verde National Park during the last 15 years, and extensive portions of burns were invaded by non-native plant species. The most threatening weed species include Carduus nutans, Cirsium arvense, and Bromus tectorum, and if untreated, they persist at least 13 years. We investigated patterns of weed distribution to identify plant communities most vulnerable to post-fire weed invasion and created a spatially explicit model to predict the most vulnerable sites. At the scale of the entire park, mature piñon–juniper woodlands growing on two soil series were most vulnerable to post-fire weed invasion; mountain shrublands were the least vulnerable. At a finer scale, greater richness of native species was correlated with greater numbers of non-native species, indicating that habitats with high native biodiversity are at the greatest risk of weed invasion. In unburned areas, weed density increased with greater soil nitrogen and phosphorus, and lower salinity. In burned areas weed density correlated with soil nitrogen status and textural class. We also evaluated the effectiveness of a variety of weed mitigation methods; aerial seeding of targeted high-risk areas with native grasses was the most effective treatment tested. We recommend a conservative mitigation plan using natives grass seed on only the most invasible sites.

Environmental variability and the initiation of dispersal: turbulence strongly increases seed release.

Dispersal is a critical process in ecology. It is an important biological driver of, for example, invasions, metapopulation dynamics, spatial pattern formation and pathogen movement. Much is known about the effect of environmental variability, including turbulence, on dispersal of diaspores. Here, we document experimentally the strong but under-explored influence of turbulence on the initiation of dispersal. Flower heads of two thistle species (Carduus nutans and Carduus acanthoides) with ripe seeds were exposed to series of laminar and turbulent air flows of increasing velocity in a wind tunnel. Seed release increased with wind speeds for both laminar and turbulent flows for both species. However, far more seeds were released, at significantly lower wind speeds, during turbulent flows. These results strongly suggest a need for more quantitative studies of abscission in the field, as well as dispersal models that incorporate variability in the diaspore release phase.

CONTEXT-DEPENDENT BIOLOGICAL CONTROL OF AN INVASIVE THISTLE

Carduus nutans (nodding or musk thistle) is an important invasive plant of Eurasian origin. Biological control of this species, using insects that attack rosettes or developing seed heads, has met with varied success in different parts of its invaded range. Here we develop and compare simple demographic matrix models for populations of this species in Australia and New Zealand, to explore reasons for these differences. In a New Zealand population, rapid population growth of C. nutans is driven by early life history transitions. In an Australian population, fecundity of C. nutans is of reduced importance, and survivorship of rosettes plays an increased role. These differences suggest how biocontrol agents that are successful at providing control in one situation may fail in another. Theoretical explorations of the models show which life history transitions drive the differences in matrix elasticities. We suggest that characteristics of the invaded community also play a role in invasion success of this species, and develop theoretical and empirical approaches to assess what factors may drive population growth, and hence what control methods are most likely to work, under different circumstances.

Migratory flight and pre-diapause feeding of Coccinella septempunctata (Coleoptera) adults in agricultural and mountain ecosystems of Central Italy

The research on pre-diapause feeding of C. septempunctata L. adults was carried out from 1998 to 2002 in the Tiber Valley, in the Botanical Garden of Perugia University, and in the Sibillini Mountains National Park (Central Italy). The species reproduces in spring feeding mostly on aphids infesting Graminaceae. In the second half of June, newly emerged adults have various migratory behaviours. Some of them remain in the valley while some leave the cereals and fly to other crops, where they feed both on 15 aphid species (e.g., Aphis gossypii Glov. on melon, Aphis fabae Scop. on sugarbeet), and also on pollens of wild plants such as Carduus nutans L., and fungi spores. However, most of the adults migrate to mountain pastures to find different food sources. During this migration, strong south-western storms often force the ladybirds to shelter in urban habitats. When the sunny weather returns again, due to the mistral (north-western wind), the adults continue the flight in swarms to mountain areas. In 1999, on Mt. Vettore (2478 m) we witnessed a "shower" of ladybirds, some of which hid in the turf. When reaching high altitude locations still covered with snow (Lake Pilato, 1940 m, on Mt. Vettore), the adults, exhausted by the long flight, usually remain motionless on the snow. Due to the warming of the sun they sink to a depth of 5-6 cm, so that they remain trapped in the frozen snow during the night. At this site C. septempunctata was found together with Vibidia duodecimguttata (Poda) and Leptinotarsa decemlineata (Say). We investigated the gut content of adults from this location. After aggregation, the hungry adults disperse to flowering plants at lower elevations where they feed on 8 aphid species (e.g. broom aphid) and pollens of several plants (e.g., Laserpitium garganicum Tem., Centaurea rupestris L., Cachrys ferulacea L. The first mating was observed on broom. At "Pian Grande" location, adults were found in crop fields (lentil, mustard) and wild pastures (cornflower, gentian) mating and feeding on aphids, pollens, nectar and fungal spores. Haymaking forces ladybirds to move towards higher locations in search of food. Echinops ritro L., Eryngium amethystinum L., Carlina acaulis L., Cirsium eriophorum (L.), Galium verum L. and Euphorbia cyparissias L. were major food sources at 1800 m altitude. Gut content of C. septempunctata adults showed that pre-diapause diet consisted of aphids, pollen of Compositae and Umbelliferae, and spores of fungi (e.g., Alternaria spp., Cladosporium spp.). There was no statistical difference between the number of adults containing remains of aphids, fungal spores, and pollens. We found four significant correlations between the different food sources observed. In August some adults stopped feeding and in diapause condition sheltered under stones and among leaves or seed capsules of plants where they had fed. Others migrated to nearby woods and aggregated between juniper needles.

Can an integrated management approach provide a basis for long‐term prevention of weed dominance in Australian pasture systems?

SummaryBroad‐leaved weeds in pasture, such as Carduus nutans, Onopordum spp. and Echium plantagineum are a major problem for graziers in southern Australia. Previous attempts to combat these weeds with a single technique have only resulted in short‐term success. An approach to long‐term control, combining biological control with different grazing and herbicide strategies, was evaluated in an integrated weed management (IWM) programme, in south‐eastern Australia. This IWM study was one of the few that has focused on biological control agents. During the field trials, the impacts of grazing and herbicide treatments on the weed and biological control agents, as well as on pasture composition, were monitored. This paper concentrates on the part of the study that focuses on the role and importance of pasture composition as part of weed management. The main pasture components were monitored using botanal, a sampling technique for estimating species composition and pasture yield in the field. IWM is a long‐term ecological approach and after 3 years, major trends were just becoming apparent. This study shows that pasture composition can be manipulated to increase productivity and sustainability. It demonstrates that broad‐leaved weeds can be reduced when high level pasture background management and chemical control are combined.

EVALUATION OF ECOLOGICAL RISK TO POPULATIONS OF A THREATENED PLANT FROM AN INVASIVE BIOCONTROL INSECT

Controversy exists over estimation of ecological risk in biological control. At present, the risk to the rare, federally listed Pitcher's thistle (Cirsium pitcheri )i nNorth America from Rhinocyllus conicus ,a biological control weevil now feeding on many native thistles, is unknown. We hypothesized that quantification of host specificity and potential phenological overlap between insect and plant would improve assessment of the magnitude of risk. In laboratory host specificity tests, we found no significant difference in R. conicus feeding or oviposition preference between the rare C. pitcheri and the targeted exotic weed (Carduus nutans )o rbetween C. pitcheri and Platte thistle (C. canescens), a closely related native North American species known to be affected by R. conicus .I na garden environment, R. conicus spontaneously found, oviposited, and developed completely on C. pitcheri .T aller plants with more flower heads were significantly more vulnerable, suggesting that the greatest impact is likely to be on individuals that generally contribute the most to recruitment and population persistence. For eight sites in two national parks over three years, the calculated period of expected R. conicus activity overlapped 99% and 78% of the flower heads initiated by C. pitcheri in the southern and the northern park, respectively. A de- mographic model suggests that population growth rate ( )o fC. pitcheri will decrease from 0.9897 to 0.8686, while time to halve the population will decrease from 66.9 to 4.9 years, under the conservative assumption that oviposition by R. conicus on C. pitcheri will occur at the same rate as on the related C. canescens. Calculated decreases in and t0.5 are larger if the rate of oviposition actually observed in the laboratory tests is used. These results indicate that the weevil poses a serious quantitative, demographic risk to the threatened C. pitcheri. The study supports the suggestion that ecological data can be used to improve the quantification of risk to native nontarget plant populations within the potential physiological host range of a biological control insect.

Quantifying the impact of a weed in a perennial ryegrass–white clover pasture

Any plant not sown from seed is often labeled a weed in improved pastures of New Zealand. Most improved pastures are a mix of perennial ryegrass and white clover but generally are infested with broadleaf weeds. Changes in forage production due to individual weeds were determined using measurements of perennial ryegrass and white clover before and after dairy cattle, beef cattle, or sheep grazing under, near, and far from individual plants of six rosette-forming weed species throughout a growing season. The larger weeds, bull thistle and musk thistle, reduced the amount of forage utilized 42 and 72%, respectively, in beef cattle– and sheep-grazed hill-country pastures. Forage production under and near Canada thistle, hedge mustard, broadleaf plantain, and hairy buttercup in a dairy pasture was greater (136, 140, 178, and 450%, respectively) than in the control areas. Although the dairy pasture was grazed following recommended grazing procedures, our results indicate that this grazing system was not maximizing forage yield potentials of perennial ryegrass and white clover and that these weeds served as an indicator that the pasture was being overgrazed.

Supplemental risk evaluations and status of Puccinia carduorum for biological control of musk thistle

Abstract Tests of seven rare and endangered native North American Cirsium species and four modern artichoke lines were requested in response to a proposal for introduction of Puccinia carduorum into the United States for biological control of musk thistle ( Carduus nutans ssp. leiophyllus ). These tests were supplemental to an earlier extensive host-range study that established P. carduorum from musk thistle as host specific, useful for biological control, and suitable for limited field tests in Virginia. Test plants in the current study were evaluated in support of a proposal to use the rust in the western United States, and particularly, in California. None of the test plants in this study had been evaluated in previous assessments and all were either rare, endangered or threatened in California. Tests were conducted in both field and greenhouse settings. Field tests were run for two seasons, and test plants were inoculated by natural spread of the pathogen from source plants inside rings of test plants. Greenhouse tests involved direct inoculation under optimal conditions of dew and temperature (18–20 °C, 16 h) for infection. None of the seven Cirsium species or subspecies tested became infected by P. carduorum , either in field or greenhouse tests, compared to infection of 98% of the individual musk thistle plants ( n = 102) from all the studies. Modern artichoke cultivars were tested only by direct inoculation under optimal greenhouse conditions. All artichoke plants ( n = 115) either were immune (no macroscopic symptoms, n = 69) or at most, resistant ( n = 46); pustules on all but two of the resistant plants were very small (⩽0.30 mm diam). Despite infections on artichokes, P. carduorum could not be maintained on artichokes under optimal greenhouse conditions. These results confirm earlier findings from host-range tests and risk assessments of P. carduorum . This information suggests that rare, threatened, or endangered Cirsium spp. and modern artichoke cultivars are not likely to be adversely affected by the use of P. carduorum for biological control of musk thistle. These data have been reviewed by grower groups and regulatory agencies in a proposal for permission to use the rust for musk thistle control throughout the United States.

Modeling for Management of Invasive Species: Musk Thistle (Carduus nutans) in New Zealand1

Mathematical modeling is an important tool with which managers can understand, and hence plan to control, invasive species. A matrix model for the invasive musk thistle, derived from demographic studies in New Zealand, is used to conduct a retrospective analysis of the relative merits of three biological control agents released to control this species: Rhinocyllus conicus, Urophora solstitialis, and Trichosirocalus horridus. The analysis of musk thistle illustrates the utility of modeling approaches in management and demonstrates how model-derived information could shift the focus of future management efforts. By placing management in an adaptive framework, where active learning is used to improve management decisions, existing management strategies can be further improved to cope in situations where optimal control methods may depend on local conditions.Nomenclature: Musk thistle, Carduus nutans L. #3 CRUNU.Additional index words: Adaptive management, biological control, nodding thistle, optimal management, uncertainty.Abbreviation: AAM, active adaptive management.

Local- Vs. Landscape-Scale Indirect Effects of an Invasive Weed on Native Plants1

Insect-mediated indirect interactions between native plant species recently have been shown to be important determinants of plant performance in a number of ecological communities. However, the potential indirect effects of exotic plant invasion on native plant species are not well understood. We examined whether the presence or proximity of the targeted exotic weed, musk thistle, influences the magnitude of attack on native thistles by the introduced biological control, flowerhead weevil. At the local scale, we quantified weevil egg densities on heads of the native wavyleaf thistle growing at different distances (0 to 100 m) from patches of the exotic thistle. Densities were significantly higher when the native thistle occurred within, vs. 30 to 50 m or 80 to 100 m from, patches of the exotic thistle, indicating a strong local ''spillover effect.'' At larger scales, we measured egg densities on wavyleaf thistle within grassland landscapes (2.4 3 2.4 km 2 ) with varying infestation densities of the invasive musk thistle. We found that egg densities increased significantly with increasing invasive thistle densities measured at larger site and landscape scales. Because flowerhead weevil feeding substantially reduces seed production of wavyleaf thistle, exotic thistle populations are likely to have indirect negative effects on these native thistles. Our results provide strong empirical evidence that exotic plants can increase the attack on native plant species by maintaining populations of a shared insect herbivore. This finding suggests that persistence of exotic weeds in less-successful biocontrol programs will magnify the nontarget effects of weed biocontrol insects. Nomenclature: Musk thistle, Carduus nutans spp. leiophyllus (Petrovic) Stoj. & Stef. # 3 CRUNU;

EXOTIC WEED INVASION INCREASES THE SUSCEPTIBILITY OF NATIVE PLANTS TO ATTACK BY A BIOCONTROL HERBIVORE

Landscape change has great, yet infrequently measured, potential to influence the susceptibility of natural systems to invasive species impacts. We quantified attack by an invasive biological control weevil (Rhinocyllus conicus) on native thistles in relation to two types of landscape change: agricultural intensification and invasion by an exotic thistle, Carduus nutans, the original target of biological control. Weevil egg load was measured on native thistles in three landscape types: (1) agriculture dominated, (2) grassland dominated with exotic thistles, and, (3) grassland dominated without exotic thistles. We found no difference in egg load on native thistles within grassland landscapes without exotic thistles vs. within agricultural landscapes, suggesting that agricultural intensification per se does not influence levels of weevil attack. However, attack on the native Cirsium undulatum increased significantly (three- to fivefold) with increasing exotic thistle density. Within-patch exotic thistle density explained >50% of the variation in both the intensity and frequency of weevil attack. Since R. conicus feeding dramatically reduces seed production, exotic thistles likely exert a negative indirect effect on native thistles. This study provides some of the first empirical evidence that invasion by an exotic plant can increase attack of native plants by shared insect herbivores.

Individual and combined effects of Trichosirocalus horridus and Rhinocyllus conicus (Coleoptera: Curculionidae) on musk thistle

Field experiments were conducted in Northeast Kansas under conditions of limited plant competition to evaluate the individual and combined impact of the imported weevils Rhinocyllus conicus Froelich and Trichosirocalus horridus (Panzer) on the introduced weed, musk thistle (Carduus nutans L.). No effects on seed production occurred when low larval densities of T. horridus (<20 per plant) fed in rosettes, with or without the later-arriving, head-feeding weevil, R. conicus, present. In contrast, under high T. horridus larval densities (66 per plant), production of flower heads was reduced compared to uninfested plants. This indirect effect resulted in a 30% loss in viable seed per plant (with R. conicus present). Plant survival was not affected by T. horridus within the range of larval densities tested. R. conicus, when present alone, reduced viable seed by approximately 45%, whereas high densities of both weevils reduced viable seed by 59%. Thus, under conditions of low plant competition, neither weevil substantially limited seed production by musk thistle. This suggests the need for a multi-faceted management program for musk thistle involving additional forms of stress, such as interspecific plant competition.

Grazing of pasture weeds by goats and sheep. 1. Nodding thistle (Carduus nutans)

Goats and sheep grazed replicate plots of perennial pasture infested with nodding thistle (Carduus nutans). Neither goats nor sheep ate the vegetative thistle until it reached ~30 cm in diameter at which time goats consumed outer leaves on some plants. Active goat grazing commenced at flowering as the capitula matured from green to purple, with subsequent preferential grazing of the purple capitula in the presence of abundant pasture (2500 kg green DM/ha). The stage of maturation of the thistle influenced the proportion of purple capitula eaten by goats. Over the extended (5 month) 1991–92 flowering period induced by above average summer rain, dispersal of viable seeds was prevented in goat-grazed plots as goats ate 100% of the flowering stems. In the short 1992–93 flowering season (1.5 months) dry conditions resulted in seedheads rapidly maturing from green to brown. Goat numbers were insufficient to eat 100% of capitula as they reached the palatable stage. Consequently viable seeds were dispersed. Sheep consumed the thistle capitula only when pasture became limiting, allowing viable seeds to be dispersed in both flowering seasons. Spraying with non-lethal doses of herbicide increased the proportion of vegetative thistles consumed by both sheep and goats. Goats selectively removed maturing capitula then grazed the stems of the nodding thistle. As goats consumed thistle capitula throughout the flowering period, the quantity of pasture in goat plots exceeded that in sheep plots, principally due to an increase in the proportion of clover. The stocking density of goats required to control nodding thistle is proportionate to the level of weed infestation and must be modified as a response to prevailing seasonal conditions and rate of removal of purple capitula. Grazing of the purple capitula of nodding thistle by goats in lieu of pasture indicates the potential for mixed animal species grazing of weed-infested pastures.

Indirect effect of early-season infestations of Trichosirocalus horridus on Rhinocyllus conicus (Coleoptera: Curculionidae)

Abstract Field experiments were conducted to determine whether early vegetative feeding by Trichosirocalus horridus alters musk thistle ( Carduus nutans ) as a resource for the later-arriving Rhinocyllus conicus . Results showed that thistles with high larval densities of T. horridus (66 per plant) produced more and shorter flower stems, significantly fewer flower heads, and delayed flowering by 1 week when compared to uninfested thistles. Also, colonization and oviposition on flower heads by adult R. conicus were as much as three and five times lower, respectively, on thistles heavily infested with T. horridus than on uninfested thistles. As a result, 63% fewer R. conicus adults developed from T. horridus -infested thistles. Musk thistles that were infested with lower densities of T. horridus larvae ( R. conicus to T. horridus -infested and uninfested plants. In a greenhouse experiment, the mortality rate of R. conicus was higher on musk thistles that were heavily infested by T. horridus , implying a change in musk thistle quality. However, this effect was apparent only at low R. conicus larval densities. T. horridus can indirectly and negatively affect R. conicus through a variety of mechanisms, but only when densities of T. horridus are high. This finding may have important implications for the continued redistribution of both weevils, which were imported for the classical biological control of musk thistle, a noxious weed in pastures and rangeland.

Conifer Regeneration Problems in Boreal and Temperate Forests with Ericaceous Understory: Role of Disturbance, Seedbed Limitation, and Keytsone Species Change

Conifer regeneration failure in the presence of dense ericaceous cover resulting from the removal of canopy trees by forest harvesting observed in boreal and temperate forest has been attributed to allelopathy, competition, and soil nutrient imbalance. Ecosystem-level alleopathic effect has been argued as a cause for conifer regeneration failure by citing examples from a species-poor boreal forest in northern Sweden with ground vegetation dominated by crowberry (Empetrum hermaphroditum, Ericales) and New Zealand dairy pastures invaded by nodding or musk thistle (Carduus nutans). This article aims to explain the phenomenon of vegetation shift from conifer forest to ericaceous heath by extending the argument of ecosystem-level impact of ericaceous plants and linking the disturbance-mediated regeneration strategies of the dominant conifer species and the understory ericaceous species with the quality of seedbed substrate that influence the direction of secondary succession. It has been argued that fire severity plays a pivotal role in controlling seedbed quality and the regeneration mechanisms of conifers, which in turn determines the direction of post-disturbance succession. The post-fire-dominated ericaceous plants and their habitat-modifying effects have been explained from the point of view of keystone species concept and their role as ecosystem engineers. In the absence of high severity natural fires the canopy keystone species (conifer) fails to regenerate successfully mainly due to limitation of favorable seedbed. On the other hand, the understory ericaceous plants regenerate vigorously by vegetative methods from the belowground components that survived the fire. Forest harvesting by clearcutting or selective cutting also create similar vigorous vegetative regrowth of ericaceous plants, but conifer regeneration suffers from the lack of a suitable seedbed. Thus in the absence of successful conifer regeneration, the vigorously growing understory ericaceous plants become the new keystone species. The new keystone ericaceous species bring about a significant long-term habitat change by rapid accumulation of plyphenol-rich humus. Ericaceous phenolic compounds have been found to inhibit seed germination and seedling growth of conifers. By forming protein-phenol complexes they cause a further reduction of available nitrogen of the already nutrient-stressed habitat. A low pH condition in the presence of phenolic compounds causes the leaching of metallic ions and forms hard iron pans that impair soil water movement. The phenolic allelochemicals of ericaceous humus are also inhibitory to many conifer ectomycorrhizae.On the other hand, ericaceous plants perpetuate in the community by their stress-tolerating strategies as well as their ability to acquire nutrients through ericoid mycorrhizae. Three mechanisms working at the ecosystem level can be suggested as the cause of vegetation shift from forest to ericaceous heath. These are (1) the absence of high severity natural fire and the limitation of suitable conifer seedbed in the presence of thick humus, (2) increased competition resulting from the rapid vegetative regeneration of understory ericaceous plants after forest canopy opening by harvesting or nonsevere fire, and (3) habitat degradation by phenolic allelochemicals of ericaceous plants causing a soil nutrient imbalance and iron pan formation. Thus, a shift in keystone species from conifer to ericaceous plant in the post-disturbance habitat may induce a retrogressive succession due to ecosystem-level engineering effects of the new keystone species.Vegetation management in conifer-ericaceous communities depends on land management objectives. If the objective is to produce timber and other forest products then the control of ericaceous plants and site preparation is necessary after forest harvesting. Ploughing and liming followed by conifer planting and repeated N fertilization has been applied successfully to promote afforestation of Calluna heathlands in Britain. However, such practice has not been proven successful in the reforestation of Kalmia-dominated sites in eastern Canada. If, on the other hand, the land management objective is to maintain heathlands for herbivore production or conservation of cultural landscape, as in the case of certain Calluna-dominated heathland in Western Europe, then moderately hot prescribed burning is useful as a management tool.

Successful Biological Control of the Musk Thistle in Oklahoma Using the Musk Thistle Head Weevil and the Rosette Weevil

Rhinocyllus conicus Froelich, the musk thistle head weevil, was released in northeastern Oklahoma beginning in 1991 for biological control of Carduus nutans L., the musk thistle. By 2001, weevils had been released in 34 counties. Trichosirocalus horridus (Panzer), the rosette weevil, was released in six counties in 1998; and additional introductions in 2000 and 2001 brought the total number of county releases for this species to 29. Release areas were surveyed in 2000 and 2001 to determine the level of weevil establishment. Head weevils were recovered from 30 of 34 counties. Thistle populations were considered well infested if 30% of the heads had >4 larvae or pupae present. In 63% of the counties, ≥25% of the sites were well infested. Thistle densities had been reduced by 25–90% in 13 counties in Oklahoma where weevils had been released. Rosette weevils were recovered in three of the six original release counties and in one county where no releases were made until 2001. A combination of head and rosette weevils in the rosette weevil recovery areas provided a synergistic reaction with thistle density reductions occurring faster than in sites where only head weevils were released. Head weevils are established in Oklahoma and are effectively reducing thistle infestations. Rosette weevils also are established in several of the 1998 release areas.

Weed establishment in the second year after high pasture sowing rates

A field and an outdoor tray experiment were conducted to determine whether weed control benefits of high sowing rates of perennial ryegrass (Lolium perenne) continue into the second year after sowing Seeds of Scotch thistle (Cirsium vulgare) nodding thistle (Carduus nutans) and hedge mustard (Sisymbrium officinale) were sown into perennial ryegrass swards that were established 12 months earlier at differing densities Emerging weed seedlings were mapped at regular intervals to enable their fate to be followed Weed emergence and survival did not decline with increased ryegrass sowing rates indicating that 12 months after sowing the competitive ability of pasture sown at low density towards weeds equalled that of pasture sown at high density