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Weed Populations (weed + population)
Selected AbstractsContrasting infection frequencies of Neotyphodium endophyte in naturalized Italian ryegrass populations in Japanese farmlandsGRASSLAND SCIENCE, Issue 2 2010Masayuki Yamashita Abstract Neotyphodium endophytes often confer benefits to their host grasses and may enhance invasiveness of some grasses. The knowledge of infection frequencies of endophytes among invading weed populations is necessary to understand the relationships between endophyte infection and invasiveness. Here we present data on infection frequencies of Italian ryegrass (Lolium multiflorum Lam.), an important weed in some farmlands in Japan, persisting in contrasting farmlands: a terraced paddy field and a wheat-soybean double-cropped field in the western region of Shizuoka prefecture, Japan. The terraced paddy site is a mosaic of several landscape elements such as paddy fields, levees, fallow and abandoned fields, with a high percentage of non-crop area. Rice (Oryza sativa L.) has been cultivated for more than a decade with no application of chemical fertilizers, pesticides and fungicides. The wheat-soybean field is characterized by the aggregation of large-scaled fields that were originally reconstructed paddy fields, showing a low percentage of non-crop area. Wheat and soybean have been grown as winter and summer crops, respectively, using chemical fertilizers and herbicides. We examined the presence or absence of endophytes in a total of 1200 seeds sampled from the two Italian ryegrass populations. The terraced paddy population exhibited a markedly high infection frequency (91.0%), due possibly to selective feeding of non-infected seeds by insects. In contrast, the wheat-soybean farmland population showed almost no infection (1.1%), whereas the putative source of the invasion in the proximity exhibited a relatively high infection rate (64.4%). Such a micro-scale variation in infection frequencies may be attributable to a loss in endophyte viability within the wheat-soybean field. The findings suggest that endophyte infection frequency may markedly differ among the Italian ryegrass populations even within the same region, presumably depending on the abundance of the seed-eating insects, farmland management regimes and/or environmental conditions such as soil humidity. [source] Seasonal patterns in post-dispersal seed predation of Abutilon theophrasti and Setaria faberi in three cropping systemsJOURNAL OF APPLIED ECOLOGY, Issue 5 2006ANDREW H. HEGGENSTALLER Summary 1Post-dispersal seed predation is an important source of mortality for arable weed populations that can potentially contribute to ecologically based management strategies. The extent to which cropping practices influence rates of seed predation is not well established. 2Removal of Abutilon theophrasti and Setaria faberi seeds by predators was measured in 2-year (maize/soyabean), 3-year (maize/soyabean/triticale + red clover) and 4-year (maize/soyabean/triticale + lucerne/lucerne) crop rotations in Iowa, USA, during 27 48-h sampling periods conducted during the 2003 and 2004 cropping seasons. 3Predation of S. faberi seeds was equal (eight sampling periods) or greater (19 sampling periods) than predation of A. theophrasti seeds, but the influence of crops on seasonal predation patterns was generally similar for the two weeds. 4Seasonal patterns in seed predation were crop-specific and complementary. In maize and soyabean, seed predation was low in spring, high in summer and low in autumn. In triticale,legume intercrops, seed predation was high in spring, low in summer and moderate in autumn. In lucerne, seed predation fluctuated from high to low, matching the periodic harvest and regrowth cycle of the crop. 5Measurements of crop canopy light interception taken in 2004 were positively correlated with rates of seed removal for both A. theophrasti (r = 0·54, P < 0·001) and S. faberi (r = 0·25, P < 0·001), suggesting that vegetative cover promotes weed seed predation. 6Synthesis and applications. The results indicate that crop vegetation can be managed to promote weed seed predation. Diversified farming systems that include a range of phenologically dissimilar crop species are likely to provide the greatest opportunities for weed seed destruction by predators. [source] Are weed population dynamics chaotic?JOURNAL OF APPLIED ECOLOGY, Issue 5 2002Robert P. Freckleton Summary 1There have been suggestions that the population dynamics of weeds may show chaotic dynamics, and that therefore it will not be possible to predict the impact of changing management regimes on weed abundance. The instability of weed populations is presumed to result either from overcompensating yield,density responses or from threshold management. 2Using theoretical arguments and empirical evidence we argue that this contention is likely to be incorrect. 3Overcompensating yield,density responses are unlikely in plant populations and this point has been extensively discussed. Such responses have only been observed in high-density artificially sown stands of weed populations. The form of chaos that results from threshold management is a consequence of high population growth resulting from the cessation of management when weed densities are lower than a threshold level. Consequently the dynamics of such populations may be argued to be extrinsically rather than intrinsically driven. 4There are many studies that have shown weed populations to be dynamically stable, both spatially and temporally. Here we present an analysis of data from the Broadbalk experiment that demonstrates long-term stability of 12 species of common weeds over a 12-year period. Using parameter estimates derived from the literature we show that the stability of these populations is similar to other annual species, both weedy and non-weedy. 5We argue that weed population dynamics are more generally better viewed as resulting from the impacts of broad-scale types of management, as well as temporal variability in population numbers. The significance of chaotic dynamics is likely to be minimal. [source] The irreversible cattle-driven transformation of a seasonally flooded Australian savannaJOURNAL OF BIOGEOGRAPHY, Issue 5 2003Ben R. Sharp Abstract Aim ,Anecdotal historical and photographic evidence suggests that woody vegetation is increasing dramatically in some northern Australian savanna habitats. Vegetation change in savannas has important implications for pastoral land-use, conservation management, and landscape-scale carbon storage, and informs theoretical debates about ecosystem function. This study seeks to determine the nature, extent and cause(s) of woody vegetation change in a seasonally flooded alluvial savanna habitat. Location ,The study area is located within the seasonally inundated alluvial zone of the tidal portion of the Victoria River, Northern Territory, Australia. The study area has been grazed by domestic stock since c. 1900, prior to which the area was inhabited and more likely regularly burnt by Aboriginal people for thousands of years. Methods ,Digital georeferenced aerial photographic coverages were used to examine and quantify woody vegetation change between 1948 and 1993. Transect surveys of woody and herbaceous vegetation were carried out to ground-truth air-photo results and determine the nature and causes of observed vegetation changes. Results ,There has been a dramatic increase in woody vegetation cover throughout the study area. Vegetation change patterns are roughly uniform across the full range of edaphic habitat variation and are unrelated to the depositional age of fluvial sediments. Two woody species, Eucalyptus microtheca and Excoecaria parvifolia, are predominantly responsible for observed increases. Demographic analyses reveal that woody invasions have been episodic and indicate that in most locations peak woody species establishment occurred in the mid-1970s. Grasses are almost absent in a majority of habitats within the study area. Instead, large areas are covered by scalded soil, dense invasive weed populations, and unpalatable forbs and sedges. What grasses do occur are predominantly of very low value for grazing. The condition of the herbaceous layer renders most of the study area almost completely non-flammable; what fires do burn are small and of low intensity. Main conclusions ,Multiple working hypotheses explaining observed patterns of woody vegetation increase were considered and rejected in turn. The only hypothesis consistent with the evidence is as follows: (1) observed changes are a direct consequence of extreme overgrazing by cattle, most likely when stocking rates peaked in the mid-1970s; (2) prolonged heavy grazing effected the complete transformation of much of the herbaceous vegetation to a new state that is not flammable; and (3) in the absence of regular fire mortality, woody vegetation increased rapidly. The relatively treeless system that existed in 1948 was apparently stable and resilient to moderate grazing levels, and perhaps also to episodic heavy grazing events. However, grazing intensity in excess of a sustainable threshold has forced a transition that is irreversible in the foreseeable future. Stable-state transitions such as this one inform debates at the heart of ecological theory, such as the nature of stability, resilience, equilibrium and carrying capacity in dynamic savanna ecosystems. [source] Tracing back seed and pollen flow within the crop,wild Beta vulgaris complex: genetic distinctiveness vs. hot spots of hybridization over a regional scaleMOLECULAR ECOLOGY, Issue 6 2004Frédérique Viard Abstract Hybrids between transgenic crops and wild relatives have been documented successfully in a wide range of cultivated species, having implications on conservation and biosafety management. Nonetheless, the magnitude and frequency of hybridization in the wild is still an open question, in particular when considering several populations at the landscape level. The Beta vulgaris complex provides an excellent biological model to tackle this issue. Weed beets contaminating sugar beet fields are expected to act as a relay between wild populations and crops and from crops-to-crops. In one major European sugar beet production area, nine wild populations and 12 weed populations were genetically characterized using cytoplasmic markers specific to the cultivated lines and nuclear microsatellite loci. A tremendous overall genetic differentiation between neighbouring wild and weed populations was depicted. However, genetic admixture analyses at the individual level revealed clear evidence for gene flow between wild and weed populations. In particular, one wild population displayed a high magnitude of nuclear genetic admixture, reinforced by direct seed flow as evidenced by cytoplasmic markers. Altogether, weed beets were shown to act as relay for gene flow between crops to wild populations and crops to crops by pollen and seeds at a landscape level. [source] Managing arable weeds for biodiversityPEST MANAGEMENT SCIENCE (FORMERLY: PESTICIDE SCIENCE), Issue 6 2007Jonathan Storkey Abstract As a result of the recent intensification of crop production, the abundance and diversity of UK arable weeds adapted to cultivated land have declined, with an associated reduction in farmland birds. A number of questions need to be addressed when considering how these declines can be reversed. Firstly, can the delivery of crop production and biodiversity be reconciled by spatially separating cropping from designated wildlife areas? A number of subsidised environmental schemes in the UK take this approach and are focused on establishing vegetation cover on uncropped land. However, because of the lack of regular disturbance in these habitats, they are dominated by perennials and they therefore have limited potential for promoting the recovery of annual weed populations. A number of farmland bird species also rely on the provision of resources in field centres, and it is therefore likely that the recovery of their populations will rely on weed management options targeted at the cropped areas of the field. This raises two further questions. Firstly, is it possible to identify beneficial weed species that are relatively poor competitors with the crop and also have biodiversity value? Secondly, are the tools available to manage these species at acceptable levels while controlling pernicious weeds? A number of approaches are being employed to answer these questions, including predicting yield loss from weed competition models and exploiting herbicide selectivity. The further development of these tools is crucial if farmer opposition to managing weeds in crops is to be overcome. Copyright © 2007 Society of Chemical Industry [source] Differential response of Amaranthus tuberculatus (Moq ex DC) JD Sauer to glyphosatePEST MANAGEMENT SCIENCE (FORMERLY: PESTICIDE SCIENCE), Issue 10 2005Ian A Zelaya Abstract Midwest USA farmers have reported inconsistent control of Amaranthus tuberculatus (= rudis) (Moq ex DC) JD Sauer by glyphosate in glyphosate-resistant crops. The potential of selection for decreased A tuberculatus sensitivity to glyphosate was therefore investigated in a reportedly resistant Everly, IA population (P0,EV). Evaluation of six A tuberculatus populations from the Midwest USA estimated a seedling baseline sensitivity of 2.15 mM glyphosate. Based on these results, three generations of divergent recurrent selection were implemented on P0,EV to isolate resistant and susceptible populations. A seedling assay was developed to screen large amounts of seeds and thus expedite the selection process. Whole-plant and seedling rate responses of P0,EV and a known pristine A tuberculatus population from Paint Creek, OH (P0,WT) identified no significant difference in response to glyphosate; however, greater phenotypic variance was ostensibly evident in P0,EV. The first recurrent generation selected for resistance at 3.2 mM glyphosate (RS1,R) had a 5.9- and 1.7-fold resistance increase at the seedling and whole-plant levels, respectively, compared with the susceptible generation selected at 32 µM glyphosate. After three cycles of recurrent selection, 14.6-fold difference in resistance at the seedling level and 3.1-fold difference at the whole-plant level were observed when comparing the populations selected for resistance (RS3,R) and susceptibility (RS3,S). Overall, recurrent selection increased the frequency of resistant individuals and decreased the variability to glyphosate at the population level. Nevertheless, variability for glyphosate resistance was still evident in RS3,R. Results herein suggested that A tuberculatus is inherently variable to glyphosate and that selection decreased the sensitivity to glyphosate. We purport that evolved glyphosate resistance in A tuberculatus may require multiple cycles of selection under field conditions. Historic estimated use of glyphosate alludes to the evolution of tolerant weed populations. Copyright © 2005 Society of Chemical Industry [source] | ||||||||||