Effects of Soil Fumigation and Seeding Regimes on Secondary Succession in a Semiarid Shrubland

In order to ascertain the degree to which disturbance and subsequent recovery of belowground biotic components (mycorrhizal and decomposer systems) influence successional dynamics, a portion of semiarid shrubland in northwest Colorado was mechanically disturbed in 1984. Two treatments, soil fumigation (fumigated with methyl bromide and not fumigated) and seeding (early-seral species, late-seral species, and not seeded) were imposed on 500 m2 plots in each of four replications. Aboveground annual production by species, and decomposition rates were measured in 1991, 7 years after the initial disturbance and treatments. Mycorrhizal infection potential had been previously measured at this site from 1984-1988. Seeding to late-seral species resulted in greater production rates (and greater community composition) of late-seral species, whereas seeding to early-seral species had only a minor effect when compared to unseeded plots 7 years after disturbance. Fumigation slowed the rate of succession on unseeded plots, primarily by decreased production of perennial grasses. Mycorrhizal infection potential, initially greatly reduced by the disturbance and by fumigation, recovered to near-background levels on unfumigated plots but not on fumigated plots after 4 years. Decomposition experiments suggest that seeding may have facilitated recovery of the decomposer system after initial disturbance and fumigation. Our results suggest that the redevelopment of soil biotic communities following disturbance has a significant impact on successional dynamics, particularly the replacement of early-seral annuals by mid-seral perennial grasses.     

Genetic correlations between competition traits and traits scored at breeding field-tests in Icelandic horses

Relationships between breeding field-test traits and competition traits were studied to investigate whether the latter could be usefully included in the genetic evaluation of Icelandic horses. The current method of genetic evaluation is based on records from breeding field-tests only. The breeding field-test data included 16 401 individual records of Icelandic horses evaluated in 11 countries during 1990–2005. Competition results included 18 982 records of 3790 horses competing in sport and gæðinga competitions in Iceland and Sweden during 1998–2004. In the breeding field-tests, eight conformation traits and eight riding ability traits were scored; height of withers was also recorded. These traits were analysed together with the competition traits tölt(comp), 4-gait, 5-gait and pace test, in bivariate analyses. Animal models were used; the fixed effects for breeding field-test traits included sex by age interaction and country by year interaction. For the competition traits the model included fixed effects of sex, age and event, and a random permanent environmental effect. Estimated heritabilities and genetic correlations for breeding field-test traits were consistent with earlier results; heritabilities ranged from 0.20 to 0.67, and moderate to high genetic correlations were estimated between many of the riding ability traits, and between riding ability traits and some conformation traits. The estimated heritabilities for competition traits were about 0.20, and genetic correlations between competition traits varied from − 0.12 to 1.00. In general, high genetic correlations were estimated between breeding field-test riding ability traits and competition traits. Moderately positive genetic correlations were found between some breeding field-test conformation traits and competition traits. Competition traits add information relating to the breeding goal of the Icelandic horse; they should therefore be added to genetic evaluation in future.     

低丘红壤南酸枣一花生复合系统物种问水肥光竞争的研究—Ⅱ．南酸枣与花生利用光能分析

试验研究结果表明低丘红壤南酸枣遮荫使复合系统花生光合有效辐射减弱以及花生产量和生物量明显下降 ,与单作花生相比间作各行花生相对光合有效辐射值日变化主要与距南酸枣树行远近有关 ,与复合系统花生产量和生物量中间行高而两侧行低的趋势相一致 ,两者呈显著正相关。     

Optimising the legume symbiosis in stressful and competitive environments within southern Australia—some contemporary thoughts

In the managed agricultural ecosystems of southern Australia, if an edaphic environment is not stressful to root-nodule bacteria (hereafter rhizobia), it is likely to become a competitive environment for nodulation (although not always detrimentally so) soon after the introduction of an inoculated legume. We suggest that stressful environments limit rhizobial communities to less than 100 cells g⁻¹ soil at some time during the season. This overview puts forward the hypothesis that in perturbed ecosystems (i.e. those that are intensively managed) such as in the 25 million ha of the southern Australian grain and grazing belts, the rhizobial community is still substantially immature in an evolutionary sense. The rhizobial community is representative of only a few species, primarily those of Mediterranean origin that were accidentally introduced, or have been fostered by legume development programs, or remnants of the populations associated with native legumes. We consider there is little inter-specific competition for substrates because of this relative immaturity, but suggest that intra-specific competition for nodulation is commonplace wherever abiotic stress is absent. We nominate two primary abiotic stresses that are permanently present that have limited rhizobial colonization or legume nodulation for some species in southern Australia and four secondary (temporary) abiotic stresses. We believe that selection of adapted symbioses, or where warranted adapted elite rhizobial strains or legume host genotypes, can overcome these stress factors. We emphasise that where several abiotic stress factors are present they may act synergistically, but that this net effect is still likely to be symbiosis-specific. We acknowledge that genetic transformation in situ is providing new strain variability with which we must contend. We also put forward the suggestion that opportunities exist for the managed introduction of selected genotypes of agricultural legumes that effectively interact with rhizobial communities to achieve optimal N-fixation. In doing so, we give more precise definition to the widely used terms ‘exclusive’, ‘selective’ and ‘promiscuous’ nodulation.     

Ecology and use of arctic fox Alopex lagopus dens in Norway: tradition overtaken by interspecific competition?

Arctic fox Alopex lagopus dens in Norway are mostly large and well-defined structures of great age. Examinations of these dens have been used in surveys of the breeding and population status of the arctic fox in Fennoscandia. In this study, dens all over north Norway and in some parts of south Norway were surveyed (n=214), and several aspects of den characteristics, localisation and use were recorded. Red fox Vulpes vulpes dens were also recorded whenever found, including red fox use of arctic fox dens (total n=211). Most of the arctic fox dens were of the “classic” type dug out in sandy soil (n=210). The average number (±1 SD) of open entrances (at least half open) was 27.4±19.0, and the average size was 363.1±288.7 m². Regional variation was found in both these measurements, as well as in two variables classifying the vegetation at the den. However, no consistent south-north gradient was found, although the most southern and the most northern regions were extremes in several respects. A positive correlation was found between den size and the number of entrances, as well as between these two variables and the two vegetation variables. For each arctic fox den, height above the tree line, distance to the tree line, distance between arctic fox dens, distance to the nearest red fox den, distance to a main road and distance to other human activity were estimated. Regional variation was found in all these variables, but no consistent south-north gradient. Again, the most southern region deviated most. There were large differences in the proportion of dens used by arctic foxes between the nine regions, with one region outstanding in its production of arctic fox litters and paucity of red foxes. The proportion sum of arctic fox entrances to sum of red fox entrances reliably, but not perfectly, identified the best arctic fox regions. Arctic fox dens that were known to have produced litters at least once during 1980-2001 had about twice as many entrances as dens without pups, were 32% larger and had a more lush vegetation. They were also situated further from a red fox den. Distance to red fox dens and height above the tree line were the most important factors regarding arctic fox use of dens. Thus, the red fox was identified as a threat to the arctic fox population.     

Savanna herbivore dynamics in a livestock-dominated landscape. II: Ecological, conservation, and management implications of predator restoration

Conserving African wildlife in human-occupied landscapes requires management intervention that is guided by a mechanistic understanding of how anthropogenic factors influence large-scale ecological processes. In Laikipia District, a dry savanna region in northern Kenya where wildlife share the landscape with humans and livestock, we examined why five of nine wild ungulate species suffered protracted declines on properties receiving the greatest conservation investment. Of 10 alternative causes examined, only an increase in predation, interacting with brief periods of high and low rainfall, was consistent with the timing, synchrony, duration and species composition of observed ungulate declines.The principal factor causing predation to increase was a shift in land use from cattle ranching, under which predators and plains zebras were severely suppressed, to wildlife conservation and ecotourism. This prompted a 5-fold increase in plains zebra abundance, and created a demand for living predators. Plains zebras ultimately comprised more than half the available prey biomass, and supported a substantial predator community, but were not limited by predators. We infer that increasing predation pressure caused predator-susceptible prey species to decline, via mechanisms that included apparent competition.Herbivore dynamics in Laikipia shared features with previously reported responses by prey communities to predator manipulation in Kruger and Serengeti National Parks. All featured one or a few numerically dominant herbivore species, which were primarily limited by rainfall and density, supporting a predator community that in turn limited the abundance of other prey species. In each case, predation had a profound effect, but on only a subset of prey species, reducing the evenness component of prey diversity.The presence of cattle in the landscape may affect predator-prey dynamics in both direct and indirect ways, depending on rainfall. In extreme years (floods or drought), episodic die-offs temporarily subsidize scavenging predators. In low rainfall years, competition between plains zebras and cattle, which negligibly support predators, may indirectly limit predator carrying capacity. Consequently, removal of cattle may favor not only zebras, but also their predators, and further depress predator-susceptible prey species.     

Pecan production increased by larger vegetation-free area surrounding the tree

Vegetation control with herbicides is a common practice in pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards that benefits both non-bearing and bearing trees. However, the appropriate size for the vegetation-free area surrounding bearing trees had not been resolved. Vegetation-free circles centered on the Tree 0, 0.9, 1.8, 3.7, and 7.3 m in diameter were established at planting and tree performance evaluated. This study reports the first six years of production that began in the sixth growing season. Yield was not affected by treatment in the first two years of production while yield was low and some trees had not begun to produce. The next four years were characterized by a strong positive relationship between vegetation-free diameter with annual yield and with cumulative yield. Weight per nut and kernel percentage were rarely affected by treatment. Leaf elemental concentrations of selected essential nutrients were not affected by treatment. Trunk diameter was similar among treatments each year. This indicates that yield was improved by the vegetation-free area impacting pistillate flower induction or flower and fruit retention rather than improved tree growth increasing the bearing surface.     

兰州市果品生产现状与提升市场竞争力的措施

在对兰州市果品产业发展现状及存在问题调研与分析的基础上 ,从加大树种品种结构调整力度、应用优质栽培技术、推行无公害生产技术、健全市场流通体系和销售网络、建设果品龙头企业、实施名牌战略等方面提出了提升兰州市果品市场竞争力的措施     

Assessing the impact of increasing carbon dioxide and temperature on crop-weed interactions for tomato and a C3 and C4 weed species

Based on the carboxylation kinetics of the C₃ and C₄ photosynthetic pathway, it is anticipated that C₃ crops may be favored over C₄ weeds as atmospheric CO₂ increases. In the current study, tomato (Lycopersicon esculentum), a C₃ crop species, was grown at ambient (~400 μmol mol⁻¹) and enhanced carbon dioxide (~800 μmol mol⁻¹) with and without two common weeds, lambsquarters (Chenopodium album), a C₃ weed, and redroot pigweed (Amaranthus retroflexus), a C₄ weed, from seedling emergence until mutual shading of crop-weed leaves. Because growth temperature is also likely to change in concert with rising CO₂, the experiment was repeated at day/night temperatures of 21/12 and 26/18 °C. For both day/night temperatures, elevated CO₂ exacerbated weed competition from both the C₃ and C₄ weed species. A model based on relative leaf area following emergence was used to calculate potential crop losses from weeds. This analysis indicated that potential crop losses increased from 33 to 55% and from 32 to 61% at the 21/12 and 26/18 °C day/night temperatures, for ambient and elevated CO₂, respectively. For the current study, reductions in biomass and projected yield of tomato appeared independent of the photosynthetic pathway of the competing weed species. This may be due to inherent variation and overlap in the growth response of C₃ and C₄ species, whether weeds or crops, to increasing CO₂ concentration. Overall, these results suggest that as atmospheric CO₂ and/or temperature increases, other biological interactions, in addition to photosynthetic pathway, deserve additional consideration in predicting competitive outcomes between weeds and crops.     

竞赛激励式教学方法在园林工程课程教学改革中的应用

根据园林工程课程的特点,分析了进行课程改革的必要性,阐述了竞赛激励式教学模式在园林工程专业课程教学中的应用,结果表明,该模式应用于教学中的效果明显,有效激发了学生的学习积极性和主动性,增强了学生的学习自信心。