Predicting the spread of the American grey squirrel (Sciurus carolinensis) in Europe: A call for a co-ordinated European approach

The introduction of the American eastern grey squirrel (Sciurus carolinensis) in Europe is causing a dramatic decline of the native European red squirrel (Sciurus vulgaris), due to competition. We used spatially explicit population dynamics models to illustrate the potential expansion of the grey squirrel from Italy, the only continental country with the species, into neighbouring countries. Simulations started with the known grey squirrel distributions in 1996 and modelled the spread for the next 100 years in two different scenarios. One “best case” in which a random seed crop pattern of poor, good and mast seed crops was simulated with concomitant effects on squirrel fecundity and mortality and one “worst case” scenario in which poor years were not included. In the best case scenario, it will take 30-40 years for grey squirrels to start invading the Alps and about 70-75 years to cross the border between Italy and France. First populations in Switzerland are predicted in 2051-2066. In the worst case scenario, the spread will be significantly faster: the species is predicted to reach France in 2026-2031 and Switzerland in 2031-2041. Removing two of the three populations of grey squirrel will likely postpone the invasion of Switzerland but not of France. Large areas of Europe contain potentially good habitat for grey squirrels, and expansion from Italy, to France, Switzerland and belong to most of Eurasia is simply a matter of time. This will represent a serious risk for the conservation of the native red squirrel and will have implications for national forest operations. However, a European policy towards introduced species is still lacking. There is therefore a need for a co-ordinated European approach to the management of the grey squirrel and other introduced species.     

Influence of biopores on root growth,water uptake and grain yield of wheat (Triticum aestivum) based on predictions from a computer model

Summary The use of vertical biopores by wheat (Triticum aestivum) seminal roots for easy access to the subsoil and the consequences for plant water supply and yield has been investigated by computer simulation. Parameters included were: biopore density and diameter, depth of cultivation and strength of the subsoil — all under a wide range of seasonal weather conditions. The model predicts that biopores add significantly to root penetration at depth, even at a density of 0.1% v/v of small, vertical pores, while 1.5% to 2.0% v/v can ensure maximum root penetration. When the growing season is shorter a larger number of biopores is needed to ensure timely root penetration to depth. With shallow tillage, biopores occur closer to the soil surface, and their importance is increased. Deeper root penetration invariably gives greater water uptake and transpiration, but may have a negative effect on grain yield, especially under the driest climatic conditions. An increase in early water use may result in less soil water being available during the grain-filling period. The effect of biopores on plant transpiration varies from year to year, depending on the amount of rain and its distribution in time, and on the amount of soil water stored at time of sowing.     

Assessment of the FAO AquaCrop model in the simulation of rainfed and supplementally irrigated maize, sugar beet and sunflower

Farming in Serbia is traditionally rainfed. Analyses show that drought events of varying severity are frequent in this region, although there is no specific pattern. There is a distinct need for an objective assessment of the impact of drought on strategic field crops, to solve the dilemma whether irrigation is required or not. For this reason, and based on available field data, the FAO AquaCrop water driven model was selected to simulate yield and irrigation water use efficiency (IWUE) for three major field crops (maize, sunflower, and sugar beet), under two scenarios: (1) natural water supply and adequate supply of nutrients, and (2) supplementary irrigation and adequate supply of nutrients. The experiments presented here were conducted between 2000 and 2007 in northern Serbia, where chernozem soil is prevalent. Data of 2003 cropping seasons were used for local calibration, whereas the remaining years for validation. Results were such that local calibration resulted in very minor changes of AquaCrop coefficients (e.g., maize basal crop coefficient, sunflower harvest index, etc.). Simulated maize yield levels exhibited the greatest departure from measured data under irrigation conditions (−3.6 and 3.3% during an extremely dry and an extremely wet year, respectively). Simulated sunflower yield levels varied by less than 10% in 8 out of 10 comparisons. The most extreme variation was noted during the extremely wet year. The difference between simulated and measured values in the case of sugar beet was from −10.2 to 12.2%. Large differences were noted only in two or three cases, under extreme climatic conditions. Statistical indicators - root mean square error (RMSE) and index of agreement (d) - for all three crops suggested that the model can be used to highly reliably assess yield and IWUE. This conclusion was derived based on low values of RMSE and high values of d (in the case of maize and sugar beet 0.999 for both yield and IWUE, and in the case of sunflower 0.999 for yield and 0.884 for IWUE). It is noteworthy that under wet conditions, the model suggested that sunflower and sugar beet do not require irrigation, as confirmed by experimental research. These data are significant because they show that the AquaCrop model can be used in impartial decision-making and in the selection of crops to be given irrigation priority in areas where water resources are limited.     

MecaNiCAL, a supply–demand model of carbon and nitrogen partitioning applied to defoliated grass: 2. Parameter estimation and model evaluation

The MecaNiCAL model of carbon and nitrogen partitioning over a period of regrowth for a mean grass plant as described in Part 1 was fitted and evaluated against data from an experiment under controlled conditions with contrasting light and nitrogen nutrition. In the first investigation, the rate of nitrogen uptake was imposed to limit the compensation of errors between process. The root:shoot ratio was simulated well, with a good estimation of the nitrogen allocation to shoots. Some underestimation of the root weight after the cut and the final shoot weight under the low nitrogen treatment were consequences of both the lack of remobilisation and the high priority given to shoots. The good agreement obtained with the expected trend in simulations conducted with the simple nitrogen aborption submodel in extended conditions (contrasting initial content of nitrogen substrate and temperature change in regrowth) ensures the general consistency of the model in the limit of its area of validity. Partitioning to shoots and forage production seem to be sensitive mainly to the size of the carbohydrate substrate pool and its availibility, particularly under conditions of low light. Estimating the initial carbohydrate substrate pool by biochemical analysis may improve the accuracy of the model. Compared with more simple assumptions of partitioning, MecaNiCAL shows a higher level of agreement with the partitioning response to a wide range of environmental conditions. A simple assumption relating partitioning to plant nitrogen content does not seem to be sufficient to simulate the decrease in the root:shoot ratio observed under conditions of low light. Nevertheless, more data from experiments conducted under a wider range of contrasting conditions are required to estimate the validity of MecaNiCAL attached to the nitrogen absorption submodel and the advantage of its more complex assumptions with even greater accuracy.     

A simulation model for evaluating technical and economic aspects of an industrial eucalyptus-based agroforestry system in Minas Gerais, Brazil

This study analyses the economic aspects of a eucalypt-based agroforestry system established in the savanna region of Brazil. In 1993,eucalypt clones were planted at a spacing of 10 × 4 m, to facilitate their future association with crops and pasture grasses. The results show that the establishment and maintenance costs represent 37% of the total costs associated with the system. More than half of the revenues are from the sale of the wood products obtained in the 11-year rotation. Variations of ±20% of the sale price of the cattle significantly affected the sensitivity analysis, closely followed by the variations of the price of wood for lumber and energy. Variations of ±20% of the establishment costs of the forest component significantly affected the economic indicators. The viability of the system is also sensitive to a slight change in interest rate. Finally, the agroforestry system adopted by the company is economically more attractive than eucalypt monoculture.This revised version was published online in November 2005 with corrections to the Cover Date.     

拖拉机运输机组制动性能的计算机仿真

运用多刚体力学的分析方法对轮式拖拉机运输机组(以下称机组)的制动过程进行了力学及运动学分析,建立了制动数学模型,编制了制动性能仿真程序并进行了实例计算,通过道路试验对仿真结果进行了验证。