Comparing landscape scale vegetation dynamics following recent disturbance in climatically similar sites in California and the Mediterranean basin

A long line of inquiry on the notion of ecological convergence has compared ecosystem structure and function between areas that are evolutionarily unrelated but under the same climate regime. Much of this literature has focused on quantifying the degree to which animal morphology or plant physiognomy is alike between disjunct areas. An important property of ecosystems is their behavior following disturbance. Yet, this aspect of ecosystems has not been investigated in a comparative study of convergence. If different ecosystems are under similar environmental controls, then one would predict that the rates and patterns of response to disturbance would also be similar. The objective of this study is to compare landscape dynamics following disturbance using spatiotemporal models to quantify vegetation change in Mediterranean ecosystems found in California and Israel. We model the process of tree and shrub regeneration at the landscape scale in two similar study sites in Israel (Mount Meron) and California (Hasting Nature Reserve). During the periods studied (1964-1992 for Israel and 1971-1995 for California), average annual change in tree cover was 5 times larger in Israel than in California. Based on multiple regression models, differences were found in the relative importance of specific variables predicting vegetation change. In Hastings (California), initial tree cover accounted for most of the explained variability in 1995 tree cover (partial R² = 0.71), while in Meron (Israel), grazing type and intensity, topography indices, and initial vegetation each accounted for about a third of the explained variability. These findings support the notion that traits such as regeneration pattern and rate, both at the individual level and at the landscape level, were largely affected by the human land use history of the region.     

Assessing variability in fecundity of Amaranthus powellii using a simulation model

Estimates of weed fecundity and its variability are critical for the development of population dynamic models and for evaluating the long‐term consequences of weed management practices. The purpose of this research was to estimate Amaranthus powellii fecundity across years, seasons and competitive environments using a mechanistic model. Existing models were modified to account for weed responses to shade, and to dynamically simulate seed production among subthreshold densities of A. powellii. The model was parameterized and tested using five sets of field data in which A. powellii was grown either alone or with transplanted broccoli. The model overestimated A. powellii height, but predicted both dry weight and fecundity well. Mean simulated fecundity for A. powellii ranged from 0 to 268 000 seeds per plant depending on year, crop maturity date, relative time of emergence and location. Year‐to‐year variability in simulated fecundity was large with coefficients of variation ranging from 77 to 128 under low and high competitive environments respectively. Our results suggest that estimates of weed fecundity based on 1 or 2 years of empirical data may result in significant errors in population dynamic models, and that the use of economic optimum thresholds based on weed density alone entails considerable risks.     

Sheep farming systems and nature management of rangeland in French Mediterranean mountain areas

For the past ten years, sheep farmers from the French Mediterranean area have been able to use agri-environmental farming schemes to prevent the development of forest on rangelands and to preserve biodiversity in these open areas. These new environmental concerns are the background for an investigation into the management of long-term interactions between farm practices and systems on grazed land. This includes an analysis of how farmers change their management practices.Considering these practices in their socio-technical dimension, we carried out a survey among sheep farmers within the Luberon Natural Regional Park to analyse the process of transformation in practice, as sheep farm sizes are also roughly increasing. We will show how changes in practice are linked with the ability of farmers to integrate these into new farming systems. But it is also related to the social status of these practices in relation to local professional groups, who define the social framework concerning farming activities and who attribute value to different practices.Integrating these links between the technical and social dimensions of practices is a challenge for producing indicators able to evaluate the abilities of farming systems to change in order to participate in the medium term environmental management.     

Species Dynamics in Disturbed Landscapes: When does a Shifting Habitat Mosaic Enhance Connectivity?

Although landscape ecology emphasizes the effects of spatial pattern on ecological processes, most neutral models of species–habitat relationships have treated habitat as a static constraint. Do the working hypotheses derived from these models extend to real landscapes where disturbances create a shifting mosaic? A spatial landscape simulator incorporating vegetation dynamics and a metapopulation model was used to compare species in static and dynamic landscapes with identical habitat amounts and spatial patterns. The main drivers of vegetation dynamics were stand-replacing disturbances, followed by gradual change from early-successional to old-growth habitats. Species dynamics were based on a simple occupancy model, with dispersal simulated as a random walk. As the proportion of available habitat (p) decreased from 1.0, species occupancy generally declined more rapidly and reached extinction at higher habitat levels in dynamic than in static landscapes. However, habitat occupancy was sometimes actually higher in dynamic landscapes than in static landscapes with similar habitat amounts and patterns. This effect was most pronounced at intermediate amounts of habitat (p = 0.3?0.6) for mobile species that had high colonization rates, but were unable to cross non-habitat patches. Differences between static and dynamic landscapes were contingent upon the initial metapopulation size and the shapes of disturbances and the resulting habitat patterns. Overall, the results demonstrate that dispersal-limited species exhibit more pronounced critical behavior in dynamic landscapes than is predicted by simple neutral models based on static landscapes. Thus, caution should be exercised in extending generalizations derived from static landscape models to disturbance-driven landscape mosaics.     

Simulating Spatial Nitrogen Dynamics in a Forested Reference Watershed, Hubbard Brook Watershed 6, New Hampshire, USA

We demonstrate that available information on spatial heterogeneity in biotic, topographic, and climatic variables within a forested watershed, Hubbard Brook Experimental Forest (HBEF) Watershed 6, New Hampshire, USA, was sufficient to reproduce the observed elevational pattern in stream NO₃ concentration during the 1982–1992 period. Five gridded maps (N mineralization factor, N uptake factor, precipitation, elevation, and soil depth factor) were created from spatial datasets and successively added to the spatially explicit model SINIC-S as spatially varying input parameters. Adding more spatial information generally improved model predictions, with the exception of the soil depth factor. Ninety percent of the variation in the observed stream NO₃ concentration was explained by the combination of the spatial variation of the N mineralization and N uptake factors. Simulated streamflow NO₃ flux at the outlet point was improved slightly by introducing spatial variability in the model parameters. The model exhibited substantial cell-to-cell variation in soil N dynamics and NO₃ loss within the watershed during the simulation period. The simulation results suggest that the spatial distributions of forest floor organic matter and standing biomass are most responsible for creating the elevational pattern in stream NO₃ concentration within this watershed.     

Effect of historic landscape change on the genetic structure of the bush-cricket Metrioptera roeseli

This study investigates the impact of past and present landscape structure on the current genetic structure of the bush-cricket Metrioptera roeseli (Orthoptera, Tettigoniidae) in a rural landscape in Germany. Assuming that land-use types, such as grassland, arable land and forest, as well as linear structures, mainly roads, differentially affect the connectivity of the bush-cricket's habitat and therefore migration and gene flow, we correlated landscape parameters between sampling locations as derived from GIS-maps with genetic similarities between individual bush-crickets as estimated by RAPD-PCR. Fifty bush-crickets were sampled with distances between sampling locations varying between 15 m and 2 km. Corresponding landscape configurations were recorded in 8 years between 1945 and 1998. Landscape configuration 50 years ago appeared to have influenced the present genetic structure of the bush-cricket (R ² = 0.18). Crossing roads and land use other than grassland along the transect between sampling locations tended to decrease genetic similarity, whereas grassland and parallel roads tended to increase genetic similarity between bush-crickets. Following shifts in land use during 1953–1973 the correlation between landscape and present genetic structure decreased gradually. Our study suggests that it needs time for the landscape to build a visible effect on the genetic structure of the bush-cricket population, and that this effect cannot be detected if the landscape changes faster than the genetic structure responds to it.     

川西北线叶嵩草草地群落动态规律及其与环境因子的效应分析

对线叶嵩草草地群落产量和营养动态及试验地的积温、降水、太阳辐射等气象因子进行了观测,研究了营养动态规律及其与环境因子的相关性规律。结果表明：线叶嵩草草地地上生物量的增长曲线呈Ｓ形。从产量分布看,试验期间对群落产量贡献最大的主要为莎草科和杂类草。试前禾本科和豆科占群落总产的１６．９０％,莎草科和杂类草占群落总产的８３．１０％;１９９４年禾本科和豆科占群落产量的１２．５３％,莎草科和杂类草占群落产量的８７．４７％。粗蛋白的含量在１１．００％～１３．５０％范围内;粗纤维的变化随生育期的增加呈上升趋势,在８月达到３５．７８％;粗脂肪在整个生育期内变化不明显,８月最高,为２．５１％;能量值的变化范围为１７．７２～１８．９４ＭＪ／ｋｇ。线叶嵩草草地牧草的利用时间最好在７月以前。这样不仅可以获得较高的牧草产量,而且可以获得较多的营养物质。牧草粗蛋白、粗纤维、粗脂肪和能量与环境因子间存在着显著的多元线性回归关系。     

龙眼长跗萤叶甲成虫田间消长规律及其防治

通过田间观察及气象因子分析,龙眼长跗萤叶甲成虫的发生与龙眼抽梢时间密切相关,而与温湿度无密切关系。该虫在广西南宁一年出现两个高峰期:3月下旬至4月中旬,6月下旬至9月下旬。室内外药剂筛选结果表明,90％晶体敌百虫、80％敌敌畏乳油、2.5％溴氰菊酯乳油、10％氯氰菊酯乳油、20％速灭杀丁乳油、48％乐斯本对龙眼长跗萤叶甲成虫有较好的防治效果。     

Genetic structure of a population of Ganoderma boninense on oil palm

Ganoderma boninense (the causal agent of basal stem rot of oil palm in Papua New Guinea) has a tetrapolar mating system with multiple alleles. Investigations into the population structure of G. boninense , using interfertility between isolates as a marker, revealed that the population on oil palm was comprised predominantly of genetically distinct individuals, although a number of isolates were found to share single mating alleles. No direct hereditary relationship was found between isolates on neighbouring or spatially separated diseased palms, indicating that outcrossing had probably occurred over several generations in the founder population prior to colonization of oil palm. In this study, a total of 81 A and 83 B mating type alleles (factors) were detected with 18 allelic repeats at the A locus and 17 at the B locus. Alleles appeared to be randomly dispersed throughout the population in each study block, although there was a significantly ( P  < 0·01) higher proportion of allelic repeats between estates separated by several kilometres compared to within estates. This is the first study on the population structure of this species and confirms that sexual reproduction is a significant component of the epidemiology of this disease.     

基于横向和垂向动力学的整车模型与仿真

通过分析汽车行驶时转向运动和垂直运动的相互影响、协调作用及综合动力学特性,建立了基于横向和垂向动力学的整车非线性系统模型,并在此基础上进行系统解耦和仿真计算。仿真结果表明,该模型可较好地反映同时考虑横向和垂向运动时的汽车动力学特性。