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排序方式: 共有727条查询结果,搜索用时 15 毫秒
71.
土壤侵蚀预报模型中的植被覆盖与管理因子研究进展 总被引:41,自引:6,他引:35
植被覆盖和土地管理措施是土支离破碎中侵蚀的主要抑制因素之一,通用土壤流失方程(USLE)中的植被覆盖与田间管理因子(C)是评价这种抑制作用的有效指标,并被广泛采用和深入研究。我国在这方面的研究还不能满足土壤侵蚀预报和水土保持规划的要求,在系统回顾该因子的研究历史的基础上,介绍了USLE和RUSLE中C因子估计方法及其不断改进和完善的过程,并概述了中国的覆盖与田间管理因子的研究进展。此外,总结国内在(C)因子估计值以及植被覆盖度,地面覆盖物和植被根系对土壤侵蚀的影响等方面研究成果,分析了目前我国覆盖与田间管理因子研究中存在的3个方面问题,并提出了前景展望。 相似文献
72.
73.
R. Ortiz Z. Huaman 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》2001,103(5):792-796
The need for broadening a crop’s genetic base may be determined by comparing allele frequencies within the gene pools of farmer
selections in their centers of diversity with that of modern breeding populations. The genetic structure of Andean and Chilean
potato farmer selections was investigated with the aid of nine isozymes, which have been studied in detail and used to characterize
North American cultivars and advanced breeding lines. These isozymes are associated with the most-important agronomic or quality
characters in the North American gene pool. By comparing these data with previous analyses of the North American gene pool,
allozyme frequency changes for nine loci were monitored. Allozyme frequency changes were not always due to genetic drift,
but resulted also from directional selection of isozyme marker linked quantitative trait loci (QTLs) affecting agronomic or
quality characters. Changes in allozyme frequency can also occur as a consequence of pleiotropy, i.e. the isozyme itself may
be involved in the expression of a phenotype. These allozyme frequency changes may reflect the manipulation of the potato
genome by breeders. There were allozymes in some North American cultivars that were not observed in the farmer selections
from the Andes and Chile. This confirms that breeders have already introgressed exotic genes from wild and other primitive
cultivated tuber-bearing Solanum species. On this basis, the need for broadening the genetic base for specific chromosomes (or chromosome regions) should be
based on analysis with these and other genetic markers available in potato.
Received: 20 November 2000 / Accepted: 27 December 2000 相似文献
74.
Effects of the northern pocket gopher (Thomomys talpoides) on alpine soil characteristics, Niwot Ridge, CO 总被引:2,自引:0,他引:2
Effects of the northern pocket gopher (Thomomys talpoides) on surface soilcharacteristics were examined at the alpinesite of Niwot Ridge, CO. We measured erosionof soil from gopher mounds and compared thecharacteristics of gopher mound (disturbed) andundisturbed soils in two major plant communitytypes. Our measurements of erosion indicatelong-term susceptibility of gopher-disturbedsoils to redistribution by water and/or wind inthis ecosystem. Ecosystem heterogeneityintroduced by the gopher is reflected insignificantly lower SOM in gopher mounds thanin surrounding undisturbed soils, acharacteristic which appears to be causallyassociated with other effects of gopherdisturbance including changes in soil textureand significantly lower clays, total C, totalN, total P, and labile P. In contrast toplant-available P, NO3
– was higherand steadily increased for the short term in both gopher mound soils and those beneath the mounds. These pools of NO3
– thendecreased to pre-disturbance levels by thefollowing spring. Collectively our resultsindicate that, through the physicalmanipulation of soil and subsequent effects onsoil resources, the northern pocket gopherfunctions as an agent of increased ecosystemheterogeneity and soil mass and nutrientredistribution at Niwot Ridge. 相似文献
75.
Urbanization,sedimentation, and the homogenization of fish assemblages in the Etowah River Basin,USA 总被引:4,自引:4,他引:0
We tested the hypothesis that urbanization alters stream sediment regimes and homogenizes fish assemblages in 30 sub-basins of the Etowah River. Sediment variables included average particle size (mean phi) of the stream bed, percent fines (<2 mm) in riffles, and baseflow turbidity (NTU). Homogenization was quantified as ratios of endemic to cosmopolitan species richness (Er:Cr) and abundance (Ea:Ca). High NTU and fine stream beds were associated with homogenized assemblages (i.e., lower E:C ratios). Mean phi and NTU were significantly correlated with E:C ratios (r = –0.74 to –0.76) and, when combined using multiple regression, accounted for 73% of the variance in ratios. Stream slope strongly covaried with mean phi (r = –0.92) and percent fines in riffles (r = –0.79), but multiple regression models showed that urbanized sites had finer beds and riffles than predicted by slope alone. Urban land cover was the primary predictor of NTU (r
2 = 0.42) and, combined with slope in multiple regression, explained 51% of the variance in NTU. Our results indicate that stream slope is a background variable predicting particle size and E:C ratios in these streams. Urbanization disrupts these relationships by transforming clear streams with coarse beds into turbid streams with finer beds. These conditions favor cosmopolitan species, ultimately homogenizing fish assemblages. Bed texture was linked to urbanization; however, NTU was the best indicator of urban impacts because it was statistically independent from slope. 相似文献
76.
T. Ellis § B. North † A. P. Scott N.R. Bromage † M. Porter † D. Gadd ‡ 《Journal of fish biology》2002,61(3):493-531
There is increasing public, governmental and commercial interest in the welfare of intensively farmed fish and stocking density has been highlighted as an area of particular concern. Here we draw scientific attention and debate to this emerging research field by reviewing the evidence for effects of density on rainbow trout. Although no explicit reference to ‘welfare’ has been made, there are 43 studies which have examined the effects of density on production and physiological parameters of rainbow trout. Increasing stocking density does not appear to cause prolonged crowding stress in rainbow trout. However, commonly reported effects of increasing density are reductions in food conversion efficiency, nutritional condition and growth, and an increase in fin erosion. Such changes are indicative of a reduced welfare status—although the magnitude of the effects has tended to be dependent upon study‐specific conditions. Systematic observations on large scale commercial farms are therefore required, rather than extrapolation of these mainly small‐scale experimental findings. There is dispute as to the cause of the observed effects of increasing density, with water quality deterioration and/or an increase in aggressive behaviour being variously proposed. Both causes can theoretically generate the observed effects of increasing density, and the relative contribution of the two causes may depend upon the specific conditions. However, documentation of the relationship between density and the effects of aggressive behaviour at relevant commercial densities is lacking. Consequently only inferential evidence exists that aggressive behaviour generates the observed effects of increasing density, whereas there is direct experimental evidence that water quality degradation is responsible. Nevertheless, there are contradictory recommendations in the literature for key water quality parameters to ensure adequate welfare status. The potential for welfare to be detrimentally affected by non‐aggressive behavioural interactions (abrasion, collision, obstruction) and low densities (due to excessive aggressive behaviour and a poor feeding response) have been largely overlooked. Legislation directly limiting stocking density is likely to be unworkable, and a more practical option might be to prescribe acceptable levels of water quality, health, nutritional condition and behavioural indicators. 相似文献
77.
Abstract Patch dieback occurred in an almost monospecific Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Léonard woodland in the Northern Province, South Africa, following severe droughts in 1988–1989 and 1991–1992. Discrete patches of dieback and adjacent paired areas of ‘healthy’ vegetation lost an average of 87 and 13% of basal area to mortality, respectively. Whole trees mostly died on ‘dead’ plots, while single‐stem mortality prevailed on ‘live’ plots. Tree mortality decreased with increasing stem number per tree. Patch dieback did not occur on sandy soils. On fine‐textured soils, variation in soil type, topography or slope did not affect dieback. Dieback was influenced by vegetation structure, soil surface condition and soil chemistry. Intense intertree competition, shown by self‐thinning occurring prior to dieback, was a precondition for dieback. Intertree competition had heightened during the 30 years prior to dieback because of an increase in woody cover. Dieback patches had changed from functioning as sinks of water and sediment to sources of these as a result of loss of perennial herbaceous cover, decreased water retention on bared surfaces, and accelerated erosion during 50 years of livestock ranching. Vegetation had thus become increasingly drought‐prone, exacerbated in places by soils with a high sodium concentration. Dieback had occurred because the water requirements of C. mopane could no longer be met during drought years on the dysfunctional patches. 相似文献
78.
79.
In 1996 and 1997, the spawning behavior of fluvial white-spotted charr Salvelinus leucomaenis, was observed in the upstream area of an erosion control dam. A small number of males with relatively large body size mated successfully with females as a pair, while almost all satellite males did not sneak successfully, resulting in a non-random mating system. The low sneaking success of subordinate males, in addition to the monopolization of spawning opportunities by a few dominant males, is one of the most important causes of skewed reproductive success among males. The total number of adult fishes in the study area (N: approximately half of the whole tributary above a dam) was estimated as 148 and 102 in 1996 and 1997, respectively. Based on these findings and some further assumptions, the estimated effective population size (Ne) was low in both years. The Ne/N ratio ranged from 0.33 to 0.36 in both years. In addition to reduced population size by construction of an impassable dam, the above-dam population suffered low Ne due to skewed reproductive success among males. The low Ne may be one cause of extinction in above-dam populations of fluvial charr, especially just after the construction of impassable barriers. 相似文献
80.
Gregory A. O'Neill Ian Dawson Carmen Sotelo-Montes Luigi Guarino Manuel Guariguata Dean Current John C. Weber 《Biodiversity and Conservation》2001,10(6):837-850
Forestry practices and high rates of land clearance for agriculture are causing genetic erosion of valuable tree species in the Peruvian Amazon, thereby endangering the economic sustainability of rural communities and limiting Peru's opportunities for the development of new timber and non-timber forest products. The potential utility and limitations of six low-input interventions to help forestall further genetic erosion in the region are discussed, with a focus on local community involvement. Improved agroforestry systems may help reduce deforestation by increasing farm productivity, although methods to increase the currently low adoption rate of these technologies need to be developed. Use of strategic tree domestication techniques can also improve farm productivity and prevent inadvertent genetic drift and inbreeding associated with traditional domestication practices, although to have a major impact, current programs need to be extended across the region. Woodlot forestry could supplant selective extraction of timber and offers an attractive opportunity for poverty alleviation if appropriate credit and land tenure policies can be developed. However, it may also result in increased deforestation if activities on public land cannot be controlled. The implementation of improved seed collection systems and simple seed transfer guidelines would help to reduce the collection of seed of poor quality and low genetic diversity, and avoid maladapted plantings, although such programs are difficult to monitor and seed costs may increase. Strategic identification and design of in situ conservation areas would help to ensure the viability of conserved populations, but requires the forfeiture of significant revenue from timber concessions. 相似文献