全文获取类型
收费全文 | 657篇 |
免费 | 103篇 |
国内免费 | 72篇 |
出版年
2024年 | 1篇 |
2023年 | 20篇 |
2022年 | 4篇 |
2021年 | 15篇 |
2020年 | 22篇 |
2019年 | 37篇 |
2018年 | 27篇 |
2017年 | 34篇 |
2016年 | 37篇 |
2015年 | 51篇 |
2014年 | 35篇 |
2013年 | 43篇 |
2012年 | 27篇 |
2011年 | 31篇 |
2010年 | 22篇 |
2009年 | 39篇 |
2008年 | 47篇 |
2007年 | 45篇 |
2006年 | 36篇 |
2005年 | 33篇 |
2004年 | 26篇 |
2003年 | 22篇 |
2002年 | 16篇 |
2001年 | 23篇 |
2000年 | 18篇 |
1999年 | 19篇 |
1998年 | 12篇 |
1997年 | 12篇 |
1996年 | 12篇 |
1995年 | 4篇 |
1994年 | 10篇 |
1993年 | 5篇 |
1992年 | 10篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有832条查询结果,搜索用时 15 毫秒
101.
Luiza Galbarczyk-Gąsiorowska Michał Gąsiorowski Krystyna Szeroczyńska 《Hydrobiologia》2009,631(1):173-183
Short sediment cores from two small oxbow lakes located in the Vistula valley near Warsaw (Poland) were analysed for chemical
composition and zooplankton (Cladocera) remains. The results of these analyses were evaluated using ordination techniques
and the results were used to reconstruct the history of human impact of the lakes. The cores were dated using the 210Pb method. These analyses show that the collected sediments (60 cm- and 67 cm-long-cores) were deposited during the last 150–200 years.
Cladocera assemblages were different in the two lakes; however, Bosmina longirostris was dominant among subfossil Cladocera species in both lakes. The presence of this species indicates the high trophic state
of the lakes. Changes in Cladocera species composition suggest that a significant increase in the lakes’ trophic levels has
occurred since the 1920s. The change could be related to the construction of flood dikes along the river Vistula, the isolation
of the studied lakes from the river, changes in land use, or some combination of these factors. The chemical composition of
sediments, particularly the increase in phosphorus and sulphur concentrations, confirms that the construction of dikes against
flood was the major factor inducing eutrophication of the oxbow lakes. This study shows that the Warsaw small water bodies
are especially sensitive to local disturbances caused by human activity, mainly water management, land use changes and agriculture.
Guest editors: K. Buczkó, J. Korponai, J. Padisák & S. W. Starratt
Palaeolimnological Proxies as Tools of Environmental Reconstruction in Fresh Water 相似文献
102.
Water levels were recorded weekly from six natural vernal pools and 10 created vernal pools at two forested wetland complexes
in central Ohio. Vernal pool median water depth and duration of inundation were significantly greater at the created vernal
pools than at the natural vernal pools (α = 0.05, P < 0.05). The average period of inundation for created pools was 309 ± 32 days, compared with 250 ± 16 days for natural pools.
The created pools produced a range of inundation times, from 163 to 365 days in length, with three pools permanently inundated. 相似文献
103.
Forecasting the combined effects of urbanization and climate change on stream ecosystems: from impacts to management options 总被引:2,自引:0,他引:2
104.
Shifts in climate regime have provoked substantial trophic‐ and species‐dependent changes within ecosystems. With growing concerns of present global warming, we examined potential lake ecosystem responses, natural hierarchy responses (i.e. immediate responses at lower system levels as opposed to delayed responses at higher system levels), and possible shifts among abiotic (physics, nutrients) and biotic (phytoplankton, zooplankton) system components. Specifically, we analyzed decadal data collected from Müggelsee, a lake in Berlin, Germany, for climate‐induced abiotic and biotic changes, their timing and type, and classified them as abrupt permanent, gradual permanent, abrupt temporary, or monotonic. We further categorized variable changes as a function of system hierarchy, including lake physics (ice, temperature, stratification), nutrients (phosphorus, nitrogen, silicate), plankton, and levels of integration (i.e. species, taxonomic groups, and total plankton). Contrary to current theory, data suggest abrupt responses did not occur in a hierarchy‐dependent manner, nor was a clear pattern observed among functional system‐based categories. Abrupt permanent changes were the most prominent response pattern observed, suggesting they may be driven by large‐scale climatic oscillations and by surpassed thresholds, as noted in previous case studies. Gradual changes coincided with affected abiotic parameters spanning an expansive time range; for example, climatic effects in spring preceded changes in nutrient limitation. Variables displaying no long‐term changes pointed to compensation processes caused by, e.g., simultaneously acting forces of warming trends and climate‐independent changes in trophic state. Nevertheless, the complexity of response patterns at the single system level manifested clear chronological regime shifts in abiotic and biotic parameters in spring and, to a lesser extent, in summer. With regard to projected global warming, the majority of currently unaffected system levels may face impending thermal thresholds, achievement of which would result in an accelerated shift in ecosystem state. 相似文献
105.
In this study the possible effects of two predicted climate change scenarios on soil water regime of Hungarian Calcic Chernozem
soils has been investigated. Soil profiles classified as Calcic Chernozem — in total 49 — were selected from the MARTHA soil
physical database that incorporates soil data at national scale. These profiles were subdivided into three groups (sandy loam,
loam and clayey loam) in accordance with their mechanical composition. Soil water retention curves were scaled separately
for each of the three textural groups, using similar media scaling in order to represent the variability of soil hydrophysical
data with one parameter, the scaling factor (SF). Reference soil profiles were chosen according to the cumulative distribution
function of the scaling factor, six for each textural group. Daily downscaled meteorological data from A2 and B2 climate scenarios
of the Hadley Centre (2070–2100) and data from a reference period (RF, 1961–1990) were used in this study to characterize
different climatic situations. Nine representative years were selected in case of all the three scenarios, using the cumulative
probability function of the annual precipitation sum. Scenario analyses were performed, validating the SWAP soil water balance
simulation model for the 18 reference soil profiles and 27 representative years in order to evaluate the expected changes
in soil water regime under different from the present (RF) climatic conditions (A2 and B2 scenarios). Our results show that
the scaling factor could be used as a climate sensitivity indicator of soil water regime. The large climate sensitivity of
the majority of Chernozem soil subtypes water regime has been proven. 相似文献
106.
Eduardo Gonzlez Etienne Muller Francisco Antonio Comín María Gonzlez-Sanchis 《Perspectives in Plant Ecology, Evolution and Systematics》2010,12(4):246-266
Plants growing in infertile environments are able to produce more biomass per unit of nutrient taken up than plants of fertile habitats, and also to minimize nutrients loss by resorbing them from senescing leaves. The leaf nutrient concentration variability of two co-existing riparian tree genera (Populus and Tamarix) along a flood inundation gradient was examined to infer nutrient limitation and to compare nutrient use strategies in the two genera. To that end, seasonal and spatial variability in leaf nitrogen (N) and phosphorus (P) concentration (i.e., % dry mass of N and P) were analyzed in 720 samples of leaves (2 tree genera × 3 seasons × 12 sites × 10 tree replicates). Both Populus and Tamarix showed strong seasonal variability in leaf N and P concentrations, with values decreasing throughout the growing season. However, while N:P atomic ratio remained seasonally constant in Populus (N:P = 33), Tamarix shifted from N:P = 29 in spring to N:P = 36 and 37 in summer and fall. %N, %P and N:P atomic ratios were also spatially variable, but leaf litter N and P concentration (i.e., nutrient resorption proficiency) and leaf litter N:P generally followed the local flood inundation gradient as shown by linear mixed effects models. In particular, nutrient resorption was usually less proficient (higher terminal nutrient concentrations) at higher flood durations (in gravel bars and natural levees), whereas N:P increased in the drier sites (floodplain terrace). At floodplain level, a P-limitation that is higher than N-limitation seems to characterize the plant nutrient circulation in the riparian ecosystem studied. Tamarix was slightly more proficient in P resorption than Populus. The study shows that leaf nutrient concentration (e.g., N and P) derived from nutrient availability is partly controlled by the flood inundation regime and can be used as an indicator of nutrient limitation in forested floodplains. Subtle differences between tree genera provide an additional, novel explanation for the recent expansion of Tamarix in many arid and semi-arid rivers with altered hydrogeomorphic regimes. 相似文献
107.
YOSEPH N. ARAYA JONATHAN SILVERTOWN DAVID J. GOWING KEVIN MCCONWAY PETER LINDER GUY MIDGLEY 《Austral ecology》2010,35(7):818-824
Consistent, repeatable segregation of plant species along hydrological gradients is an established phenomenon that must in some way reflect a trade‐off between plants' abilities to tolerate the opposing constraints of drought and waterlogging. In C3 species tissue carbon isotope discrimination (δ13C) is known to vary sensitively in response to stomatal behaviour, reflecting stomatal limitation of photosynthesis during the period of active growth. However, this has not been studied at fine‐spatial scale in natural communities. We tested how δ13C varied between species and sexes of individuals in the family Restionaceae growing along a monitored hydrological gradient. Twenty Restionaceae species were investigated using species‐level phylogeny at two sites in the Cape Floristic Region, a biodiversity hotspot. A spatial overlap analysis showed the Restionaceae species segregated significantly (P < 0.001) at both sites. Moreover, there were significant differences in δ13C values among the Restionaceae species (P < 0.001) and between male and female individuals of each species (P < 0.01). However, after accounting for phylogeny, species δ13C values did not show any significant correlation with the hydrological gradient. We suggest that some other variable (e.g. plant phenology) could be responsible for masking a simple response to water availability. 相似文献
108.
109.
Evaluating future success of whitebark pine ecosystem restoration under climate change using simulation modeling
下载免费PDF全文
![点击此处可从《Restoration Ecology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Robert E. Keane Lisa M. Holsinger Mary F. Mahalovich Diana F. Tomback 《Restoration Ecology》2017,25(2):220-233
Major declines of whitebark pine forests throughout western North America from the combined effects of mountain pine beetle (Dendroctonus ponderosae) outbreaks, fire exclusion policies, and the exotic disease white pine blister rust (WPBR) have spurred many restoration actions. However, projected future warming and drying may further exacerbate the species' decline and possibly compromise long‐term success of today's restoration activities. We evaluated successes of restoration treatments under future climate using a comprehensive landscape simulation experiment. The spatially explicit, ecological process model FireBGCv2 was used to simulate whitebark pine populations on two U.S. Northern Rocky Mountain landscapes over 95 years under two climate, three restoration, and two fire management scenarios. Major findings were that (1) whitebark pine can remain on some high mountain landscapes in a future climate albeit at lower basal areas (50% decrease), (2) restoration efforts, such as thinning and prescribed burning, are vital to ensure future whitebark pine forests, and (3) climate change impacts on whitebark pine vary by local setting. Whitebark pine restoration efforts will mostly be successful in the future but only if future populations are somewhat resistant to WPBR. Results were used to develop general guidelines that address climate change impacts for planning, designing, implementing, and evaluating fine‐scale restoration activities. 相似文献
110.