全文获取类型
收费全文 | 1319篇 |
免费 | 218篇 |
国内免费 | 134篇 |
出版年
2023年 | 47篇 |
2022年 | 17篇 |
2021年 | 49篇 |
2020年 | 59篇 |
2019年 | 82篇 |
2018年 | 77篇 |
2017年 | 68篇 |
2016年 | 65篇 |
2015年 | 78篇 |
2014年 | 61篇 |
2013年 | 75篇 |
2012年 | 56篇 |
2011年 | 38篇 |
2010年 | 43篇 |
2009年 | 91篇 |
2008年 | 79篇 |
2007年 | 76篇 |
2006年 | 69篇 |
2005年 | 63篇 |
2004年 | 56篇 |
2003年 | 42篇 |
2002年 | 35篇 |
2001年 | 41篇 |
2000年 | 47篇 |
1999年 | 37篇 |
1998年 | 38篇 |
1997年 | 33篇 |
1996年 | 16篇 |
1995年 | 20篇 |
1994年 | 18篇 |
1993年 | 15篇 |
1992年 | 12篇 |
1991年 | 12篇 |
1990年 | 15篇 |
1989年 | 8篇 |
1988年 | 11篇 |
1987年 | 1篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有1671条查询结果,搜索用时 15 毫秒
141.
To select appropriate plant materials for a phylogeography of deciduous broad-leaved forests in Japan, we surveyed intraspecific chloroplast DNA variation in 34 species found in these forests. A relatively large number of intraspecific cpDNA variations were detected in ten species: Carpinus japonica (nucleotide diversity π=0.00083), C. laxiflora (π=0.00221), Magnolia obovata (π=0.00134), Lindera triloba (π=0.00255), L. obtusiloba (π=0.00289), Pourthiaea villosa var. leavis (π=0.00263), Acer japonicum (π=0.00170), A. micranthum (π=0.00237), Euonymus oxyphyllus (π=0.00322) and Styrax obassia (π=0.00100). 相似文献
142.
Natural disturbances are an important source of environmental heterogeneity that have been linked to species diversity in
ecosystems. However, spatial and temporal patterns of disturbances are often evaluated separately. Consequently, rates and
scales of existing disturbance processes and their effects on biodiversity are often uncertain. We have studied both spatial
and temporal patterns of contemporary fires in the Sierra Nevada Mountains, California, USA. Patterns of fire severity were
analyzed for conifer forests in the three largest fires since 1999. These fires account for most cumulative area that has
burned in recent years. They burned relatively remote areas where there was little timber management. To better characterize
high-severity fire, we analyzed its effect on the survival of pines. We evaluated temporal patterns of fire since 1950 in
the larger landscapes in which the three fires occurred. Finally, we evaluated the utility of a metric for the effects of
fire suppression. Known as Condition Class it is now being used throughout the United States to predict where fire will be
uncharacteristically severe. Contrary to the assumptions of fire management, we found that high-severity fire was uncommon.
Moreover, pines were remarkably tolerant of it. The wildfires helped to restore landscape structure and heterogeneity, as
well as producing fire effects associated with natural diversity. However, even with large recent fires, rates of burning
are relatively low due to modern fire management. Condition Class was not able to predict patterns of high-severity fire.
Our findings underscore the need to conduct more comprehensive assessments of existing disturbance regimes and to determine
whether natural disturbances are occurring at rates and scales compatible with the maintenance of biodiversity. 相似文献
143.
The present and future well-being of the world’s forest dwelling populations depends on their ability to gain livelihood resources
from their immediate environment. Sustainable extraction of non-timber forest products has been promoted by conservationists
and development agencies as a feasible strategy for forest dwellers that does not compromise the resource base. Yet surveys
of actual resource use suggest that for poorer resource-dependent communities without access to markets, non-timber forest
products can only ever represent a safety-net activity and a supplementary income source. Others argue that resource availability,
in terms of the diversity and productivity of the forest, is the key parameter in realizing a contribution of forest products
to well-being. This paper examines the scope and heterogeneity of forest product use to reveal whether resource availability
necessarily provides the context for significant contributions to well-being of forest dwellers. We present data from an area
of tropical rainforest, close to Iquitos in Peru, which was previously shown to have high potential value. We find, through
a census survey of households within a forest reserve area, that non-timber forest products provide only a relatively small
portion of income and that only a small proportion of available products are actually commercialized, despite apparent market
availability. We show that the low rates of commercialization can be explained by unequal access capital assets used for extraction,
to natural resources themselves, and to product markets. They are also explained by the concentration of capital-poor households
on subsistence gathering activities. The value of destructive uses of forests, both logging and agriculture, remain higher
than returns from non-timber products. This research demonstrates that although non-timber forest products are an important
livelihood source, market integration and commercialization is not everywhere an appropriate or realistic strategy. 相似文献
144.
In forest ecosystems the single largest respiratory flux influencing net ecosystem productivity (NEP) is the total soil CO2 efflux; however, it is difficult to make measurements of this flux that are accurate at the ecosystem scale. We examined patterns of soil CO2 efflux using five different methods: auto-chambers, portable gas analyzers, eddy covariance along and two models parameterized with the observed data. The relation between soil temperature and soil moisture with soil CO2 effluxes are also investigated, both inter-annually and seasonally, using these observations/results. Soil respiration rates (R
soil) are greatest during the growing season when soil temperatures are between 15 and 25 °C, but some soil CO2 efflux occurs throughout the year. Measured soil respiration was sensitive to soil temperature, particularly during the spring and fall. All measurement methods produced similar annual estimates. Depending on the time of the year, the eddy covariance (flux tower) estimate for ecosystem respiration is similar to or slightly lower than estimates of annual soil CO2 efflux from the other methods. As the eddy covariance estimate includes foliar and stem respiration which the other methods do not; it was expected to be larger (perhaps 15–30%). The auto-chamber system continuously measuring soil CO2 efflux rates provides a level of temporal resolution that permits investigation of short- to longer term influences of factors on these efflux rates. The expense of building and maintaining an auto chamber system may not be necessary for those researchers interested in estimating R
soil annually, but auto-chambers do allow the capture of data from all seasons needed for model parameterization. 相似文献
145.
Aim For a large set of forest herbs we tested: (1) whether there is a positive relationship between local abundance and geographical range size; (2) whether abundance or range size are affected by the niche breadths of species or niche availability; and (3) whether these are affected by the species life‐history traits. Location Northwestern Germany. Methods We measured abundance as mean density in 22 base‐rich deciduous forests and recorded range size as area of occupancy on four different spatial scales (local to national). Niche breadth was expressed in terms of habitat specificity (specialists, generalists) and of the ability to grow across a broad range of soil pH. The species’ pH niche position was used as a measure of the importance of habitat availability. As life‐history traits we used diaspore mass and number, plant height, seed longevity, lifespan/clonality, pollination mode, dispersal capability and flowering time. Results There were mainly no positive relationships between the abundance of species and their range size, as tested across species and across phylogenetically independent contrasts. Forest specialists were generally distributed less widely than generalists, but habitat specificity was not related to local abundance. Species with a broader pH niche breadth were more common, but the positive relationships between niche breadth and abundance or range size disappeared when accounting for sample size effects. Clonal species with few and heavy diaspores were most abundant, as well as early‐flowering species and those lacking dispersal structures. Local and regional range size were determined largely by habitat availability, while national range was positively affected by plant height and diaspore mass. Main conclusions Different processes determine the local density of species and their range size. Abundance within habitat patches appears to be related mainly to the species life histories, especially to their capacity for extensive clonal reproduction, whereas range size appears to be determined strongly by the availability of suitable habitat. 相似文献
146.
Deciduous forests may respond differently from coniferous forests to the anthropogenic deposition of nitrogen (N). Since fungi,
especially ectomycorrhizal (EM) fungi, are known to be negatively affected by N deposition, the effects of N deposition on
the soil microbial community, total fungal biomass and mycelial growth of EM fungi were studied in oak-dominated deciduous
forests along a nitrogen deposition gradient in southern Sweden. In-growth mesh bags were used to estimate the production
of mycelia by EM fungi in 19 oak stands in the N deposition gradient, and the results were compared with nitrate leaching
data obtained previously. Soil samples from 154 oak forest sites were analysed regarding the content of phospholipid fatty
acids (PLFAs). Thirty PLFAs associated with microbes were analysed and the PLFA 18:2ω6,9 was used as an indicator to estimate
the total fungal biomass. Higher N deposition (20 kg N ha−1 y−1 compared with 10 kg N ha−1 y−1) tended to reduce EM mycelial growth. The total soil fungal biomass was not affected by N deposition or soil pH, while the
PLFA 16:1ω5, a biomarker for arbuscular mycorrhizal (AM) fungi, was negatively affected by N deposition, but also positively
correlated to soil pH. Other PLFAs positively affected by soil pH were, e.g., i14:0, a15:0, 16:1ω9, a17:0 and 18:1ω7, while
some were negatively affected by pH, such as i15:0, 16:1ω7t, 10Me17:0 and cy19:0. In addition, N deposition had an effect
on the PLFAs 16:1ω7c and 16:1ω9 (negatively) and cy19:0 (positively). The production of EM mycelia is probably more sensitive
to N deposition than total fungal biomass according to the fungal biomarker PLFA 18:2ω6,9. Low amounts of EM mycelia covaried
with increased nitrate leaching, suggesting that EM mycelia possibly play an important role in forest soil N retention at
increased N input. 相似文献
147.
John C. Denton Charlotte L. Roy Gregory J. Soulliere Bradly A. Potter 《The Journal of wildlife management》2012,76(2):422-432
Clearing of hardwood forests was widespread in the north central region of the United States at the turn of the 20th century, but largely subsided by the 1920s. Hardwood trees in the region have since regenerated and matured into sizes capable of producing nest cavities suitable for cavity-nesting ducks. We estimated regional nest-site abundance for cavity-nesting ducks during 2008, 2018, and 2028 from cavity density and tree-abundance estimates obtained at 4 hardwood forest sites in conjunction with Forest Inventory and Analysis data and tree-growth modeling software from the United States Forest Service (Forest Vegetation Simulator). Land cover data were used to determine area of hardwood forests ≤0.5 km, 0.5–1 km, 1–1.5 km, 1.5–2 km, and >2 km from wetlands and open water available to cavity-nesting ducks. We estimated 13.2 million, 17.0 million, 19.0 million, and 20.1 million potential duck nest cavities available ≤0.5 km, ≤1 km, ≤1.5 km, and ≤2 km of water, respectively, in the region and predicted nest cavity abundance will increase 41% from 2008 to 2028. Hardwood forests in Indiana, Michigan, Ohio, and Wisconsin currently have the highest abundances of potential nest sites, but cavity-bearing forests in Minnesota, Michigan, and Wisconsin were more commonly proximate to wetlands and open water. Because current and future estimates indicate sufficient nest sites to support growing cavity-nesting duck populations in the north central United States, we recommend regional management efforts focus on protecting, restoring, and maintaining quality wetlands in proximity to hardwood forests. © 2011 The Wildlife Society. 相似文献
148.
Experimental study of the effects of projected climate change on plant phenology allows us to isolate effects of warming on life‐history events such as leaf out. We simulated a 2 °C temperature increase and 20% precipitation increase in a recently harvested temperate deciduous forest community in central Pennsylvania, USA, and observed the leaf out phenology of all species in 2009 and 2010. Over 130 plant species were monitored weekly in study plots, but due to high variability in species composition among plots, species were grouped into five functional groups: short forbs, tall forbs, shrubs, small trees, and large trees. Tall forbs and large trees, which usually emerge in the late spring, advanced leaf out 14–18 days in response to warming. Short forbs, shrubs, and small trees emerge early in spring and did not alter their phenology in response to warming or increased precipitation treatments. Earlier leaf out of tall forbs and large trees coincided with almost 3 weeks of increased community‐level leaf area index, indicating greater competition and a condensed spring green‐up period. While phenology of large trees and tall forbs appears to be strongly influenced by temperature‐based growth cues, our results suggest that photoperiod and chilling cues more strongly influence the leaf out of other functional groups. Reduced freeze events and warmer temperatures from predicted climate change will interact with nontemperature growth cues to have cascading consequences throughout the ecosystem. 相似文献
149.
Radiative forcing of natural forest disturbances 总被引:1,自引:0,他引:1
Thomas L. O'Halloran Beverly E. Law Michael L. Goulden Zhuosen Wang Jordan G. Barr Crystal Schaaf Mathew Brown José D. Fuentes Mathias Göckede Andrew Black Vic Engel 《Global Change Biology》2012,18(2):555-565
Forest disturbances are major sources of carbon dioxide to the atmosphere, and therefore impact global climate. Biogeophysical attributes, such as surface albedo (reflectivity), further control the climate‐regulating properties of forests. Using both tower‐based and remotely sensed data sets, we show that natural disturbances from wildfire, beetle outbreaks, and hurricane wind throw can significantly alter surface albedo, and the associated radiative forcing either offsets or enhances the CO2 forcing caused by reducing ecosystem carbon sequestration over multiple years. In the examined cases, the radiative forcing from albedo change is on the same order of magnitude as the CO2 forcing. The net radiative forcing resulting from these two factors leads to a local heating effect in a hurricane‐damaged mangrove forest in the subtropics, and a cooling effect following wildfire and mountain pine beetle attack in boreal forests with winter snow. Although natural forest disturbances currently represent less than half of gross forest cover loss, that area will probably increase in the future under climate change, making it imperative to represent these processes accurately in global climate models. 相似文献
150.
Aim To develop and test a simple climate‐based ecophysiological model of above‐ground biomass – an approach that can be applied directly to predicting the effects of climate change on forest carbon stores. Location Humid lowland forests world‐wide. Methods We developed a new approach to modelling the aboveground biomass of old‐growth forest (AGBmax) based on the influences of temperature on gross primary productivity (GPP) and what we call total maintenance cost (TMC), which includes autotrophic respiration as well as leaf, stem and other plant construction required to maintain biomass. We parameterized the models with measured carbon fluxes and tested them by comparing predicted AGBmax with measured AGB for another 109 old‐growth sites. Results Our models explained 57% of the variation in GPP across 95 sites and 79% of the variation in TMC across 17 sites. According to the best‐fit models, the ratio of GPP to maintenance cost per unit biomass (MCB) peaks at 16.5 °C, indicating that this is the air temperature leading to the highest possible AGBmax when temperatures are constant. Seasonal temperature variation generally reduces predicted AGBmax, and thus maritime temperate climates are predicted to have the highest AGBmax. The shift in temperatures from temperate maritime to tropical climates increases MCB more than GPP, and thus decreases AGBmax. Overall, our model explains exactly 50% of the variation in AGB among humid lowland old‐growth forests. Main conclusions Temperature plays an important role in explaining global variation in biomass among humid lowland old‐growth forests, a role that can be understood in terms of the dual effects of temperature on GPP and TMC. Our simple model captures these influences, and could be an important tool for predicting the effects of climate change on forest carbon stores. 相似文献