全文获取类型
收费全文 | 8428篇 |
免费 | 1622篇 |
国内免费 | 2505篇 |
出版年
2024年 | 9篇 |
2023年 | 264篇 |
2022年 | 197篇 |
2021年 | 368篇 |
2020年 | 519篇 |
2019年 | 545篇 |
2018年 | 501篇 |
2017年 | 490篇 |
2016年 | 490篇 |
2015年 | 496篇 |
2014年 | 449篇 |
2013年 | 536篇 |
2012年 | 405篇 |
2011年 | 434篇 |
2010年 | 413篇 |
2009年 | 475篇 |
2008年 | 541篇 |
2007年 | 579篇 |
2006年 | 556篇 |
2005年 | 497篇 |
2004年 | 464篇 |
2003年 | 391篇 |
2002年 | 370篇 |
2001年 | 355篇 |
2000年 | 331篇 |
1999年 | 288篇 |
1998年 | 266篇 |
1997年 | 195篇 |
1996年 | 159篇 |
1995年 | 151篇 |
1994年 | 130篇 |
1993年 | 104篇 |
1992年 | 106篇 |
1991年 | 89篇 |
1990年 | 82篇 |
1989年 | 59篇 |
1988年 | 38篇 |
1987年 | 39篇 |
1986年 | 38篇 |
1985年 | 22篇 |
1984年 | 25篇 |
1983年 | 20篇 |
1982年 | 26篇 |
1981年 | 4篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1976年 | 3篇 |
1958年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Sam Mugume Colin A. Chapman Gilbert Isabirye‐Basuta Emily Otali 《African Journal of Ecology》2015,53(4):465-472
Tropical forests contain much of the world's biodiversity, yet their rate of decline is increasing. The strategy most frequently used to protect this biodiversity is to make parks and reserves. While there is a great deal of research on the effectiveness of parks for protecting biodiversity, there is little research on how well extractive reserves conserve biodiversity. Here, we evaluate the effectiveness of four forest reserves in western Uganda at maintaining populations of primates and compare census data from the reserves to data from the neighbouring well‐protected Kibale National Park. The relative abundance of the five most common primates in the park was approximately four times that of the forest reserves. In the forest reserves, evidence of new human encroachment was seen every 500 m, while in the park it was seen every 100,000 m. Two recommendations emerge from our research: (i) for forest reserves, such as those studied here, to have conservation value for primates, extraction must be reduced and (ii) until the long‐term viability of the populations in forest reserves can be ascertained, they should not be considered in estimates of the sizes of endangered species protected ranges. 相似文献
82.
Traditional ecological knowledge (TEK) is a potential source of ecological information. Typically TEK has been documented at the species level, but habitat data would be equally valuable for conservation applications. We compared the TEK forest type classification of ribereños, the non-indigenous rural peasantry of Peruvian Amazonia, to a floristic classification produced using systematically collected botanical data. Indicator species analysis of pteridophytes in 300 plots detected two forest types on non-flooded tierra firme, each associated with distinct soil texture and fertility, and one forest type in areas subject to flooding. Nine TEK forest types were represented in the same set of plots. Each TEK forest type was consistently (>82%) associated with one of the three floristic classes and there were also clear parallels in the ecological characterizations of the forest types. Ribereños demonstrated clear preferences for certain forest types when selecting sites for slash-and-burn agriculture and hunting. Our results indicate that the non-tribal inhabitants of Amazonia possess valuable TEK that could be used in biodiversity inventories and wildlife management and conservation for characterizing primary rain forest habitats in Amazonia. 相似文献
83.
Godlove Ambe Neba David McClintock Newbery George Bindeh Chuyong 《Ecology and evolution》2016,6(1):125-142
In the ectomycorrhizal caesalpiniaceous groves of southern Korup National Park, the dominant tree species, Microberlinia bisulcata, displays very poor in situ recruitment compared with its codominant, Tetraberlinia bifoliolata. The reported ex situ experiment tested whether availabilities of soil potassium and magnesium play a role. Seedlings of the two species received applications of K and Mg fertilizer in potted native soil in a local shade house, and their responses in terms of growth and nutrient concentrations were recorded over 2 years. Amended soil concentrations were also determined. Microberlinia responded strongly and positively in its growth to Mg, but less to K; Tetraberlinia responded weakly to both. Added Mg led to strongly increased Mg concentration for Microberlinia while added K changed that concentration only slightly; Tetraberlinia strongly increased its concentration of K with added K, but only somewhat its Mg concentration with added Mg. Additions of Mg and K had small but important antagonistic effects. Microberlinia is Mg‐demanding and apparently Mg‐limited in Korup soil; Tetraberlinia, whilst K‐demanding, appeared not to be K‐limited (for growth). Added K enhanced plant P concentrations of both species. Extra applied Mg may also be alleviating soil aluminum toxicity, and hence improving growth indirectly and especially to the benefit of Microberlinia. Mg appears to be essential for Microberlinia seedling growth and its low soil availability in grove soils at Korup may be an important contributing factor to its poor recruitment. Microberlinia is highly shade‐intolerant and strongly light‐responding, whilst Tetraberlinia is more shade‐tolerant and moderately light‐responding, which affords an interesting contrast with respect to their differing responses to Mg supply. The study revealed novel aspects of functional traits and likely niche‐partitioning among ectomycorrhizal caesalps in African rain forests. Identifying the direct and interacting indirect effects of essential elements on tropical tree seedling growth presents a considerable challenge due the complex nexus of causes involved. 相似文献
84.
Liana is a life form that possesses high importance in many Neotropical forests. Density of climbers apparently increases with the intervention rate (e.g. logging). The aim of this work is to characterize the structure, floristic composition and soils of a sector classified as Liana Forest (LF). We identified an LF sector in a not-logged area; three 1 ha square plots were measured (individuals ≥ 10 cm dbh, “diameter at breast height”). In each plot, we evaluate four 100 m2 square understory subplots (all spermatophyta individuals < 10 cm dbh). LF has a low canopy ( < 15 m) and is dominated by Alexa imperatricis and Pentaclethra macroloba. Basal area (20.4 m2ha? 1) and diversity (H′ = 2.6) are lower than other surrounding plots. Understory is dominated by gnarled climbers, and the most important are Cheiloclinium hippocrateoides and Bauhinia scala-simiae. Soil is extremely acidic, with very low fertility but is similar to neighboring places. We conclude that LF was neither originated by edaphic restrictions nor logging; LF probably suffered a hurricane wind that fell down most of the canopy trees, thick individuals of climber species also disappeared, and the current successional stage favors a recovery dominated with thin individuals of this life form. 相似文献
85.
86.
87.
What is the optimal number of leaves when measuring leaf area of tree species in a forest community? 下载免费PDF全文
《植物生态学报》2018,42(9):917
植物形态性状叶面积简单易测, 能够反映植物对环境的适应与响应, 指示生态系统的功能与过程。在野外测定叶面积时, 叶片取样数量往往采用约定俗成的10-20片, 但到底采集多少叶片才是最优和最具代表性, 却少有探究。该研究以浙江金华山常绿落叶阔叶混交林的优势树种木荷(Schima superba)与枫香树(Liquidambar formosana)为研究对象, 通过对5个胸径等级植株和每个植株6个方位开展大批量叶片取样(>2 500个), 分析两个树种的叶面积变异特征, 探讨叶片取样数量为多少才能最代表该森林类型的叶片大小性状规律。结果表明, 常绿乔木木荷平均叶面积与变幅均小于落叶乔木枫香树。木荷叶面积与胸径无显著相关性, 而枫香树叶面积与胸径有较显著相关性, 但两个树种均在中胸径等级(15-20 cm)差异不显著; 两个树种的叶面积与采样方位无显著相关性, 但在东、西和底部的差异不显著。因此, 综合考虑代表性与野外可操作性, 叶片采集首选中胸径成树的底部叶片。随机抽样统计可知, 树木叶面积测定的最适叶片采集数量因物种而异, 木荷的最适叶片采集数量为40, 而枫香树最少为170片。因此, 在叶面积测定时, 叶片采集的数量应该不能只局限在10-20片, 在人力、物力和时间等条件允许的情况下, 应该尽可能多地测定较多叶片的叶面积。 相似文献
88.
A new Oberonia species, O. muthikulamensis (Orchidaceae), is described and illustrated from Muthikulam forest, Western Ghats of Kerala, India. It is compared with a closely morphologically similar species, O. sebastiana, endemic to Western Ghats. 相似文献
89.
90.