全文获取类型
收费全文 | 309篇 |
免费 | 12篇 |
出版年
2017年 | 2篇 |
2016年 | 2篇 |
2014年 | 5篇 |
2013年 | 7篇 |
2012年 | 9篇 |
2011年 | 8篇 |
2010年 | 10篇 |
2009年 | 13篇 |
2008年 | 7篇 |
2007年 | 19篇 |
2006年 | 12篇 |
2005年 | 14篇 |
2004年 | 2篇 |
2003年 | 3篇 |
2002年 | 2篇 |
2001年 | 2篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1993年 | 4篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1987年 | 3篇 |
1985年 | 3篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1968年 | 1篇 |
1959年 | 7篇 |
1958年 | 12篇 |
1957年 | 6篇 |
1956年 | 14篇 |
1955年 | 23篇 |
1954年 | 16篇 |
1953年 | 18篇 |
1952年 | 15篇 |
1951年 | 12篇 |
1950年 | 11篇 |
1949年 | 2篇 |
1948年 | 6篇 |
1947年 | 2篇 |
1946年 | 1篇 |
排序方式: 共有321条查询结果,搜索用时 468 毫秒
91.
92.
93.
94.
95.
96.
97.
98.
99.
R. GUERRIERI M. MENCUCCINI L. J. SHEPPARD M. SAURER M.P. PERKS P. LEVY M. A. SUTTON M. BORGHETTI J. GRACE 《Global Change Biology》2011,17(5):1946-1962
This study aimed to evaluate the effects of long‐term repeated aerial nitrogen (N) and sulphur (S) misting over tree canopies of a Sitka spruce plantation in Scotland. We combined δ13C and δ18O in tree rings to evaluate the changes in CO2 assimilation (A) and stomatal conductance (gs) and to assess their contribution to variations in the intrinsic water‐use efficiency (WUEi, i.e., the A/gs ratio). Measurements of δ15N enabled shifts in the ecosystem N cycling following misting to be assessed. We found that: (i) N applications, with or without S, increased the ratio between A and gs in favour of A, thus supporting a fertilizer effect of added N. (ii) After the treatments ceased, the trees quickly adjusted to the reductions of N deposition, but not to the reduction in S deposition, which had a negative effect on WUEi by reducing A. This indicates that the beneficial role of N deposition may be negated in forests that previously received a high load of acid rain. (iii) δ15N in tree rings reflected the N dynamics caused by canopy retention, with the fingerprint also present in the litter, after the experiment stopped. (iv) Both our results (obtained using canopy applications) and a collection of published data (obtained using soil applications) showed that generally WUEi increased in response to an increase of N applications, with the magnitude of the changes related to soil conditions and the availability of other nutrients. The shifts observed in δ15N in tree rings also suggest that both the quantity of the applied N and its quality, mediated by processes occurring during canopy N retention, are important determinants of the interactions between N and C cycles. Stable isotopes are useful probes to understand these processes and to put the results of short‐term experiments into context. 相似文献
100.
ANTONIO HERNÁNDEZ‐MATÍAS JOAN REAL ROGER PRADEL ALAIN RAVAYROL NICOLAS VINCENT‐MARTIN 《Ibis》2011,153(4):846-857
Survival typically contributes most to population trends in long‐lived birds and its accurate estimation is therefore vital for population management and conservation. We evaluated the effects of age, territoriality and reproduction on survival in Bonelli’s Eagle Aquila fasciata through multistate capture‐mark‐recapture analyses on a long‐term dataset. Monitoring was carried out in southeast France (1990–2008) and involved the surveying of territorial Eagles, the marking of fledged chicks, and the recording of resightings and recoveries of marked non‐territorial and territorial birds. Survival improved with age, but territoriality was not retained in the best model; yearly survival was estimated at 0.479 for fledglings (to 1 year of age), 0.570 for 1‐ and 2‐year‐olds, and 0.870 for 3‐year‐old and older individuals. The second best model supported a further increase in survival from 3‐year‐olds (0.821) to older individuals (0.880). In the third best supported model, territoriality enhanced survival, but only in 2‐year‐olds (0.632 vs. 0.562 for non‐territorial). We found no correlation between the previous breeding stage and future survival, consistent with the long lifespan of the study species. Nevertheless, 4‐year‐old and older successful breeders were more likely to breed the following year than failed adult breeders (0.869 vs. 0.582), suggesting that the cost of reproduction is small in comparison with the variation in quality among individuals or their territories. 相似文献