全文获取类型
收费全文 | 173篇 |
免费 | 5篇 |
国内免费 | 4篇 |
专业分类
林业 | 72篇 |
农学 | 5篇 |
基础科学 | 5篇 |
6篇 | |
综合类 | 73篇 |
农作物 | 6篇 |
畜牧兽医 | 9篇 |
园艺 | 5篇 |
植物保护 | 1篇 |
出版年
2022年 | 1篇 |
2021年 | 1篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2017年 | 11篇 |
2015年 | 5篇 |
2014年 | 7篇 |
2013年 | 9篇 |
2012年 | 13篇 |
2011年 | 11篇 |
2010年 | 15篇 |
2009年 | 15篇 |
2008年 | 16篇 |
2007年 | 14篇 |
2006年 | 14篇 |
2005年 | 6篇 |
2004年 | 6篇 |
2003年 | 4篇 |
2002年 | 4篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 1篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1988年 | 1篇 |
排序方式: 共有182条查询结果,搜索用时 15 毫秒
111.
黄山市城区行道树结构特征分析 总被引:3,自引:0,他引:3
行道树是城市森林的重要组成部分,城市行道树结构特征的研究,有助于城市规划和建设,为正确评价城市生态系统的服务功能提供依据。本文通过对黄山市城区15条主干道路行道树的抽样调查,发现有乔木树种24种,灌木树种37种,分析行道树的组成、直径和冠幅结构及行道树物种多样性。结果表明,黄山市城区行道树组成优势种明显,直径在6~14cm的行道树占55.7%,街道之间行道树冠幅差异明显,新安北路的冠幅最大,为11.31m,天都大道冠幅最小,为2.4m,滨江中路树种多样性最大,H指数为2.6850,黄山东路树种多样性指数最小,H值为0.7498。 相似文献
112.
113.
Allometric equations predict tree seedling biomass from non-destructively measured variables such as stem diameter (D), height (H) and seedling silhouette area (A), measured by digital imaging. This study investigates whether one general allometric equation can predict biomass of radiata
pine (Pinus radiata D.Don) seedlings grown under three levels of photosynthetic photon flux density (PPFD). It also identifies which commonly
used variables (A, D
2
H or D
2) were the best for predicting seedling biomass under these conditions. Radiata pine seedlings were grown with constant daytime
(12 h d−1) PPFD = 500, 250 or 125 μmol m−2 s−1 for 11 weeks. Seedlings were randomly selected every 10 d for measurement. Analysis of covariance tested whether the relationship
between seedling biomass and A, D
2
H or D
2 varied for each PPFD level. PPFD levels influenced the relationship between biomass and A, D
2
H or D
2. As a result, “full” allometric models which varied with PPFD levels were more accurate and precise at predicting biomass
than “reduced” models which did not vary with PPFD level, although a “reduced” model using D
2 also performed well. 相似文献
114.
Wild pistachio (Pistacia atlantica Desf.) is the most economically important tree species in many rural areas in the west of Iran. The species produces resin used for a wide variety of traditional uses. Because the resin can be harvested non-destructively, the trees are maintained until mortality occurs from natural causes. The result is that natural, managed stands include a variety of age classes. In recent years, a lack of smaller size classes has been observed in the Qalajeh forest, which is located in the Zagros Mountain region of western Iran. We established a series of plots in an area typical of Qalajeh forest to characterize the diameter distribution of the wild pistachio component. We confirmed a deficit of stems <30 cm dbh, based in the expectation that the landscape-level diameter distribution should be characterized by a negative exponential curve. For trees ≥30 cm dbh, de Liocourt's equation closely fit the diameter distribution (r2 = 0.93), translating to a q-factor of 1.34. We used this curve to estimate the deficit number of stems in diameter classes <30 cm. We estimate that this forest should have 19–24 wild pistachio trees/ha in the 5–25 cm classes, as compared to about 5 trees/ha found currently. Based on local conditions, we recommend that at least 30 seedlings/ha should be planted to allow 6–8 trees to reach to the 5 cm class. 相似文献
115.
Individual-tree diameter growth model for sugar maple trees in uneven-aged northern hardwood stands under selection system 总被引:1,自引:0,他引:1
An individual-tree diameter model was developed for sugar maple (Acer saccharum Marsh.) in northern hardwood stands managed under selection system. We fitted long-term remeasurement data to a linear mixed model to account for the temporal autocorrelation of the remeasurements. The model was evaluated using independent data from two physiographic regions and representing a range of tree diameter classes, residual basal areas and years since cut. We compared our model to several individual-tree models based on data from stands with varied management histories. Several competition indices were also tested for an improvement in model fitting and prediction. Our model had lower bias and prediction error when compared to two previous models, as it better accounted for the increased diameter growth that occurred in trees from appropriately managed stands. The addition of a tree-specific competition index failed to improve model fit and predictive ability over stand-level basal area. 相似文献
116.
苏州光福木荷林林分特征及经营对策 总被引:1,自引:0,他引:1
通过样地调查对苏州光福自然保护区木荷林的林分特征、直径分布和生长情况进行研究.结果表明,木荷种群在林分乔木层中占绝对优势,并处于旺盛生长期.以Weibull分布拟合光福自然保护区木荷林乔木树种直径,发现Weibull分布均可以较好地描述光福木荷林主要乔木树种分布;偏度为偏左,峰度值为正.林分蓄积量67.211~246.088m3/hm2,其中木荷平均胸径6.473~11.480 cm,平均树高6.35~13.53m,蓄积量32.517~205.598 m3/hm2,占全林蓄积量的31.17%~99.87%. 相似文献
117.
118.
林分径阶蓄积量与材种出材量测算的研究 总被引:2,自引:0,他引:2
研究了林分径阶蓄积量与材种出材量的估测模型,其基本原理是应用相对树高曲线法测定林分径阶蓄积量,应用一致性削度方程法计算径阶单木材种出材率;相对树高曲线法与一致性削度方程法是基于胸径和树高两个测树因子的二元立木材积测定系统,通过建立较完善的数学模型改进计算方法,提高估测效果. 相似文献
119.
川滇高山栎种群统计特征的海拔梯度变化 总被引:2,自引:2,他引:2
川滇高山栎广泛分布于横断山地区,属于亚高山硬叶栎林,是中国植被非常特殊的类型.种群统计是种群生态学研究的核心,主要研究种群在时间和空间上的数量变化规律.本文采用标准地法和样方收获法,对卧龙林区五个海拔梯度的川滇高山栎种群的大小结构、高度结构和生物量结构等方面探讨川滇高山栎种群的海拔梯度变异性规律.结果表明(1)川滇高山栎种群径级分布表现为两类,一类为海拔3000 m以下地带的双峰型,径级峰值出现在2 cm~4 cm,约占70.33%;另一类为海拔3000 m以上地带的单峰型,径级峰值出现在1 cm~3 cm,约占76.35%.(2)川滇高山栎种群高度级分布表现为两类,一类为海拔3000 m以下地带,高度级峰值出现在2 m~3 m左右,约占65.38%;另一类为海拔3000 m以上地带,随高度级增大,高度级百分率呈递减趋势.(3)川滇高山栎种群总生物量随海拔升高呈递减趋势,其平均总生物量为26 408.7 kg·hm-2,各器官平均生物量大小排序为干>枝>叶>皮,其生物量占种群总生物量的百分率分别为29.85%、18.54%、18.54%和6.91%.海拔2 720 m~2 920 m地带川滇高山栎种群地上部分器官生物量,干枝叶1皮的比例约为4321;海拔3 020 m~3 120 m地带川滇高山栎种群地上部分器官生物量,干枝叶皮的比例约为4221. 相似文献
120.