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91.
Abstract. The number of annual weeds were recorded in 752 field experiments in spring‐sown cereal crops conducted in Sweden 1972–1993. Two null hypotheses were tested regarding how the sowing date influenced the weed flora. 1. There is no relationship between the weed flora composition and sowing date. A pCCA (with geographic regions, crop species and soil types as covariables) clearly refuted this hypothesis. Hence, the composition of the weed flora varied depending on sowing date. 2. Species classified as summer annuals, winter annuals and germination generalists (that can germinate substantially in both spring and autumn) do not differ in their placement along the first ordination axis in the pCCA, i.e. according to sowing date. An ANOVA was unable to reject this hypothesis. Hence, germination syndrome classification did not explain the observed community differences related to sowing date. These results illustrate the importance of the date of disturbance for any secondary succession involving a seed bank and also the importance of annual dormancy cycles in seed banks. 相似文献
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小兴安岭通河林区原麝夏季对生境的选择 总被引:16,自引:1,他引:15
2004年8 ~ 9月,在通河县龙口林场采用样线样方法对原麝夏季生境的选择进行了研究。共设235个样方,其中原麝利用样方55 个,采用卡方(Chi-square) 统计进行显著性检验,结果表明原麝偏好利用针阔混交林,避免杂木林;倾向于选择离水源近、远离人为干扰的上坡位;一般喜欢有石砬子的山坡;对坡向的利用无选择性。对原麝利用样方和非利用样方进行比较,发现原麝生境具有海拔高、郁闭度小、食物丰富和坡度陡等特征。对各种生境因子的主成分分析表明,前6个特征值的累积贡献率达到73.30%,可以较好地反映原麝生境特征,根据载荷系数绝对值大小将原麝夏季生境选择影响因子分别命名为食物丰富度因子、植被因子、干扰因子、隐蔽度因子、地理因子和倒木因子。 相似文献
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Abstract. The effects of grazing upon the composition of a grassland were examined in a replicated grazing experiment with sheep over eight years. Grazing was controlled to produce two levels of grazing in each of winter, spring and summer, combined in a 2×2×2 factorial experimental design. Particular species were either increased or decreased by winter grazing while, in general, dicotyledonous species were favoured by grazing in both winter and spring. A model fitting the main effects of grazing in three seasons showed the importance of grazing in winter and spring to achieve sward diversification. 相似文献
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Summer diapause in the cotton bollworm, Helicoverpa armigera (Hübner), which prolongs the pupal stage, particularly in males, is induced by high temperatures. In the laboratory, summer-diapausing pupae of H. armigera were induced at high temperatures (33-39 degrees C) with a photoperiod of LD8:16; winter-diapausing and non-diapausing pupae, cultured at 20 degrees C with a photoperiod of LD8:16 and at 27 degrees C, LD16:8, respectively, acted as a control. Retention time of eye spots, weight, and lipid and glycogen levels were compared. At high temperatures, both body weight and energy storage capacity were much higher in summer-diapausing pupae than in non-diapausing pupae reared at 33-39 degrees C. At temperatures (>33 degrees C) high enough to maintain summer diapause, the eye spots of summer-diapausing pupae did not move during the 30-day experiment. However, eye spots of summer-diapausing pupae placed at 30 degrees C began to move about 10 days after they were transferred, significantly later than in non-diapausing pupae reared at 33-39 degrees C or non-diapausing pupae reared at 27 degrees C, which initiated eye spot movement 2 days after pupation. The differences in retention time of eye spots between summer- and winter-diapausing pupae shows that winter diapause is more intense than summer diapause in this insect. The weight loss, and lipid and glycogen metabolism curves indicate that the summer-diapausing pupae's metabolism is very low. We conclude that summer diapause in the cotton bollworm is a true diapause and that the summer diapause enables the cotton bollworm to withstand the high temperatures of summer. 相似文献
99.
北京小龙门自然保护区四种苔藓鸟巢的观察 总被引:4,自引:0,他引:4
2004年8月20~22日在北京小龙门自然保护区,分别对宝兴歌鸫(Turdus mupinensis)、黄眉姬鹪(Ficedula narcissina)、白腹蓝鸫(Cyanoptila cyanomelana)和北红尾鸲(Phoenicurusauroreus)4种鸟的鸟巢的结构和巢材进行了观察,发现他们均用藓类植物作为巢材。经过鉴定,这些藓类植物共7种,分属于4科,其中最多是小牛舌藓(Anomodon minor)。该文还对巢材中藓类植物的分布、生物学特性和鸟类取材的行为等进行了初步分析。 相似文献
100.
湖南八大公山夏季鸟类名录及其多样性 总被引:5,自引:0,他引:5
2002—2004年采用样带法对湖南八大公山自然保护区的天平山、杉木界、斗蓬山3个区域夏季鸟类进行了调查,结果表明八大公山的夏季鸟类共有77种,隶属10目27科,其中留鸟55种(占71.43%),夏候鸟22种(占28.51%);国家Ⅱ级保护动物6种,湖南省级保护动物7种;东洋界鸟类43种(占55.84%),古北界18种(占23.38%),广布种16种(占20.78%),其地理区划属东洋界-华中区-西部山地高原亚区。天平山、杉木界、斗蓬山鸟类的多样性指数和均匀性指数与3个区域的下述因素有关海拔低,则栖息地生境多样化程度高,鸟类的多样性指数也高;人类干扰大,则鸟类的均匀性指数较低。八大公山夏季鸟类的多样性指数和均匀性指数分别为1.5838和0.8395,鸟类的种数、多样性和均匀性水平均低于海拔较低的壶瓶山自然保护区,鸟类多样性和均匀性水平高于发展了旅游业的武陵源自然保护区。 相似文献