黄土高原森林林隙特征及植物多样性

通过对马栏林区林隙的调查研究,发现其林隙大小以20～40 m2和40～60 m2居多,其中40～60 m2所占数量比例和面积比例均最大.林隙的年龄结构是中前期和中后期数量较多,分别占到林隙总数的42.31%和38.46%.林隙形成方式以砍伐树桩占的比重最大,说明该地区现有的林隙大多是人为干扰形成的.林隙形成木中辽东栎(Quercus liaotungensis)和油松(Pinus tabulaeformis)的株数相对较多,且主要是由中小径级的树木构成.不同大小林隙的植物多样性变化表现为:乔木层面积为40～60 m2时其多样性较高,灌木层和草本层面积均为20～40m2时其生物多样性较高.在不同发育阶段中,草本层的多样性指数在林隙发育的中前期最高,而乔木层和灌木层均在林隙发育到中后期时最高.     

黄土丘陵沟壑区植被自然恢复过程中土壤种子库特征

采用野外调查取样和室内实验相结合的方法,研究了黄土丘陵沟壑区植被自然恢复过程中土壤种子库的物种组成、密度、物种多样性以及相似性特征.结果表明:阴、阳坡10个样地中,共萌发了55种2203株幼苗,隶属于23科48属;总密度达到5 505±625.537粒/m2,且0～5 cm土层远远大于5～10 cm土层的土壤种子库密度,约是其4.6倍;丰富度、多样性、均匀度指数均未与恢复年限达到显著相关水平,但生态优势度随恢复年限的增加呈上升趋势,且阳坡上升的趋势比阴坡明显;组成成分相似性系数变化范围为0.0952～0.7143,恢复年限相距越大,其相似性越小,说明植被恢复演替的过程也是土壤种子库空间异质性加大的过程.     

天山北坡呼图壁河流域近313年降水的重建与分析

分析发现,天山北坡呼图壁河流域树轮年表与呼图壁气象站上年8月至当年7月的平均降水相关性较好,最高单相关系数为o.484(a=0.001),且具有明显的树木生理学意义.利用赛热克巴依萨、希热克久热特、喀音萨依的标准化年表能够较好地重建呼图壁河流域近313年的上年8月至当年7月平均降水序列,方差解释量达41%,并通过交叉检验表明重建结果稳定可靠.     

黄土高原地区水资源承载力动态变化分析——以山西、陕西、宁夏、甘肃4省为例

水资源承载力对地区经济可持续发展和群众生活的物质基础具有重要影响,是达到地区社会、经济和环境不断优化的关键.采用模糊综合评价方法,选取水资源利用率、降水量、人均占有水量和生态用水量为评价指标,对黄土高原4省1994-2004年的水资源承载力进行了综合及分区评价.结果表明:黄土高原4省水资源承载力在1994-2004年总体上呈缓慢上升趋势,说明水资源有向良性发展趋势;陕西和山西省水资源承载力较高,2省水资源开发利用有较大空间,甘肃和宁夏水资源承载力已趋于饱和,实施跨流域调水和节水措施势在必行.     

黄土高原降水资源特征与林木适宜度研究

黄土高原位于干旱半干旱地区,水资源匮乏,如何根据当地的降水条件来确定适宜的植被类型是生态建设中的一个关键问题.通过对黄土高原68个气象台站1961-2000年逐月降水量的统计,结果表明:黄土高原的降水特征总体表现为年平均降水量区域差异大,由东南部的大于600 mm逐渐递减到西北部的不足200 mm;植物生长期间的降水占全年80%以上,且季节分配不均,表现为春旱、夏多、秋少.依据黄土高原的降水资源特征,从林地水量平衡理论出发,以生长季期间林地的水分收支状况来确定林木的适生程度,并以县为单位对黄土高原的林木适宜性进行分区.结果表明,黄土高原适宜于乔木林和灌木林的区域各占1/4左右.     

黄土高原东部近45年气温变化研究

利用山西省68个站的年平均气温资料,运用累积距平、聚类分析等统计学方法,研究了近45年来黄土高原东部山西省的气温变化。结果表明:最高气温出现在1999年,次高年出现在1998年,最低出现在1984年,这三年的气温距平均超过了两倍标准差的异常标准;80年代和70年代相比气温还略有降低;仅从距平我们很难较为准确地分析出气温的冷暖时段,而采用累积距平方法则清楚地显示出1986年和1996年是该地区近45年气温出现转折的关键两年,以此为界将这一地区45年的气温变化划分为三个时段;由于1987-1996年10年气温演变的差别,导致了三类不同演变的站点,突出的是南部的大部分站点从90年代中期开始才进入暖期,北部升温高于南部。     

民勤荒漠区近50a气温、降水对全球变暖的响应

近些年来,全球气温变暖已成为一个全世界广泛关注的问题。民勤属于我国最典型的干旱荒漠化地区。文中对民勤荒漠区1961~2007年气温和降水资料进行了分析,结果表明:民勤沙区近47年的年平均气温增高幅度高于全国水平。在过去47年当中,2月份平均气温增高3.01℃,这一增加幅度在其他文献中未见报道,当地冬季趋暖现象明显。近47年来当地降水增加的趋势并不显著,干旱期占据了植物的整个生长季节。     

乌鲁木齐市近47年气温降水特征与突变分析

利用乌鲁木齐市的气温和降水资料,运用气候趋势系数、一元回归分析法和5年趋势滑动,进行了气候变化的趋势分析。结果表明:乌鲁木齐市气温和降水的变化趋势基本与西北地区气候变化趋势一致;春季和夏季气温呈下降趋势,秋冬季呈上升趋势,冬季增温幅度最大;四季降水量呈增加趋势,冬季降水量增加趋势最明显。在此基础上,利用滑动T检验法、YAMAMOTO检验信噪比(SNR)、Cram er法和M ann-Kendall法进行气候突变分析,结果表明:气温突变比降水突变明显,气温在1962年发生突变。     

Response of Stipa bungeana and Pennisetum flaccidum to urine of sheep in steppe grassland of north‐western China

Urine [12·3 g nitrogen (N) L^?1], collected from native Tan sheep in rangeland of the Loess Plateau in north‐western China, was applied to vegetation patches dominated by Stipa bungeana (C₃ grass) or Pennisetum flaccidum, (C₄ grass) at either 0, 1·0, 2·0 or 4·0 L urine m^?2 in early‐, mid‐, or late‐growing season, and herbage mass and tiller number per sample recorded, allowing calculation of the respective contributions of increases in tiller weight and tiller density to the response from N in urine. Such records were made three times in the growing season of application of urine, and at the end of the following growing season. Responses to urine were of the order of 1 and 5 kg herbage DM kg^?1 N applied for S. bungeana and P. flaccidum, respectively. Except for early‐season application, responses of S. bungeana were mainly expressed in the season following urine application whereas responses of P. flaccidum tended to be expressed within the year of urine application, although with a residual response in the following year. The general pattern was for a tiller density‐mediated response in the period immediately after application, followed later by a tiller weight‐mediated response. Taken together with other recent research, S. bungeana can be considered a stress‐tolerating species with a limited response to N application and P. flaccidum an opportunist species able to capitalize on increased N supply.     

Productivity and water use of alfalfa and subsequent crops in the semiarid Loess Plateau with different stand ages of alfalfa and crop sequences

Alfalfa (Medicago sativa L.) plays an important role in crop–livestock mixed farming on marginal land in the semiarid Loess Plateau. However, the duration, yield performance and water use of long-term alfalfa stands and choice of appropriate subsequent crops are not clear. A 5-year field experiment was conducted at Zhonglianchuan, Gansu Province, China from 2001 to 2005. Productivity and water use were determined and compared between (1) three alfalfa stands that were 1–5 (A1–5), 6–10 (A6–10) and 11–15 (A11–15) years old during the trial; (2) alfalfa using conventional cultivation and a water-harvesting technique (RA1–5); and (3) conventional crop rotation (CK) and four 5-year crop sequence rotations sown after 10-year-old alfalfa had been ploughed, being millet–wheat–potato–pea–potato (MWLPL); millet–corn–corn–wheat–wheat (MCCWW); millet–potato–wheat–corn–corn (MLWCC) and millet–fallow–pea–potato–pea (MFPLP). Forage yield peaked in 7-year-old alfalfa (5740 kg ha⁻¹), but 9-year-old alfalfa had the maximum forage yield profit (4477 kg ha⁻¹ y⁻¹) in terms of whole growing years. Soil water use efficiency (WUE_S in terms of forage yield and soil water use) of alfalfa increased dramatically up to the 11th year, and then leveled off from year 12 to 15. Forage yield and WUE_B/ET (WUE in terms of aboveground biomass and evapotranspiration) of alfalfa were significantly higher using water harvesting compared with conventional cultivation, but were significantly lower than CK. Soil water content did not change in CK as stand age increased, but it decreased in conventional alfalfa stands. After 10 years of alfalfa, a fallow year was not necessary before planting annual crops as soil water was greatly restored after sowing subsequent annual crops. Yield of some crops in the four crop sequence rotations did not differ significantly from CK. MWLPL and MLWCC had more aboveground biomass than MCCWW and MFPLP but the choice of crop sequence needs to be further considered.