Spatial pattern of plant species diversity and the influencing factors in a Gobi Desert within the Heihe River Basin,Northwest China

Understanding the spatial pattern of plant species diversity and the influencing factors has important implications for the conservation and management of ecosystem biodiversity. The transitional zone between biomes in desert ecosystems, however, has received little attention in that regard. In this study, we conducted a quantitative field survey(including 187 sampling plots) in a 40-km~2 study area to determine the spatial pattern of plant species diversity and analyze the influencing factors in a Gobi Desert within the Heihe River Basin, Northwest China. A total of 42 plant species belonging to 16 families and 39 genera were recorded. Shrub and semi-shrub species generally represented the major part of the plant communities(covering 90% of the land surface), while annual and perennial herbaceous species occupied a large proportion of the total recorded species(71%). Patrick richness index(R), Shannon-Wiener diversity index(H'), Simpson's dominance index(D), and Pielou's evenness index(J) were all moderately spatially variable, and the variability increased with increasing sampling area. The semivariograms for R and H' were best fitted with Gaussian models while the semivariograms for D and J were best fitted with exponential models. Nugget-to-still ratios indicated a moderate spatial autocorrelation for R, H', and D while a strong spatial autocorrelation was observed for J. The spatial patterns of R and H' were closely related to the geographic location within the study area, with lower values near the oasis and higher values near the mountains. However, there was an opposite trend for D. R, H', and D were significantly correlated with elevation, soil texture, bulk density, saturated hydraulic conductivity, and total porosity(P0.05). Generally speaking, locations at higher elevations tended to have higher species richness and diversity and the higher elevations were characterized by higher values in sand and gravel contents, bulk density, and saturated hydraulic conductivity and also by lower values in total porosity. Furthermore, spatial variability of plant species diversity was dependent on the sampling area.     

Meteorological impacts on evapotranspiration in different climatic zones of Pakistan

Arid regions are highly vulnerable and sensitive to drought. The crops cultivated in arid zones are at high risk due to the high evapotranspiration and water demands. This study analyzed the changes in seasonal and annual evapotranspiration(ET) during 1951–2016 at 50 meteorological stations located in the extremely arid, arid, and semi-arid zones of Pakistan using the Penman Monteith(PM) method. The results show that ET is highly sensitive and positively correlated to temperature, solar radiation, and wind speed whereas vapor pressure is negatively correlated to ET. The study also identifies the relationship of ET with the meteorological parameters in different climatic zones of Pakistan. The significant trend analysis of precipitation and temperature(maximum and minimum) are conducted at 95% confidence level to determine the behaviors of these parameters in the extremely arid, arid, and semi-arid zones. The mean annual precipitation and annual mean maximum temperature significantly increased by 0.828 mm/a and 0.014℃/a in the arid and extremely arid zones, respectively. The annual mean minimum temperature increased by 0.017℃/a in the extremely arid zone and 0.019℃/a in the arid zone, whereas a significant decrease of 0.007℃/a was observed in the semi-arid zone. This study provides probabilistic future scenarios that would be helpful for policy-makers, agriculturists to plan effective irrigation measures towards the sustainable development in Pakistan.     

轴流泵马鞍区水力性能与压力脉动测试与分析

为了分析轴流泵在马鞍区工况的运行特性,对一轴流泵不同工况下的外特性和压力脉动进行了测试,重点分析了轴流泵马鞍区水力特性和压力脉动特性.试验结果表明:模型泵H-Q曲线在0.50Q_d~0.60Q_d内表现出明显的马鞍形,且扬程在马鞍区内0.55Q_d工况时达到最小值,较0.60Q_d工况扬程降低0.33 m,为设计工况下扬程的5.5%;叶轮进口和泵出口处压力脉动具有较为明显的周期性,单个周期内压力脉动表现出明显的4波峰4波谷特征;0.55Q_d工况时,叶轮进口处压力脉动峰峰值为设计工况的2.3倍;各工况下导叶中间和出口处压力脉动规律较为复杂;叶轮进口压力脉动主频为叶片通过频率,0.55Q_d工况叶频处的幅值最大,高于设计工况27.6%.小流量工况下,导叶中间、导叶出口处压力脉动在频域内出现较多低频信号,压力脉动频率成分较复杂.泵出口压力脉动主频在1.00Q_d工况下明显表现为叶频.研究成果可为轴流泵不稳定运行特性的优化提供参考.     

农用无人机避障技术的应用现状及展望

随着中国农业现代化稳步推进,农业生产对农业机械的需求日益增长,农用无人机因其广泛适用于地面机械难以耕作的农业区域,在实践推广应用中已表现出明显特点和优势。随着精准农业航空技术的进一步发展,实现障碍物的自主识别与实时避障是农用无人机智能化发展的必然趋势之一。然而,农田作业环境复杂、障碍物类型多变,实现农用无人机的实时避障仍任重道远。为提高航空植保作业的安全性,对农田环境中的障碍物进行了分类,并提出了作业视场的避障分区及各区避障策略。通过对比分析国内外农用无人机避障技术的应用现状及各类避障传感器,并剖析中国农用无人机避障问题中存在的不足,展望未来农用无人机避障技术的研究方向和研究热点。该文可为中国农用无人机避障技术的有序发展提供参考。     

基于能值分析的马尾松低效林林下种草养鸡耦合模式评价

马尾松低效林是红壤侵蚀区典型植被类型。林下种草养鸡则是林下种草与林下养鸡的耦合新模式,既能解决马尾松林下水土流失问题,又能提高经济效益,实现生态种养循环,驱动水土流失治理。为科学评价马尾松低效林林下种草养鸡模式的生态经济效益,本文应用能值分析法,测评福建省长汀县三洲镇垌坝村传统林下养鸡(对照)、林下种草养鸡两种模式的能值自给率、能值投资率、净能值产出率、环境负载率、可持续发展指数和能值反馈率。结果表明,传统林下养鸡模式的能值自给率、能值投资率、净能值产出率、环境负载率、可持续发展指数和能值反馈率分别为0.860、0.166、2.125、3.380、0.629和0.026,林下种草养鸡模式上述指标值分别为0.400、1.510、0.870、1.370、0.635和0.290。林下种草养鸡模式的能值投资率、可持续发展指数比对照提高1.344和0.006个单位,显示更强的经济活力;能值自给率、环境负载率比对照下降0.460、2.010个单位,显示更低的环境负载。林下种草养鸡模式的净能值产出率低于对照1.255个单位,而能值反馈率高出对照0.264个单位。总体而言,林下种草养鸡耦合模式经济活力更强,环境负载更低,符合脆弱区林下经济发展趋势。侵蚀区土壤结构性差,限制了从系统外导入的化肥在循环系统中功能的发挥,导致系统净能值产出率、能值反馈率较低,这为该模式的优化指明了方向。     

30年来北京市土地利用时空变化特征

基于1980、1990、1995、2000、2005和2010年6期土地利用类型图,利用数理统计与空间分析相结合的方法分析市域、主体功能区的土地利用数量结构及类型转化的时空差异,以期为北京市实施土地利用精准调控提供科学依据。结果表明:1)研究期间全市耕地减少1 462.11km~2,建设用地增加1 344.64km~2,林地和草地略有增减;各土地利用类型面积变化呈"先加速后减速"趋势,1990—1995和2000—2005年建设用地扩张和耕地缩减尤为严重;2000年前,耕林草之间相互转化较活跃,后趋于稳定,2000年后耕地转为建设用地成为最主要的转化方向;2)城市功能拓展区和城市发展新区耕地降幅分别为61.75%和22.28%,建设用地增幅分别为84.32%和109.16%,较其他功能区更为剧烈;3)耕地缩减及建设用地扩张多发生在距城市中心较近(30km)的区域,且随时间变化向外推移,距城市中心65~70km有密云、平谷和延庆区建成区,耕地缩减和建设用地扩张也相对明显。过去30年北京市建设用地激增,耕地锐减,土地利用结构失调,未来应控制建设用地占用耕地,全国其他地区在城市化进程中也应重点协调建设用地扩张与耕地及生态空间保护的矛盾。     

Conserving fluxes of terrestrial invertebrates to trout in streams: A first field experiment on the effects of cattle grazing

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