莆田市城市空间扩展及其对植被覆盖变化的影响

分析城市空间扩展和植被覆盖指数变化及其质量之间的关系,可以为保护城市生态环境及在大空间尺度上为城市植被景观提升提供指导。以遥感成像技术为基础,通过提取目标区域卫星影像对应的建筑物和植被特征光谱,通过对多年特定光谱进行建模运算,即可得出城市空间拓展与植被覆盖指数之间的关系;以莆田市为例,利用Landsat卫星影像提取建筑指数NDBI和植被指数NDVI对该市2000、2010和2016年的城市空间扩展及其对植被覆盖变化的影响进行定量研究。结果表明:在16年间,莆田市城市建筑用地面积占比由22.70%增至28.84%,其中前期2000—2010年建筑用地面积占比增加了2.04%,而后期2010—2016年增加4.10%;植被覆盖指数变差的面积占37.26%,而植被覆盖指数变好的面积占28.87%,大部分退化的植被位于西北部山区和东南部平原;建筑指数和植被覆盖指数呈线性负相关关系,建筑指数每提升10%,植被覆盖指数降低0.77～0.86。总体看来,莆田市的城市空间扩展对植被景观影响较大,且呈现后期大于前期的趋势。     

Recent forest cover type transitions and landscape structural changes in northeast Minnesota,USA

Landsat TM satellite data covering an approximate 5-year interval (1990–1995) were used to quantify spatial pattern and transition rates between forest ecological states for a 2.76 million ha region in northeast Minnesota. Changes in forest cover were stratified by Ecological Subsection, management status, and by ownership categories using a 1995 digital ownership layer. Approximately 4.2% of the 1990 mature forested area was converted to early successional types by 1995. Of this 4.2%, private lands accounted for 33%, federal lands 31%, county lands 20% and state lands 16%. Notable conversion percentages by cover type category were spruce-fir (−5.3%), aspen-birch(−4.7%), jack pine (−4.6%) and black spruce(−3.0%). Transition rates were also adjusted to fit ten-year time intervals. Shannon-Weaver Eveness and edge density of cover types increased over the study period as relative contagion and interior forest area decreased. These trends suggest both smaller patches and a more even distribution of cover types. Area of upland conifers, lowland conifers and lowland hardwoods decreased while the area of mature upland hardwoods increased in most patch size classes except the > 500 ha class which showed a substantial decrease in area. The area of early successional types increased in most patch size classes. Non-industrial private forestland had the lowest proportion of interior forest of all ownership categories -decreasing by 13.5% in five years. Smaller average cut-unit size sand uncoordinated forest management is the likely cause since cutting rates between private and public forestland were similar. This revised version was published online in July 2006 with corrections to the Cover Date.     

Physical influences of landscape on a large-extent ecological disturbance: the northeastern North American ice storm of 1998

The 1998 ice storm was a large-extent ecological disturbance that severely affected the eastern Adirondack forests of northern New York. Ice damage produced widespread breakage of limbs and trunks in susceptible trees. Although ice storms are common within northeastern North American forests, the magnitude and extent of the 1998 storm far exceeded damage caused by typical ice storms in the recent past. While plot and stand-scale ecological impacts of ice storms have received attention insofar as tree species vulnerability, stand age susceptibility, and microhabitat alterations, larger-extent damage patterns have not been previously evaluated. The normalized difference vegetation index (NDVI) was employed to assess forest vigor and canopy density in atmospherically corrected Landsat Thematic Mapper (TM) satellite imagery of the Adirondacks. Digital change analysis of the baseline forest condition (1990 NDVI data), and the condition encountered in a post-storm image (1998 NDVI data) was conducted. Forest damage was separated from natural variations in canopy reflectance by employing a generalized linear model that incorporated in situ measurements. A robust empirical variogram analysis revealed that locations of tree damage were significantly correlated for distances up to 300 meters. Intensely-damaged forest exhibited greater spatial dependence, but over a smaller distance. Canopy damage was not greater proximate to stream and forest boundaries, and did not follow our hypothesis of decreasing damage with distance from the boundary. Overall, we show that local topography (elevation and aspect), forest composition (deciduous or coniferous), and the meteorological characteristics of the disturbance event acted together to determine the spatial extent of ice storm damage.This revised version was published online in May 2005 with corrections to the Cover Date.     

Interactions between fire and flooding in a southern African floodplain system (Okavango Delta, Botswana)

A series of 98 satellite images was analysed to reconstruct the fire and flood history of a floodplain system in southern Africa (Okavango Delta, Botswana). The data was used to investigate interactions between fire and flooding, and to determine the relevance of rainfall and flood-events for fire occurrences on floodplains and on drylands. The aims of the study are (1) to analyse and compare the fire frequency on floodplains and on adjacent drylands, (2) to investigate the influence of rainfall and flooding on the fire occurrence and (3) to determine correlations between fire frequency and flood frequency. The analyses show higher fire frequencies on floodplains than on drylands because of higher biomass production and fuel loads. The fire occurrence on drylands shows a correlation with annual rainfall events, while the fire frequency on floodplains is in principle determined by the flood frequency. Between floodplain types, clear differences in the susceptibility to fire where shown by analysing flood frequency vs. fire frequency. Here, the highest potential to burn was found for floodplains that get flooded about every second year. By calculating mean fire return intervals, the potential to burn could be specified for the different floodplain types.     

Observing succession on aspen-dominated landscapes using a remote sensing-ecosystem approach

In the North American upper Great Lakes region, forests dominated by the aspens (Populus grandidentata Michx. – bigtooth aspen, and P. tremuloides Michx. – trembling aspen), which established after late 19th and early 20th century logging, are maturing and succession will create a new forest composition at landscape to regional scales. This study analyzed the capabilities of Landsat ETM+ remote sensing data combined with existing ecological land unit classifications to discriminate and quantify patterns of succession at the landscape scale over the 4200 ha University of Michigan Biological Station (UMBS) in northern Lower Michigan. In a hierarchical approach first multi-temporal Landsat ETM+ was used with a landscape ecosystem classification to map upland forest cover types (overall accuracy 91.7%). Next the aspen cover type was subset and successional pathways were mapped within that type (overall accuracy 89.8%). Results demonstrated that Landsat ETM+ may be useful for these purposes; stratification of upland from wetland types using an ecological land unit classification eliminated confounding issues; multi-temporal methods discriminated evergreen conifer versus deciduous understories. The Landsat ETM+ classifications were then used to quantify succession and its relationship to landform-level ecological land units. Forests on moraine and ice contact landforms are succeeding distinctly to northern hardwoods (95% and 88% respectively); those on outwash and other landforms show greater diversity of successional pathways.     

Satellite estimate of grass biomass in a mountainous range in central Italy

One of the main problems in managing ranges used for extensive pastoralism is the difficulty of obtaining reliable estimates of grass biomass over very large areas. Estimates of grass biomass are useful as an indicator of both available forage and risk of soil erosion. Nevertheless, large scale field measurements are expensive and time-consuming. The use of satellite images may provide a complementary means of estimating grass biomass over very large areas at a reasonable cost. The aim of this study was to test the use of Landsat satellite data for estimating grass biomass in a mountainous range in central Italy used primarily for sheep breeding. During each of four ground campaigns carried out over two years, grass was cut and its biomass measured in 60-90 test plots. Four Landsat images taken simultaneously to the ground campaigns were processed to obtain several vegetation indexes calculated for each ground test plot. The vegetation indexes showed significant correlations with measured grass biomass. The Normalized Difference Vegetation Index (NDVI) provided the most accurate estimate of grass biomass. When data for each of the four ground campaigns were analyzed separately, correlations for early summer campaigns were higher than correlations for late summer campaigns, indicating that when the ratio of dry/green biomass increases, satellite estimate becomes less accurate. Overall, our results show that satellite data can provide a useful source of biomass information for the management of large ranges. This revised version was published online in June 2006 with corrections to the Cover Date.     

基于Landsat 8和随机森林的青海门源天然草地地上生物量遥感估算

草地地上生物量（above-ground biomass, AGB）的遥感监测可快速且客观地对草地生长现状进行评估,对生态环境评价和草地资源利用有重要意义。为了提高遥感估算草地AGB的准确性,基于青海省门源县的地面实测数据,利用Landsat-8 OLI遥感数据计算出的植被指数分别构建了单因素回归模型和随机森林模型（random forest, RF）,确定了AGB遥感估测最佳模型,并反演得到了研究区2019-2021年草地AGB空间分布。结果表明：1）在29个植被指数构建的单因素回归模型中,与草地AGB相关性较高的5个植被指数为NDVI、RBNDVI、TVI、GNDVI、MSR,R²均达0.49以上。其中,NDVI模型的精度最高,验证集R²为0.50,均方根误差（root mean square error, RMSE）为702.89 kg·hm^-2。2）在RF模型中,变量筛选前R²=0.61,RMSE=621.14 kg·hm^-2;经过变量筛选后模型精度有小幅度提升,R²达0.62,RMSE基本不变;二者精度均优于单因素模型,相比传统单因素最优回归模型,R²提高0.12,RMSE降低了80.95 kg·hm^-2。3）门源县AGB空间分布特征为西北部较高,东南部相对较低;大体呈中部高,四周低的分布状况。2019-2021年全县天然草地总产草量介于4.2827万~8.9776 万t,平均单产介于1063.49~1484.82 kg·hm^-2;草地类型以高寒草甸为主,2019-2021年产草量为4.0825万~5.6653 万t,平均地上AGB介于1060.38~1471.94 kg·hm^-2;山地草甸平均AGB为1036.81~1637.43 kg·hm^-2;温性草原平均AGB介于1198.72~1786.63 kg·hm^-2。     

黄河中上游部分地区土地覆盖动态变化研究

作为美国国家科学基金项目"中国生态恢复综合影响评价"的部分研究结果,对项目选定的四个评价区之一的黄河中上游地区的研究区域进行土地利用和覆盖变化研究,并以此评价退耕还林效果。利用覆盖研究区域的卫星遥感图像进行解译,以便获取研究区域土地利用和土地覆盖变化(LUCC)的历史数据,特别是大于25度坡度的LUCC以及退耕前后的LUCC数据,通过这些解译数据评价研究区域的退耕还林效果。解译后的数据显示退耕还林效果良好。选定24景跨20年(1978年至2007年)的Landsat遥感卫星图像数据,通过解译追踪研究区域LUCC历史演变,来判断退耕还林的执行效果。     

基于Landsat ETM＋影像的城市热岛研究

采用Landsat ETM＋影像数据,分别利用MODTRAN4和FLAASH对热红外波段和可见光、近红外波段进行大气校正,采用Valor的植被指数混合模型估算地表比辐射率,以普朗克公式推算地表温度,并以长沙市为研究区,分析了不同地表覆盖类型以及NDVI与地表温度之间的关系。结果表明,不同地表覆盖类型的地表温度有着显著差异,城市地表温度与NDVI之间具有明显的负相关关系,增加城市的绿地面积有助于缓解城市热岛效应。     

基于Landsat8遥感影像的地表温度反演方法对比研究

随着数据的自由共享,Landsat8系列卫星的遥感数据已经成为地表温度反演的重要数据之一。本文基于Landsat8数据的地表温度反演算法,选择大兴安岭地区漠河市作为研究区域。采用基于Landsat8热红外波段的辐射传输模型法、单窗方法、劈窗算法等不同方法对地表温度进行反演,探究高纬度地区地表温度分布规律,并运用野外实测地温数据进行验证。结果表明：针对本研究区的3种地表温度反演的拟合效果都较好,温度误差在1℃左右,其中辐射传输方程法所得结果精度更高,可为高纬度地区夏季地表温度反演与冻土退化等研究提供一定的参考价值。