Patterns Of Vegetation Cover Research Articles

基于中高分辨率遥感的植被覆盖度时相变换方法

PDF HTML阅读 XML下载 导出引用 引用提醒 基于中高分辨率遥感的植被覆盖度时相变换方法 DOI: 10.5846/stxb201305020904 作者: 作者单位: 黄河中下游数字地理技术教育部重点实验室,中国科学院遥感与数字地球研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 中国科学院知识创新工程重大项目(KZCX1-YW-08-03);水利部-官厅密云水库上游水土保持遥感监测二期工程(HW-STBC2004-03);中国博士后科学基金资助项目(20100470994) A temporal transformation method of fractional vegetation cover derived from high and moderate resolution remote sensing data Author: Affiliation: Institute of Remote Sensing Applications,Chinese Academy of Sciences,Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:植被覆盖度是衡量地表植被状况、指示生态环境变化的一个重要指标,也是许多学科的重要参数。传统的测量方法难以获取时间连续的面状数据,且耗时、耗力,很难大范围推广。遥感估算方法虽然可以弥补传统方法的不足,但由于云覆盖等天气条件的影响,获得同一时相覆盖整个研究区的遥感影像非常困难,时相的差异必然导致研究结果产生误差。针对植被覆盖度这一重要生态参数,结合低分辨率遥感数据的时间优势和中高分辨率遥感数据的空间优势,提出一种时相变换方法,将源于中高分辨率影像的植被覆盖度变换到研究需要的时相上。首先,利用像元二分模型计算MODIS尺度的时间序列植被覆盖度,并利用已经获得的SPOT影像计算其获取时相上的植被覆盖度;其次,利用土地利用图划分植被覆盖类型,并利用MODIS数据和土地利用数据之间的空间对应关系制作MODIS像元内各类植被覆盖的面积百分比数据;再次,利用面积百分比数据提取各类植被覆盖的纯像元,结合MODIS植被覆盖度时间序列,从而提取各类植被覆盖纯像元的植被覆盖度时间序列曲线;最后利用像元分解的方法提取MODIS像元内各类植被覆盖组分的植被覆盖度的变化规律,将其应用到该组分对应位置上SPOT像元的植被覆盖度上,从而将其变换到所需要的时相上。在密云水库上游进行试验,将覆盖研究区的10 景SPOT5多光谱影像计算的植被覆盖度统一变换到7 月上旬,结果显示:视觉效果上明显好转,且空间上连续一致;变换前后植被覆盖度的统计量对比结果也符合植被生长规律;利用外业样点数据与对应位置的植被覆盖度变换结果进行回归分析,结果发现各植被覆盖类型的R2均在0.8左右,表明变换结果与实测值非常接近,时相变换的效果较好,从而可以很好地促进相关研究精度的提高。 Abstract:Fractional vegetation cover (FVC) is an important index of land surface vegetation status. It is also an indicator of ecological environment changes and an important spatial parameter for various ecological modeling. The traditional methods of FVC measurement are time-consuming and labor-intensive, and thus difficult to obtain large-scale time series FVC data. Remote sensing technique is an effective approach to estimate FVC, but it is very difficult to acquire high and moderate resolution remote sensing images covering the entire study area during the same period because of the cloud cover and other weather conditions. Consequently, the FVC data derived from multi-temporal images inevitably lead to uncertain research results. To address the problem, this paper proposes a novel method to eliminate the impact of acquisition time differences on FVC from high and moderate resolution remote sensing images. For FVC data derived from images with different resolutions and acquisition dates, this proposed temporal transformation method is used to estimate high resolution FVC combined with a low-resolution time series FVC. Firstly, low and high resolution FVC data can be calculated from time series MODIS images and acquired SPOT images respectively using the dimidiate pixel model. Secondly, the vegetation cover is divided into different vegetation types based on the land use map derived from these SPOT images. And for each MODIS pixel, the area percentages of various types of vegetation cover are calculated based on the spatial overlay of MODIS image and land use data. As a result, the area percentage data represent that the area ratio of the different vegetation types within each MODIS pixel. Thirdly, the pure pixels of various types of vegetation cover can be extracted based on the area percentage data where the ratio is equal to 1, and the FVC time series curve of each type of vegetation cover can be generated based on these pure pixels and time series MODIS FVC data. Finally, the sub-pixel FVC variation of each type of vegetation cover can be extracted from MODIS pixels based on the pixel unmixing technique, and then apply them to the same location of SPOT FVC. Thus, the SPOT FVC can be transformed from its acquisition date to the specific date, which satisfies the need of our research. The feasibility of this temporal transformation method is examined in the upstream of Miyun Reservoir. The FVC data derived from 10 SPOT images are transformed to the same date of early July. The case study results show that: (1)The visual effects of the transformed FVC are significantly improved and consistent with the spatial patterns of vegetation cover; (2)The changes of FVC statistics information before and after the transformation are also in line with the laws of vegetation growth; (3) The linear regression of the FCV data on field measurement samples shows strong positive correlations between them, and the R2 is about 0.8 for each vegetation cover indicating the transformation results is close to the field measured values. The transformation results with higher precision can promote the accuracy of related researches. This method has also a certain reference value for the transformation of other parameters. 参考文献 相似文献 引证文献

Effects of slope angle and aspect on plant cover and species richness in a humid Mediterranean badland

ABSTRACTSoil erosion is one of the most severe land degradation processes in the Mediterranean region. Although badlands occupy a relatively small fraction of the Mediterranean area, their erosion rates are very high. Many studies have investigated to what extent vegetation controls soil erosion rates. This study, however, deals with the impact of erosion on vegetation establishment. In semi‐arid badlands of the Mediterranean, soil water availability constitutes the main limiting factor for vegetation development. As a consequence, south‐facing slopes are typically less vegetated due to a very large water stress. However, these findings do not necessarily apply to humid badlands. The main objective of this paper is to determine the topographic thresholds for plant colonization in relation to slope aspect and to assess the spatial patterns of vegetation cover and species richness. We surveyed 179 plots on highly eroded badland slopes in the Central Pyrenees. We defined four aspect classes subdivided into slope angle classes. Colonization success was expressed in terms of vegetation cover and species richness. Slope angle thresholds for plant colonization were identified for each slope aspect class by means of binary logistic regressions.The results show that a critical slope angle exists below which plants colonize the badland slopes. Below this critical slope angle, plant cover and species richness increase with a decreasing slope angle. The largest critical slope angles in humid badlands are observed on south‐facing slopes, which contrasts with the results obtained in semi‐arid badlands. North‐facing slopes however are characterized by a reduced overall vegetation cover and species richness, and lower topographic threshold values. The possible underlying processes responsible for this slope‐aspect discrepancy in vegetation characteristics are discussed in terms of environmental variables that control regolith development, weathering and erosion processes. Moreover, possible restoration strategies through the use of vegetation in highly degraded environments are highlighted. Copyright © 2014 John Wiley & Sons, Ltd.

Automatic identification of agricultural terraces through object-oriented analysis of very high resolution DSMs and multispectral imagery obtained from an unmanned aerial vehicle

Agricultural terraces are features that provide a number of ecosystem services. As a result, their maintenance is supported by measures established by the European Common Agricultural Policy (CAP). In the framework of CAP implementation and monitoring, there is a current and future need for the development of robust, repeatable and cost-effective methodologies for the automatic identification and monitoring of these features at farm scale. This is a complex task, particularly when terraces are associated to complex vegetation cover patterns, as happens with permanent crops (e.g. olive trees). In this study we present a novel methodology for automatic and cost-efficient identification of terraces using only imagery from commercial off-the-shelf (COTS) cameras on board unmanned aerial vehicles (UAVs). Using state-of-the-art computer vision techniques, we generated orthoimagery and digital surface models (DSMs) at 11 cm spatial resolution with low user intervention. In a second stage, these data were used to identify terraces using a multi-scale object-oriented classification method. Results show the potential of this method even in highly complex agricultural areas, both regarding DSM reconstruction and image classification. The UAV-derived DSM had a root mean square error (RMSE) lower than 0.5 m when the height of the terraces was assessed against field GPS data. The subsequent automated terrace classification yielded an overall accuracy of 90% based exclusively on spectral and elevation data derived from the UAV imagery.

Relating vegetation dynamics to temperature and precipitation at monthly and annual timescales in Taiwan using MODIS vegetation indices

To predict the responses of the timing, duration, and density of photosynthetically active plant cover to a changing climate, it is necessary to first quantitatively describe the relationships between temporal and spatial patterns of vegetation cover and both spatial and inter-annual variation in temperature and precipitation. We examined these relationships at multiple scales in Taiwan using monthly maximum composite values of MODIS-NDVI and MODIS-EVI between 2000 and 2012. The two vegetation indices were highly correlated to each other on a monthly basis for non-forest land-cover types, but correlated poorly in forests, probably due to the saturation of NDVI. However, the two indices were equally sensitive in detecting the onset and offset of growing season for all vegetation types. We found that EVI was positively related to both precipitation and temperature on a monthly timescale, although the relationships were not significant at the annual timescale. The much greater variation in monthly than in annual precipitation and temperature probably explains this difference. At low elevations, precipitation had a positive effect and temperature had a negative effect on EVI; however, at high elevations, which are mostly forested, both were positively related to EVI (although precipitation effects were not significant). We interpret this as evidence of water limitation of photosynthetic cover in the warmer, low-elevation parts of the island, whereas in the higher-elevation areas precipitation was usually adequate to satisfy evapotranspirative demand. This study illustrates the importance of examining the effects of precipitation and temperature on plant growth at a range of spatial and temporal scales. Specifically, finer spatial and temporal scales of analysis may better reveal climatic controls over vegetation growth than broader scales of analysis.

Pedogenic carbonate stable isotopic evidence for wooded habitat preference of early Pleistocene tool makers in the Turkana Basin

The origin and evolution of early Pleistocene hominin lithic technologies in Africa occurred within the context of savanna grassland ecosystems. The Nachukui Formation of the Turkana Basin in northern Kenya, containing Oldowan and Acheulean tool assemblages and fossil evidence for early members of Homo and Paranthropus, provides an extensive spatial and temporal paleosol record of early Pleistocene savanna flora. Here we present new carbon isotopic (δ13CVPDB) values of pedogenic carbonates (68 nodules, 193 analyses) from the Nachukui Formation in order to characterize past vegetation structure and change through time. We compared three members (Kalochoro, Kaitio, and Natoo) at five locations spanning 2.4–1.4Ma and sampled in proximity to hominin archaeological and paleontological sites. Our results indicate diverse habitats showing a mosaic pattern of vegetation cover at each location yet demonstrate grassland expansion through time influenced by paleogeography. Kalochoro floodplains occurred adjacent to large river systems, and paleosols show evidence of C3 woodlands averaging 46–50% woody cover. Kaitio habitats were located along smaller rivers and lake margins. Paleosols yielded evidence for reduced portions of woody vegetation averaging 34–37% woody cover. Natoo environments had the highest percentage of grasslands averaging 21% woody cover near a diminishing Lake Turkana precursor. We also compared paleosol δ13CVPDB values of lithic archaeological sites with paleosol δ13CVPDB values of all environments available to hominins at 2.4–1.4Ma in the Nachukui and Koobi Fora Formations. Grassy environments became more widespread during this interval; woody canopy cover mean percentages steadily decreased by 12%. However, significantly more wooded savanna habitats were present in the vicinity of lithic archaeological sites and did not mirror the basin-wide trend of grassland spread. Hominin lithic archaeological sites consistently demonstrated woody cover circa 40% throughout our study interval and were 4–12% more woody than coeval basin environs. We propose that Turkana Basin early tool makers may have preferred a more wooded portion of the savanna ecosystem to reduce heat stress and to gain differential access to potable water, raw materials, animal carcasses, and edible plants.

Linking vegetation cover patterns to hydrological responses using two process-based pattern indices at the plot scale

Vegetation cover pattern is one of the factors controlling hydrological processes. Spatially distributed models are the primary tools previously applied to document the effect of vegetation cover patterns on runoff and soil erosion. Models provide precise estimations of runoff and sediment yields for a given vegetation cover pattern. However, difficulties in parameterization and the problematic explanation of the causes of runoff and sedimentation rates variation weaken prediction capability of these models. Landscape pattern analysis employing pattern indices based on runoff and soil erosion mechanism provides new tools for finding a solution. In this study, the vegetation cover pattern was linked with runoff and soil erosion by two previously developed pattern indices, which were modified in this study, the Directional Leakiness Index (DLI) and Flowlength. Although they use different formats, both indices involve connectivity of sources areas (interpatch bare areas). The indices were revised by bringing in the functional heterogeneity of the plant cover types and the landscape position. Using both artificial and field verified vegetation cover maps, observed runoff and sediment production on experiment plots, we tested the indices’ efficiency and compared the indices with their antecedents. The results illustrate that the modified indices are more effective in indicating runoff at the plot/hillslope scale than their antecedents. However, sediment export levels are not provided by the modified indices. This can be attributed to multi-factor interaction on the hydrological process, the feedback mechanism between the hydrological function of cover patterns and threshold phenomena in hydrological processes.

Об использовании эколого-климатической модели в картографировании растительного покрова Якутии

Information and computer technologies of recent decades provide new approaches that allow to create maps of better quality and precise characteristics. Based on the statement that distribution patterns of vegetation cover in a given region are closely related to a certain set of ecological factors, we have tasked to analyze the accuracy of map contours by means of GIS. The study consisted in correlation of raster maps of ecological-climatic factors with the map of vegetation cover. The raster map of relief was used as supplemental as well. In this stage of the study, we analyzed the accuracy of contours depicting the distribution of the following subzones: lowlands of the arctic tundra, subarctic tundra, foresttundra, northern and middle taiga. Empirical selection of ecological-climatic factors has shown that the following factors have the most pronounced effect on distribution of vegetation cover of Yakutia: average temperatures of January and July, the sum of positive temperatures above 10 °C. Mathematic correlation of raster maps of the given factors with the vegetation map yielded ranges of temperature and elevation values that were most characteristic for each subzone. Having calculated derived values using map algebra, we simulated the spatial distribution of subzonal vegetation. Overlaying the vector map of vegetation of Yakutia has proved accurate subzone distribution in the model. This means that the abovementioned statistical samplings of temperature indices can be used for climatic characterization of each vegetation subzone. However, the model shows regions requiring special attention. They are represented by white, uncoloured patches between or within certain subzones. Analysis results of such uncharted areas state that they are characterized by a complex of temperature factors being non-typical for a given subzone. And revision of contour boundaries in these areas or vegetation types they represent is required using satellite imagery and literature data.

Vegetation covers as indicator of seismo-tectonic environment: an integrated geophysical study in north Baromura hill, India

Spatial distributions of vegetation cover are closely related to topographical characters like slope, steepness, soil types, elevations, etc. In seismo-tectonic regions fault lines or tectonic lineaments are the most prominent surface signatures that can be reflected by vegetation distribution as tectonic displacement causes the change of topographical parameters. In the present study a part of northern Baromura hill (between 23o 42′ N to 23o49 N latitudes and 91o30 E to 91o36 E longitudes), which is considered as an active seismo-tectonic region of Tripura as well as northeast India, was selected for assessing the relationship between spatial variation of plant cover and topography. Geologically, Baromura hill is a north–south extended fold belt and formed by late Tertiary to Quaternary depositions. Evidences show that tectonic events played very important role in landform evolution of this area during the geological past. In this research an extensive fieldwork was conducted in the study area to understand the geomorphic and vegetation signatures and their environmental relationships. Various spectral resolutions of Landsat ETM+ were used for digital analysis. To analyse the vegetation character of this area digital operation was done within the spectral range 0.63–1.75 μm. Band 4 and band 3 were used for NDVI operation. Band 5 or mid-wave infrared band was classified in unsupervised mode to understand the range of plant water in the study area. Overlay operation with those operated images shows the spatial distribution pattern of vegetation cover indicates the landform condition. Three clear tectonic lineaments (faults) were detected from the digital study in on the northern part Baromura hill. Structural conditions of those fault lines were verified by geo-electrical survey during fieldwork. GPS tool was used for fixing the ground control points. Resistivity characters of the study area assessed from VES profile strongly supports that vegetation cover analysis from remotely sensed data within 0.63–1.75 μm spectral ranges can be a very useful tool for detecting surface tectonic signatures of the landform.

Erfassung struktureller Waldparameter mithilfe von flugzeuggetragenem Laserscanning | Deriving structural forest parameters using airborne laser scanning

Airborne laser scanning is a relatively young and precise technology to directly measure surface elevations. With today's high scanning rates, dense 3-D pointclouds of coordinate triplets (xyz) can be provided, in which many structural aspects of the vegetation are contained. The challenge now is to transform this data, as far as possible automatically, into manageable information relevant to the user. In this paper we present two such methods: the first extracts automatically the geometry of individual trees, with a recognition rate of over 70% and a systematic underestimation of tree height of only 0.6 metres. The second method derives a pixel map of the canopy density from the pointcloud, in which the spatial patterns of vegetation cover are represented. These patterns are relevant for habitat analysis and ecosystem studies. The values derived by this method correlate well with field measurements, giving a measure of certainty (R2) of 0.8. The greatest advantage of airborne laser scanning is that it provides spatially extensive, direct measurements of vegetation structure which show none of the extrapolation errors of spot measurements. A large challenge remains in integrating these new products into the user's processing chains and workflows, be it in the realm of forestry or in that of ecosystem research.

Effects of vegetation restoration on soil organic carbon sequestration at multiple scales in semi-arid Loess Plateau, China

Soil organic carbon (SOC) sequestration by vegetation restoration is the theme of much current research. Since 1999, the program of qGrain for Greenqhas been implemented in the semi-arid Loess Plateau, China. Its scope represents the largest vegetation restoration activity in China. However, it is still unclear for the SOC sequestration effects of vegetation cover change or natural succession promoted by the revegetation efforts at different scales under the semi-arid conditions. In this study, the changes in SOC stocks due to the vegetation restoration in the middle of Loess Plateau were estimated at patch, hill slope transect and small watershed scale from 1998 to 2006. Soil samples were taken from field for the determination of cesium-137 ((137)Cs) and SOC contents. Vegetation cover change from 1998 to 2006 at the small watershed scale was assessed using Geographic Information System. The results showed that cropland transforming to grassland or shrubland significantly increased SOC at patch scale. Immature woodland, however, has no significant effect. When vegetation cover has no transformation for mature woodland (25 years old), SOC has no significant increase implying that SOC has come to a stable level. At hill slope scale, three typical vegetation cover patterns showed different SOC sequestration effects of 8.6%, 24.6%, and 21.4% from 1998 to 2006, and these SOC increases mainly resulted from revegetation. At the small watershed scale, SOC stocks increased by 19% in the surface soil layer at 0-20 cm soil depth from 1998 to 2006, which was equivalent to an average SOC sequestration rate of 19.92 t Cy(-1) km(-2). Meanwhile. SOC contents showed a significant positive correlation (Pl0.001) with the (137)Cs inventory at every soil depth interval. This implied significant negative impacts of soil erosion on SOC sequestration. The results have demonstrated general positive effects of vegetation restoration on SOC sequestration at multiple scales. However, soil erosion under rugged topography modified the spatial distribution of the SOC sequestration effects. Therefore, vegetation restoration was proved to be a significant carbon sink, whereas, erosion could be a carbon source in high erosion sensitive regions. This research can contribute to the performance assessment of ecological rehabilitation projects such as qGrain to Greenq and the scientific understanding of the impacts of vegetation restoration and soil erosion on soil carbon dynamics in semi-arid environments. (C) 2010 Elsevier B.V. All rights reserved.

Comparing Different Approaches for Mapping Urban Vegetation Cover from Landsat ETM+ Data: A Case Study on Brussels.

Urban growth and its related environmental problems call for sustainable urban management policies to safeguard the quality of urban environments. Vegetation plays an important part in this as it provides ecological, social, health and economic benefits to a city's inhabitants. Remotely sensed data are of great value to monitor urban green and despite the clear advantages of contemporary high resolution images, the benefits of medium resolution data should not be discarded. The objective of this research was to estimate fractional vegetation cover from a Landsat ETM+ image with sub-pixel classification, and to compare accuracies obtained with multiple stepwise regression analysis, linear spectral unmixing and multi-layer perceptrons (MLP) at the level of meaningful urban spatial entities. Despite the small, but nevertheless statistically significant differences at pixel level between the alternative approaches, the spatial pattern of vegetation cover and estimation errors is clearly distinctive at neighbourhood level. At this spatially aggregated level, a simple regression model appears to attain sufficient accuracy. For mapping at a spatially more detailed level, the MLP seems to be the most appropriate choice. Brightness normalisation only appeared to affect the linear models, especially the linear spectral unmixing.

Investigating dynamical trends in burned and unburned vegetation covers using SPOT-VGT NDVI data

Fires induce dynamical trends in vegetation covers. In order to investigate the effects of fires in dynamical patterns of vegetation cover, normalized difference vegetation index data from the SPOT-VEGETATION sensor over the times series 1998–2003 were analysed for burned and unburned test sites located in the Italian Peninsula. The statistical analysis was carried out by means of three different methods: (i) power spectral density (PSD), which reveals scaling as well as periodic trends; (ii) the multiple segmenting method (MSM), which is well suited to analysing scaling behaviour for short time series; and (iii) detrended fluctuation analysis (DFA), a method that allows persistence in non-stationary signal fluctuations to be captured. Results from the statistical analyses showed that the scaling exponents α of the pixel time series for fire-affected sites range around mean values of ~1.38 (PSD), ~1.19 (MSM) and ~1.22 (DFA), while those for fire-unaffected sites vary around mean values of ~0.86 (PSD), ~0.63 (MSM) and ~0.65 (DFA). The two classes of vegetation (fire affected and fire unaffected) are significantly discriminated from each other (with the t-Student test, p < 0.0001) for all three methods adopted. The scaling exponents of both fire-affected and fire-unaffected sites show the persistent character of the vegetation dynamics though the fire-affected sites show larger exponents. Such a result shows that fires contribute by increasing the persistence of the time dynamics of vegetation and, therefore, drive unstable behavioural trends in vegetation dynamics of burned areas.

Agricultural Legacies in the Great Basin Alter Vegetation Cover, Composition, and Response to Precipitation

The land-use history of an ecosystem influences current structure and possibly response to modern disturbances and stresses. In semiarid systems the nature of land-use legacies is poorly understood, confounding efforts to establish sustainable management approaches. We compare previously cultivated and non-cultivated lands in Owens Valley, California, where cultivation once extended to 34% of the valley floor but was largely discontinued by 1940, to measure the influence of past disturbance on modern vegetation. We combined historic maps of cultivated and non-cultivated land with an extensive vegetation survey, historic aerial photographs, and satellite measurements of vegetation response to precipitation variability to examine the importance of land-use history in determining the sensitivity of vegetation to annual variations in precipitation. Remote sensing analysis showed that total plant cover on previously cultivated lands was lower and fluctuations in cover were marginally more dependent on precipitation compared with plant cover on non-cultivated lands. We then compared modern plant assemblages within cultivated and non-cultivated land to determine if compositional differences could explain the current patterns of vegetation cover. We found lower species richness on previously cultivated parcels, and higher frequency and cover of perennial grasses on non-cultivated lands. Therefore, we showed differences in land-cover patterns, isolated a mechanism that could account for the differences (species differences), and developed a method for remotely analyzing land regions that have experienced historic anthropogenic disturbance.

Relationships between deer mice and downed wood in managed forests of southern British Columbia

We examined the relationship between deer mice (Peromyscus maniculatus (Wagner)) and downed wood in a low-elevation Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forest and a high-elevation Engelmann spruce (Picea engelmannii Parry ex Engelm.) – subalpine fir (Abies lasiocarpa (Hook.) Nutt.) forest in the south-central interior of British Columbia. We experimentally manipulated the volume of downed wood on clear-cut and forested sites and monitored the response of deer mice with a mark–recapture study to assess population densities and survival and reproduction rates. Populations responded positively to harvesting at the low-elevation but not the high-elevation study area. At the low-elevation study area, the population dynamics of deer mice on clear-cut and forested treatments were not positively associated with patterns of vegetation cover or increasing downed-wood volumes. Instead, populations on clearcuts appeared to increase in response to an unknown factor associated with lower volumes. No relationship was detected between population dynamics of deer mice and downed-wood volumes at the high-elevation site. The population dynamics of deer mice on forests at the high-elevation site appeared to be more closely related to vegetation cover than to downed wood. The results indicated that downed wood is not a critical habitat component for deer mice in the south-central interior of British Columbia.

Vegetation cover mapping in India using multi-temporal IRS Wide Field Sensor (WiFS) data

In this study, we explored the potential of multi-temporal IRS Wide Field Sensor (WiFS) data for mapping of vegetation cover types in India. A vegetation cover type map was generated from a hybrid approach (supervised and unsupervised) classification of 8–10 months IRS WiFS composite data (Raw bands, Max NDVI) over the period of 1998 to 1999. The study has identified 35 cover classes with a description of 22 vegetation cover including 14 forest cover types from 188 m spatial resolution WiFS data. India has a diverse range of forests: from the tropical evergreen in the south to the alpine meadows in the north, from the deserts in the west to the evergreen forests in the north-east and a diverse pattern of vegetation cover. More than half of the forest area in India is tropical-moist and dry-deciduous types distributed in central part. The WiFS vegetation cover map was compared to the estimates of forest cover area derived from IRS LISS III images (as per Forest Survey of India). There was a good agreement on spatial distribution and area of forest between the WiFS product and the LISS product, however, the WiFS product provided additional information on vegetation cover types and other land use/cover classes. Analysis of temporal NDVI profile allows identification of distinct growth pattern between the different vegetation cover types. It is evident that WiFS data can be used to provide timely and detailed vegetation cover type maps with limited ancillary data. The WiFS derived maps could be very useful as input to biogeochemical models that require timely estimation of forest area and type.

Modelling climate change‐driven treeline shifts: relative effects of temperature increase, dispersal and invasibility

1 Global warming will probably shift treelines upslope in alpine areas and towards the pole in arctic environments. However, responses of regional treelines to climatic trends over the last century do not show any clear trends. We hypothesize that these equivocal responses may partly be caused by limitation of dispersal and/or recruitment that is species-specific to particular trees with potentially expanding ranges. 2 To test this hypothesis, we established and parameterized a temporally and spatially explicit model of plant spread and analysed its sensitivity to: (a) variation in predicted climatic trends; (b) the spatial distribution of recruits around a seed source; and (c) variation in the resistance of resident non-woody vegetation to invasion. We used data from a high mountain landscape of the Northern Calcareous Alps in Austria where the treeline is dominated by Pinus mugo Turra, a shrubby pine. 3 Low growth rates and long generation times, together with considerable dispersal and recruitment limitation, resulted in an overall slow range expansion under various climate-warming scenarios. 4 Running the model for 1000 years predicted that the area covered by pines will increase from 10% to between 24% and 59% of the study landscape. 5 The shape of the dispersal curve and spatial patterns of competitively controlled recruitment suppression affect range size dynamics at least as severely as does variation in assumed future mean annual temperature (between 0 °C and 2 °C above the current mean). Moreover, invasibility and shape of the dispersal curve interact with each other due to the spatial patterns of vegetation cover in the region. 6 Ambiguous transient responses of individual treeline systems may thus originate not only from variation in regional climatic trends but also from differences in species’ dispersal and recruitment behaviour and in the intensity and pattern of resistance of resident alpine vegetation to invasion.

Erosion quantification in the small marly experimental catchments of Draix (Alpes de Haute Provence, France). Calibration of the ETC rainfall–runoff–erosion model

Erosion rates measured for 15 years in the small (ranging from 0.13 ha to nearly 1 km 2) and steep (30–75%) mountainous marly basins of Draix (South Alps, France) are over 100 tons/ha/year in a badland area devoid of vegetation. In a similar basin, reforested at the end of the 19th century, the sediment yield is considerably reduced (less than 3 tons/ha/year). The analysis of the sediment yield of these torrents reveals the importance of the erosion processes on slopes at the scale of the elementary gully, and the influence of deposition, transport and scouring processes as soon as the channel network of the basin is developed. These processes were modelled by a hydrosedimentologic model developed by Cemagref for flood torrent erosion and called ETC. ETC is an integrated, spatialized and global model, working at the event scale. The rainfall–runoff component and the solid transport component, which involves scouring, deposition and armouring, are well developed. On the other hand, the erosion component is not yet a physically based model as the knowledge on the elementary processes is not enough quantitative. The ETC model was used on a badland basin of 86 ha in area, the Laval, and has shown its ability to simulate both water and sediment flows in mountainous and steep basins. The erosion modelling highlighted the need for a better representation of the slope processes and for more information on the storing of sediments in the channel network. Observations carried out on the influence of the vegetation cover patterns and the links between slopes and channels should be taken into account to improve the model.

Hydrological processes and their seasonal controls in a small Mediterranean mountain catchment in the Pyrenees

Abstract. The Vallcebre catchments are located in a middle mountain area of the Pyrenean ranges, built up by sedimentary rocks and loamy soils. The vegetation cover is pastures and forests of Pinus sylvestris, mostly occupying former agricultural terraces. Some relatively small, heavily eroded landscapes (badlands) occur in the catchments, playing a relevant hydrological and geomorphic role. Annual precipitation is 924 mm and potential (reference) evapotranspiration is about 700 mm. Rainfall interception in forests represents about 24% of precipitation; interception rates were similar throughout the seasons because of a compensation between rainfall intensities and atmospheric conditions. Soil moisture showed a temporal pattern characterised by the occurrence of marked deficit periods in summer and also, but less pronounced, in winter. During most of the year, subsurface flows on hillslopes drove the spatial organisation of soil moisture and the occurrence of saturated areas. Nevertheless, this spatial organisation was also controlled by the patterns of vegetation cover. During dry periods, subsurface flow ceased, saturated areas disappeared and the spatial patterns of soil moisture changed. Stream flow from these catchments was dominated by storm flow, and the runoff generating mechanisms showed a clear seasonal pattern, controlled mainly by the soil moisture and the extent of saturated areas. During the dry periods, runoff was produced only on impervious areas and badlands. At the end of the dry periods, some large rainfall events generated significant runoff because of the perched saturation of the shallow soil horizons. Thereafter, runoff generation was dominated by the role of saturated areas. Stream waters in catchments with badlands had very high suspended sediment concentrations. The seasonal pattern of erosion processes in badlands was characterised by physical weathering during winter, regolith breakdown and vigorous hillslope erosion during spring and summer, and efficient transport of sediments in autumn.

Geomorphic agents versus vegetation spreading as causes of badland occurrence in a Mediterranean subhumid mountainous area

Previous evidence on the chief role of physical weathering on badland development in the Vallcebre area (South Eastern Pyrenees) led to the study of the amounts of energy available for the main geomorphic agents, and to analyze the results in the light of the patterns of vegetation cover and species distribution, in order to explain both the occurrence of these badland forms and their preference for north-facing (shady) hillslopes. Results showed that the weathering potential provided by freezing is about two orders of magnitude greater than rainfall kinetic energy, and that the former strongly depends on hillslope aspect. The study of vegetation demonstrated that vegetation is poorer in cover and species composition on shady hillslopes than on sunny ones, spontaneous revegetation of these surfaces being restrained by cold thermal conditions rather than by water availability. These results show that the occurrence of these badland areas depends on montane climatic attributes that control geomorphic and biological processes, and that are different from those that promote the occurrence of badlands in dry Mediterranean areas. The preference for northern aspects provides evidence for these differences.

Coupling dynamic models of climate and vegetation

AbstractNumerous studies have underscored the importance of terrestrial ecosystems as an integral component of the Earth's climate system. This realization has already led to efforts to link simple equilibrium vegetation models with Atmospheric General Circulation Models through iterative coupling procedures. While these linked models have pointed to several possible climate–vegetation feedback mechanisms, they have been limited by two shortcomings: (i) they only consider the equilibrium response of vegetation to shifting climatic conditions and therefore cannot be used to explore transient interactions between climate and vegetation; and (ii) the representations of vegetation processes and land‐atmosphere exchange processes are still treated by two separate models and, as a result, may contain physical or ecological inconsistencies.Here we present, as a proof concept, a more tightly integrated framework for simulating global climate and vegetation interactions. The prototype coupled model consists of the GENESIS (version 2) Atmospheric General Circulation Model and the IBIS (version 1) Dynamic Global Vegetation Model. The two models are directly coupled through a common treatment of land surface and ecophysiological processes, which is used to calculate the energy, water, carbon, and momentum fluxes between vegetation, soils, and the atmosphere. On one side of the interface, GENESIS simulates the physics and general circulation of the atmosphere. On the other side, IBIS predicts transient changes in the vegetation structure through changes in the carbon balance and competition among plants within terrestrial ecosystems.As an initial test of this modelling framework, we perform a 30 year simulation in which the coupled model is supplied with modern CO2concentrations, observed ocean temperatures, and modern insolation. In this exploratory study, we run the GENESIS atmospheric model at relatively coarse horizontal resolution (4.5° latitude by 7.5° longitude) and IBIS at moderate resolution (2° latitude by 2° longitude). We initialize the models with globally uniform climatic conditions and the modern distribution of potential vegetation cover. While the simulation does not fully reach equilibrium by the end of the run, several general features of the coupled model behaviour emerge.We compare the results of the coupled model against the observed patterns of modern climate. The model correctly simulates the basic zonal distribution of temperature and precipitation, but several important regional biases remain. In particular, there is a significant warm bias in the high northern latitudes, and cooler than observed conditions over the Himalayas, central South America, and north‐central Africa. In terms of precipitation, the model simulates drier than observed conditions in much of South America, equatorial Africa and Indonesia, with wetter than observed conditions in northern Africa and China.Comparing the model results against observed patterns of vegetation cover shows that the general placement of forests and grasslands is roughly captured by the model. In addition, the model simulates a roughly correct separation of evergreen and deciduous forests in the tropical, temperate and boreal zones. However, the general patterns of global vegetation cover are only approximately correct: there are still significant regional biases in the simulation. In particular, forest cover is not simulated correctly in large portions of central Canada and southern South America, and grasslands extend too far into northern Africa.These preliminary results demonstrate the feasibility of coupling climate models with fully dynamic representations of the terrestrial biosphere. Continued development of fully coupled climate‐vegetation models will facilitate the exploration of a broad range of global change issues, including the potential role of vegetation feedbacks within the climate system, and the impact of climate variability and transient climate change on the terrestrial biosphere.