Vegetation cover
Vegetation cover defines the percentage of soil which is covered by green vegetation. Leaf area index (LAI) is an alternative measure of plant cover which gives the area of leaves in square meters corresponding to an area of one square meter of land. In a wide range of environments, vegetation cover greatly affects both surface water runoff and sediment loss (Elwell and Stocking, 1976; Lee and Skogerboe 1985, Francis and Thornes 1990). A vegetation cover of 45-50% is considered a critical value since above this value soils are adequately protected from raindrop impact and soil erosion is significantly reduced. Vegetation cover can be measured in the field by assessing the percentage of the ground that it is covered by the existing annual or perennial vegetation. Aerial photographs or satellite images can also be used for measuring vegetation cover of extensive areas.
| Aim of the method/technique |
The derived maps of vegetation cover can be used for assessing land degradation vulnerability. Vegetation cover percentage is a crucial component for assessing soil erosion rates and land desertification risk.
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| Scale – spatial and temporal |
A detailed or semi-detailed map of vegetation cover at the scale 1:20,000 or 1:50,000 can be useful for regional studies in the LEDDRA project. Spatial and temporal changes of vegetation cover occur into the same year due to plant cycle, crop harvesting, animal grazing, plant pruning, etc. Therefore vegetation cover data, especially in cropland and grazing land is preferable to be measured once every month. Soil erosion models, such as PESERA (Kirkby 1999, Kirkby et al. 2000), requires twelve separated vegetation cover layers for assessing annual soil erosion rates.
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| Brief description |
Vegetation cover of a study area is defined as the percentage of the area covered by vegetation. It is usually assessed through the use of remote sensed data (Normalised Difference Vegetation Index NDVI, Green Vegetation Fraction-GVF, etc.). NDVI can thus be considered as a proxy to quantify vegetation primary productivity (Ricotta et al., 1998; Boelman et al. 2003, Bro-Jørgensen et al. 2008) and plant biomass (Myneni et al. 1995, Bajocco et al. 2010), to estimate vegetation performance change (Olsson et al. 2005) and land cover dynamics.
Vegetation cove at lower spatial levels, this can be based on CORINE land cover maps, whilst land cover changes, according to Feranec et al. (2010), can be analysed through the diachronically comparison between different land use/cover maps. In specific areas and time analyses remote sensing data are also used. Object-based methods (Figure 1) have been also proposed for land cover mapping and detecting changes (Mäkelä and Pekkarinen 2001).
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Figure 1. Example of segmentation method for vegetation cover estimation (Source: authors on Landsat data) |
Five classes of vegetation cover can be distinguished for the purpose of LEDDRA: (a) vegetation cover <10%, (b) vegetation cover =10-25%, (c) vegetation cover = 25-50%, (d) vegetation cover = 50-75% and (e) vegetation cover >75%. |
| Data requirements |
The required data are vegetation maps, CORINE Land Cover map, Aerial photographs, Remote sensing images at high and medium spatial resolution as Landsat ETM images (30 m of spatial resolution) or MODIS NDVI images (1000 or 250 m). |
| Main applications in cropland, grazing and forests & shrubland regions |
Vegetation cover has a wide variety of applications in the assessment of land degradation and desertification processes in forest and non-forest areas. Description of vegetation cover percentage can be useful in assessing the impacts of an applied response assemblage. Many studies have shown that both runoff and sediment loss are greatly affected by vegetation cover reducing raindrop impact. Vegetation cover is a crucial parameter for assessing desertification risk in forested areas (Ferrara et al. 2009) and in most of forest productivity models |
| Strengths and weaknesses |
Mapping vegetation cover is a very important measurement. Since many natural or man induced changes occur during the year, vegetation cover requires periodically monitoring. |
