Vegetation quality is assessed in terms of a) fire risk and ability to recover, b) erosion protection for the soil, c) drought resistance and d) plant cover. The dominant types of vegetation in the Mediterranean region (including bare land, perennial agricultural crops, annual agricultural crops, deciduous oak woodland, macchia, evergreen forests, and pine forests) are grouped into four categories according to fire risk. There are also four categories of vegetation according to degree of soil erosion protection, from annual agricultural crops (very low) to mixed macchia/evergreen forests (high). Five categories are used to classify vegetation with respect to drought resistance, and there are three categories of plant cover.

Fires have become very frequent, especially in pine-dominated forests with dramatic consequences for increased soil erosion rates and biodiversity losses. Many Mediterranean species are highly flammable and combustible due to a high content of resins and oils. It is possible for all the species of a mature ecosystem to regenerate during the first year following a fire, as they tend to be well-adapted to surviving fire, but drought, grazing by livestock or other land use changes may interrupt the process irreversibly. The two main strategies for species survival in a fire-prone ecosystem are re-sprouting from underground parts and germination of buried seeds.

Vegetation and land use are important factors controlling the intensity and frequency of overland flow and surface wash erosion. In particular, areas cultivated with rain-fed crops such as cereals. vines, almonds and olives, where the soil surface is left bare for much of the year, are very vulnerable to erosion and desertification. Trends in Mediterranean agriculture to maintain a vegetation cover, and use minimum tillage and direct seeding techniques will help to avoid increasing soil erosion.

There are two groups of vegetation well-adapted to drought resistance. One group are deciduous and avoid drought with a large photosynthetic capacity but no resistance to desiccation. The other group are evergreen (sclerophyllous) and drought enduring with low rates of photosysnthesis. The main response of plants to increased aridity is reduction in leaf area index. In the short term this reduces plant transpiration, but if the vegetation cover is reduced the likelihood of soil erosion is increased when rain eventually falls. Olive trees exhibit a high adaptation and resistance to long term droughts and can support a remarkably high diversity of flora and fauna in the understorey.

Many authors have demonstrated that in a wide range of environments both run-off and sediment loss decrease exponentially as the percentage of vegetation cover increases. A value of 40% vegetation cover is considered critical, below which accelerated erosion dominates in a sloping landscape. This threshold my be modified for different types of vegetation, rain intensity and land attributes.

Vegetation Quality Index = (fire risk*erosion protection*drought resistance*vegetation cover)**1/4.

For the components of this calculation, four classes of fire risk, four classes of erosion protection, five classes of drought resistance and three classes of plant cover must be assessed.

Brandt, J., and Thornes, J., 1996 Mediterranean Desertification and Land Use. J. Wiley & Sons, Chichester, England, 554.

Kosmas C., Ferrara A., Gerontidis, St. Bellotti B., Detsis V., Faretta S., Mancino G., and Pisante, M. 1999. A comparative analysis of the physical environment of two Mediterranean areas threatened by desertifciation. Mediterraneo M. 127-148.

Conacher, A., and Sala, M., 1998. Land Degradation in the Mediterranean Environments of the World: Nature and Extent, Causes and Solutions. J. Wiley & Sons, Chichester, 491 p.