**6. Conclusion**

In Portugal, as well as in the Mediterranean countries, important land use and cover changes have occurred since the second half of the last century. The abandoning of traditional subsistence systems based on cereal cultivation, probably the most important change, has taken place mainly in the more disadvantaged areas where farming systems in general and livestock farms in particular are often operating close to the margins of sustainability. This was originally a consequence of the difficulties associated with adopting modern farming systems. Later, it was the result of the demographic exodus from these rural areas, and more recently it has been reinforced by the implementation of CAP measures. In several areas, cereal crop soils were neglected and a natural vegetation succession occurred, increasing plant recovery and establishing shrub and woodland areas. In other areas, the adoption of measures aimed at reducing intensive agricultural methods involved afforestation schemes and conversion to grazing land.

There are important differences in the hydrological and erosional functioning of the different land uses/cover types monitored. Shrub and woodland are considered better for soil and water conservation, producing less surface runoff and therefore less soil erosion. The results obtained also show a positive trend in organic matter content, highlighting the importance of vegetation in these very shallow soils with a low clay content in increasing structural stability and avoiding soil loss. The major threat to these ecosystems is associated with controlling the frequency of wildfires.

Conversely, cereal cultivation and tree planting accelerate runoff and soil erosion, which is attributed to soil tillage which loosens the soil and reduces anti-erodibility. Erosion and land degradation became a problem in Portugal when arable farming expanded into marginal areas over the decades. The poor water and soil protection provided by young pine is attributed to the poor ground coverage under the trees.

In fact, the amount of bare soil on a site is generally a good indicator of the soil's vulnerability to erosion and degradation. Good soil coverage is an essential element in soil conservation programmes. Vegetation protects the soil from eroding in various ways. Rainfall interception by the plant has two main consequences, the most important being that it reduces the erosive power of impacting raindrops. It also reduces the volume of water reaching the soil surface. Subsequently, soil erosion can be controlled by changing land use and increasing ground coverage, which was shown to be one of the basic approaches to controlling soil erosion in all land use types.

On the basis of the experiment results, pastureland should be encouraged, particularly for the degraded soils that are used to produce cereals in order to minimize the amount of soil loss by erosion, thus avoiding slumping and promoting stability. Accommodating pasture management with a weighted number of grazing animals per area unit and extending pasture rotation times could reduce soil erosion processes effectively and ongoing land degradation could be prevented according to the 'Directive of the European Parliament and of the Council Establishing a Framework for the Protection of Soil' (Commission of the European Communities, 2006). Additionally, is important to emphasise that extensive grazing is the main focus of landscape management in marginal areas of the Mediterranean region with very low population densities, only small resident communities, little mechanised agriculture and poor communications.

In addition to better pasture management, another possible consideration may be management of native shrub land and recovering oak. Land afforestation should be supported by a set of measures to minimise the impact of site preparation techniques, forest management and fire prevention on soils. Plough use as a tool in preparation of soils for sowing seeds in dry farming systems should be replaced in favour of other less pernicious tillage techniques.

However, in the study area, as well as in the majority of Portuguese rural areas, key problems remain and are complex to solve. They include: a) How to stop the demographic exodus that took place in the middle of the last century and continues nowadays? b) How to supply an appropriate income to attract young farmers to depopulated areas where the great majority of the population are elderly? c) How to improve farm structures that consist of small scattered plots? d) How to manage systems that favour soil conservation and combat land degradation but maintain economic viability? e) How to adjust Mediterranean agriculture to climate change scenarios for the 21st century?.
