Major degradation issues
In this region, according to our field experiments, the traditional land management systems - typically involving a balanced combination of agriculture, animal husbandry and forestry under a limited human pressure - produce low amounts of overland flow and the lowest soil erosion rates. When the natural vegetation disappears, overland flow and erosion are significantly enhanced.
Due to the increasing pressure this region is undergoing major land use change. The trend is to replace natural forest with exotic species (eucalyptus); the intensification of traditional land uses with corresponding reduction of fallow periods; overgrazing in natural areas, despite the weak production of the vegetation and the poor quality of soils and their high erodibility.
The reduction of organic matter and vegetation cover results in soil compaction and higher overland flow generation in the overgrazed areas, whilst in the ploughed areas erosion rates are higher.
A fundamental role in determining soil erosion hazard is played by land use change, through the changes in the soil and water processes. On the other hand, the soil biomass management (absence of manure supply and weak cover of the soils by vegetation) contributes to the low soil carbon content. Being a surface process, erosion displaces significant amounts of organic matter, which will decrease the carbon retention by the soils, inducing important soil degradation with severe implications on soil fertility and increasing the fragility of the surface.
The increasing pressure over this marginal area (due to the use of the better soils for commercial crops) leads to the use of steep slopes under poor agricultural management. This represents the highest risk for soil sustainability.
The process of land degradation is linked to the socio-economic evolution and to the resources management. Grazing and Forests exploitation are still important, mixed with the extension of the cultivated areas. Overgrazing, over-exploitation of wood and the expansion of agriculture on unfavourable lands, lead to extreme soil erosion and a dangerous siltation of the dams' reservoirs.
The concentration of the population and flocks on vulnerable slopes explains the retreat of the vegetation cover, the rapid soil degradation and spreading of rills and gullies
The context of rural transformations include the complexity of landownership (number of regulations, predominance of smallholdership, scattering of fields due to inheritance), and the urbanisation of high-quality agricultural land. In addition, the traditional small-holder agriculture is marginalised in favour of the modern, large-scale agriculture, whereas the latter is mostly responsible for the inefficient use of groundwater.
The deforestation and the extension of the cultivated area date from the early years of the 20th century and the limits of most of the forests were fixed in the years 1920-1930. But inside the forests and in the remnant pastures, internal degradation by overgrazing and wood gathering still continues. In the cultivated area the reduction of the surface and of the period in fallow, the mechanisation and ploughing in the direction of the slope cause an ongoing land degradation due to the thinning of soils, crust formation and compaction.
All these phenomena decrease the infiltration of rainwater that otherwise may be stored and percolated to the water table. At the same time, the increasing runoff over the land surface causes soil erosion and the silting and pollution of reservoirs, which play an important role in both water storage and the regulation of water provision in the dry season.
We can interpret the environmental evolution of the Sehoul plateau as follows:
- The surface covered by a dense forest represents a heritage where the soil is in equilibrium, and is characterized by a thin organic layer on top of a leached sandy horizon. This layer is permeable enough and well structured to absorb the total amount of rain, even during exceptional events.
- The population density increased during the 20th century, through natural growth, but also through immigration to this rural area close to the cities of Rabat and Salé. This has led to the forest degradation. Several processes occurred simultaneously: overgrazing, with an over-use of the forest to feed the flocks during the dry periods, and cutting of wood for fire to use with rural population and charcoal production to the cities.
- At the southern fringes of the Mamora forest, suburban agriculture expanded (production of grains, vegetables, fruits, breeding .....) to meet the needs of an increasing population in neighbouring cities.
Inside the degraded forest, runoff leads to erosion of the upper organic horizon. The leached layer is exposed and can be removed easily by overland flow during heavy rain or by wind after a dry period, as happened in spring 1995. In some places, small dunes are formed.
[Note: Actually, this process represent the removal of an old aeolian deposit which dates from the last glacial peak ; Dryness of this period explained the building of real dunes, made of the thick leached sands developed by pedogenesis during former wet periods. By Thermoluminescence, these dunes have been dated (26 to 20 Ka BP). This heritage of the late Pleistocence is evolving again, in the same way, but as a consequence of human impact on the forest.]
The current wind erosion is not rapidly stabilized, even after a wet season. Growth of grasses does not have sufficient density on the surfaces, which already lost their organic superficial horizon. It is why inside the forest we have some bare perimeters, regularly affected by wind erosion in case of strong winds.
