Desertification

Le Houerou (1996) - Direct causes of desertification:

• Reduction of organic matter - triggered by human and physical action can cause soil deg
• Reduction in soil stability 
• Problems arise from unstable and poorly developed structure - top soil compaction, causing reduced soil porosity and permeability
• Lowered soil water levels
• Soil surface crusting (from salinisation) and sealing (from breakdown of microaggregates)
• Lower soil biological activity - less fungi and worms = less soil mixing
• Lower fertility and productivity

Such factors are initially driven by a change from natural to managed vegetation and can therefore result in:

• Greater run off causing increasing in flood events and salinity
• Reduced shade increasing surface temps and evaporation - drylands are already net evaporative environs, this intensifies the problem
• Increased surface wind speeds causing more surface erosion

Focus on the Sahel belt (Middleton and Thomas, 1997) - degradation of vegetation and soils - change from natural to managed veg:

• On marginal lands, where you are at the margins of rain fed cultivation, degradation of veg leads to degradation of soils 
• Soils become thin and sandy with low mineral content that can only support a limited biomass
• Rainfall is sporadic in space and time so high unreliability of rainfall at a particular site - makes nomadic way of life most efficient and most in equilibrium with the natural environment
• Overall reduction in rainfall since 60s - seen across Africa (Smithson et al 2002)

Occurs mainly in sub-humid, semi-arid and arid regions as climate is most prone to drought due to position of the Hadley cell (descending dry air blocking out the rain baring trade winds) (Middleton and Thomas, 1994)

Safriel (2009) - Climate change is having adverse affects on the environment -

• Reduction in precip is seen with more seasonality and irregularity  
• Increased evapo-transpiration as temps increase
• Causes a reduction in soil moisture therefore less veg can grow leading to a reduction in veg cover
• Causes soil erosion and leads to oxidisation of soil carbon increasing carbon dioxide emissions and contributing to greenhouse gases affecting climate
• Trends show a warming climate from 1891-1990 by 86oC (Le Houerou, 1996)

Grainger (1990) - Last glaciation - area of sub tropical lands was pushed towards the equator - caused changes from savannah to desert landscapes

• Created poor soils that had poor water holding capacity, poor structure and therefore were susceptible to water and wind erosion
• Type of erosion may reactivate fossil sand dunes - as done in the 1960s -80s making the area more vulnerable 

Increase in grazing causes a change from perennial to annual grasses:

• Perennial are the most fed on - provide food all year round and as a consequence many are overgrazed leaving the landscape covered in thorny scrubs and bushes - more veg cover leaving soil susceptible to erosion
• Annual grasses only come with the rainy season - increased variability and seasonality of rains make these an unreliable food source
• Results in a NUTRITIONAL DEFICIT 

Overgrazing changes the ecology balance - reduced density of overall biomass with changes to less supportive veg (perennial to thorny scrubs)

Fire - can be natural and man made:

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