Improved process descriptions in the PESERA model

Table of Contents

1. Authors
2. Abstract
3. Introduction
4. General extensions to PESERA
   1. Grazing
   2. Fire
   3. Wind Erosion
5. Particular extensions to PESERA
   1. Mulching and/or maintaining ground cover vegetation within tree crops
   2. Retention of crop residues as litter layer at harvesting of arable and other crops
   3. Irrigation and water harvesting for croplands
   4. Invasion and clearance of unpalatable species
   5. Terracing and strip cropping
   6. Nitrogen budgeting and rotations
6. Other extensions to PESERA
   1. Mass movements
   2. Data collection calibration of PESERA against reservoir data
7. Conclusions
8. References

Invasion and clearance of unpalatable species

There is clear evidence of invasion of grazing lands by unpalatable species in southern Africa, significantly reducing carrying capacities while apparently maintaining a relatively high biomass. In the American south-west there has been a historical replacement of grass in semi-arid rangelands by unpalatable shrubs, associated with a decrease in vegetative cover (though less decrease in biomass) and increased erosion. In both cases overgrazing of fragile ecosystems has been a possible cause, although there is also some debate about the role of subtle climate changes. For a given average fraction of biomass α that is consumed, we here provisionally partition the calculated biomass between a proportion, p_U of unpalatable species and a proportion (1-p_U) of palatable species. The unpalatable proportion is then estimated as p_U= α/α₀ for a parameter (to be determined) α₀ (necessarily >1) , and the palatable portion is then consumed at the increased rate α/(1-α/α₀). This expression is valid for values of α <α₀/(1+α₀). This change reflects immediately on the carrying capacity of the land, although clearly an increase in unpalatable species tends, other things being equal, to provide some protection from erosion by both eater and wind, by increasing the land cover.

This procedure allows the proportion of unpalatable shrubs to be estimated, but the process is not normally reversible, and unpalatable shrubs generally need to be removed by hand or machinery, sometimes repeatedly over a number of years.