- Fire feedback (local, established): The more grassland, the more fire, which in turn opens space for more grassland colonization.
- Grazing feedback (local, well established): If grazing pressure is high, there is not enough fuel left to carry fires of sufficient intensity and frequency to keep the seedlings from escaping above the flame zone.
- Self-organizing patchiness (local, well established): Rietkerk et al. (2004) describe several feedbacks that explain self-organized patchiness in ecosystems as a scale-dependent mechanism (Rietkerk et al. 2004). Established trees may trap and retain nutrients, and create microclimates that further improve the conditions for tree establishment and growth. Higher vegetation density allows lower evaporation and higher water infiltration through shading and root penetration respectively. These conditions allow plant recruitment.
There are several different hypotheses regarding the mechanism by which bush encroachment occurs and how their drivers interact. Different mechanisms (or combinations of mechanisms) may be important in different places.
Shift from grassy to woody savannas
Important shocks (eg droughts, floods) that contribute to the regime shift include:
- Droughts (regional, well established): In savanna ecosystems, droughts play an important role in increasing the likelihood of fires as well as decreasing the water content of the soil. While increasing fire would favor grass over shrubs, dry soils in its superficial layers would be an advantage for shrubs which have deeper rooting than grass.
- Floods (regional, well established): Contrary to droughts, floods are shocks that increase the water content in the soil and potentially suppress fire.
The main external direct drivers that contribute to the shift include:
- Grazing (local, well established): In the sustained presence of high numbers of grazers (typically cattle) accumulation of grass fuel is reduced, leading to period without intense fire for long enough that woody plants can grow beyond the fire-susceptible stage, which in turn suppresses grass production and fires, further enhancing the establishment of woody vegetation (Higgins et al. 2000, Staver et al. 2009).
- Browsing (local, well-established): elimination of browsers (especially very large browsers such as elephant and giraffe, but also the more-numerous small browsers) from the system when cattle are introduced (Dublin et al. 1990) can affect fire regimes and the competitive balance between grass and trees, in a way that favours trees. Similarly alien species, such as Prosopis in South Africa or Acacia nilotica in Australia, both deliberately introduced, can play an important role in bush encroachment by affecting fire regimes (Poynton 1990).
- Water availability (Regional, well established): both shrublands and grasslands are types of drylands characterized as water limited. Slight increases in water availability, both from rainfall or subterrain, favors tree development; while shortages favors development of grass. Grasses are thought to be more shallowly-rooted than trees, so if grass cover is reduced by overgrazing, more water available for trees, which promotes their growth and establishment, further suppressing grass growth (Noy-Meir 1982).
- Atmospheric CO2 (global, speculative): The underlying mechanism is still debated, but several possibilities have been proposed: i) that rising CO2 levels favour C3 (woody plant) photosynthesis relative to C4 (tropical grass) photosynthesis; ii) elevated CO2 may reduce transpiration of grasses, leading to greater water percolation and therefore favoring deeper rooted woody species; iii) faster growth of woody plants due to CO2 enrichment, and therefore faster escape of seedlings from susceptibility to fire; and iv) investments in carbon-based defense compounds such as tannins, which are the main defense compounds in many encroaching trees but not in grasses (Midgley and Bond 2001, Wiegand et al. 2006).
The main external indirect drivers that contribute to the shift:
- Global warming (global, well established): Global warming not only maintains high concentrations of atmospheric CO2; it also increase the likelihood and intensity of droughts in certain areas of the globe.
- Demand for food (global - regional, speculative): As human population grows, food demand increase incentives to increase efficiency of cattle ranching, particularly density. At high densities, cattle can reduce fire frequency opening a window of opportunity for shrubs to grow enough and esscape the effects of fire flames.
- Agriculture (regional, proposed): On the other hand, food demand also increase agriculture incentives to increase productivity. Sometimes it implies the intensification of irrigation use and new water structures that reduce aquifers. When deep soil water content is reduced, grass acquire an advantage over shrubs in its competition for water and nutrients.
Slow internal system changes that contribute to the regime shift include:
- Shrub density (local, well established): Shrub density change slowly but it is the best indicator of the regime shift. It responds to self-organizing patchiness feedback.
- Fire frequency (regional, well established): Fire is influenced by climate, rain variability, grazing, and direct management actions. However, its frequency change slowly as response of the landscape configuration and the memory of past burning events (Peterson 2002).