Regime Shifts DataBase

Large persistent changes in ecosystem services

Forest regime
Forests are ecosystems typically dominated by trees, perennial plants taller than 5 meters. Tropical forest includes moist and dry forests (MA, 2005). A mature tropical forest contain at least four layers: emergent layer up to 45 - 80 meters tall, the canopy among 35 - 45 meters tall, the understory layer and the floor layer. Such structure gives a variety of habitats that host roughly half of the known plants and animals biodiversity (MA, 2005).


Savanna regime
Savannas, on the other hand, are drylands dominated by a mixture of grasslands and shrublands. The canopy in savannas never closes, and the floor layer is dominated by grasses, especially C4 species. Savannas, dry forest and shrublands conform 40% of the world’s land area and host up to 42% of human population (Reynolds et al. 2007, Falkenmark and Rockström 2008). About 25% of drylands, including savannas, are covered by croplands and sustain 50% of world’s livestock (MA, 2005).

Shift from forests to savannas

The most widely recognized driver is deforestation and consequently fragmentation of forest landscape, which reduces rainfall and increases surface temperature (Da Silva et al. 2008, Nobre et al. 2009). Reduction of forest cover accelerates albedo effect, loss of evapotranspiration and roughness length (Sternberg 2001), activates fire feedback, changes ocean circulation and warms up sea surface in the Amazon case, and ultimately changes the spatial vegetation structure.

Deforestation and forest degradation is driven by a complex, case specific interaction of social and economic drivers. The most important reported drivers include agriculture expansion, infrastructure development, the logging industry and fast population growth (Geist and Lambin 2002, MA, 2005). For example, in the Amazon, illegal logging is a critical threat that besides its damage to the forest, bring with them secondary effects like expansion of hunting areas, slash-and-burn farms, mining, the establishment of new road networks and therefore more logging facilities. The MA (2005) reports that 70 countries have problems with illegal logging leading to national income losses of $5 billions and total economic losses of about $10 billion. By 2001, Laurance and Williamson (2001) reported that 80% of brazilian logging activity were illegal; however, government counterintuitively sponsored colonization through cattle ranching projects.

Climate change and global warming are expected to enhance the regime shift; and the loss of forest areas are expected to exacerbate climate change (Laurance and Williamson 2001, Bonan 2008). While Laurance and Williamson (2001) report that forests like Amazon apparently change from carbon sinks to carbon sources during ENSO events; Nobre et al. (2009) suggest that deforestation of Amazon may actually increase ENSO variability; and Bonan (2008) confirms that deforestation would enhance global warming by decrease of evaporative cooling and release of carbon dioxide.

Shift from forest to savanna

The expansion of agriculture, increase in population as well as deforestation reduces rainfall and increases surface air temperature. This has a direct affect on plants, reducing evapotranspiration and photosynthesis and decreasing the supply of water vapour. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires, which plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintains savanna state.

When tropical forests are replaced by less vegetated cover like savannas, or ultimately by sand in deserts, net radiation at the top of the atmosphere decreases inducing subsidence that inhibits precipitation (Oyama and Nobre 2004). While in the tropics land clearing affects the water balance and as consequence warms up the climate, in boreal forest such clearing affects mainly albedo and as results cools down climate (Foley et al. 2005). The albedo feedback is strengthened by changes in land cover, typically induced by deforestation for logging or agriculture activities. Warmer temperature and drier atmospheres, such as in savanna regime, result in an increase in lifting condensation level feedback that reduces the opportunity of cloud formation and therefore the likelihood of rainfalls (Pinto et al. 2009). Plants that do not have enough water responds by reducing transpiration and photosynthesis, interrupting the supply of water vapor that contribute to the recycled component of precipitation (Oyama and Nobre 2004, Saleska et al. 2007). Less evapotranspiration blocks the inland propagation of cold fronts responsible for precipitation, increasing the dry season length (Oyama and Nobre 2003, Pinto et al. 2009).

Savanna vegetation is better adapted to dry environments. Grasses usually have C4 photosynthesis type, a chemical pathway that reduces water consumption and helps to cope with nitrogen or CO2 limitations. Evapotranspiration depends on soil moisture and biomass. Thus, for instance, droughts frequency or grazing reduce biomass, weakening in turn the feedback effect (Dekker et al. 2007, Dekker et al. 2010). In addition, the spatial distribution of rainfall is affected by both the land-cover type and topography. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires. In such zones fire can be produced after few days of dry conditions. On the regional scale, fire smoke may reduce rainfall by trapping moisture and inhibiting raindrops formation (Laurance and Williamson 2001). Fire plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintain savanna state (Laurance and Williamson 2001, Hutyra et al. 2005)

Forest provides a wide range of ecosystem services. Besides being hot spots of biodiversity, forest provides soil and water protection, it prevents soil erosion, floods and landslides. For example, soil erosion may be 10-20 times higher on areas cleared of forest (MA, 2005). Depending on soil conditions, at the local scale forest can also regulate the hydrological cycles by increasing precipitation and decreasing evaporation. They regulate below grown runoff and smoothing seasonal extreme events: heavy rainfalls or dry spells. Due to its regulating function in the water cycle, forested watersheds provides water supply to one third of the world’s largest cities (MA, 2005). 

Forest sustain about 200 million people belonging to indigenous groups, who depend on forest not only as source of resources (food, fiber, fuel) but also their culture and religious traditions (MA, 2005). Forest also maintain the agroforestry industry which, including temperate forests, produces 3.3 billion cubic meters of wood (MA, 2005). 

Managerial options for the forest to savanna regime shifts requires targeting the main drivers: deforestation and landscape fragmentation. Controlling illegal logging and implementing sustainable logging plans are part of the strategy. Sustainable logging needs to take into consideration reducing fragmentation and allowing deforested patches to regrow. Likewise, the expansion of agricultural frontier and grazing areas needs to be controlled and when unavoidable, it needs to be planned in order to prevent fragmentation. The fire frequency feedback accelerates the shift from forest to savanna regime. Laurance and Williamson (2001) suggest fundamental changes in prevailing land-use practices and development policies to avoid wildfires. Such changes include the management of logging and grazing areas in order to reduce fragmentation and therefore it would reduce the fire risk. Hence, fire and fragmentation management need to be coupled strategies.