Forest to Savannas
Forest to savannas is a regime shift typical of tropical areas where forests, an ecosystem dominated by trees changes to a savanna dominated by a mixture of grasslands and shrublands. Several feedbacks play an important role in this regime shift including albedo effects, evapotranspiration and cloud formation, fragmentation and fire-prone area expansion, change in ocean circulation and self organizing vegetation patterns. However, these feedbacks are not always strong enough to produce alternative regimes. In some areas shifts are expected to occur under stochastic events like ENSO droughts or unlikely events like Earth orbit change.
Key direct drivers
- Vegetation conversion and habitat fragmentation
- Harvest and resource consumption
- Infrastructure development
- Environmental shocks (eg floods)
- Global climate change
- Small-scale subsistence crop cultivation
- Extensive livestock production (rangelands)
- Timber production
- Tropical forests
Key Ecosystem Processes
- Soil formation
- Primary production
- Nutrient cycling
- Water cycling
- Food crops
- Wild animal and plant foods
- Climate regulation
- Water regulation
- Regulation of soil erosion
- Natural hazard regulation
- Aesthetic values
- Knowledge and educational values
- Spiritual and religious
- Food and nutrition
- Health (eg toxins, disease)
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
- Social conflict
Typical spatial scale
Typical time scale
- Contemporary observations
Confidence: Existence of RS
- Well established – Wide agreement in the literature that the RS exists
Confidence: Mechanism underlying RS
- Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms
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).
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).
Drivers and causes of the regime shift
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.
How the regime shift works
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)
Impacts on ecosystem services and human well-being
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.
Bonan, G. 2008. Forests and climate change: Forcings, feedbacks, and the climate benefits of forests. Science 320:1444-1449.
Da Silva, R., D. Werth, and R. Avissar. 2008. Regional impacts of future land-cover changes on the amazon basin wet-season climate. J Climate 21:1153-1170.
Dekker, S. C., H. J. de Boer, V. Brovkin, K. Fraedrich, M. J. Wassen, and M. Rietkerk. 2010. Biogeophysical feedbacks trigger shifts in the modelled vegetation-atmosphere system at multiple scales. BIOGEOSCIENCES 7:1237-1245.
Dekker, S. C., M. Rietkerk, and M. F. P. Bierkens. 2007. Coupling microscale vegetation-soil water and macroscale vegetation-precipitation feedbacks in semiarid ecosystems. Global Change Biol 13:671-678.
Falkenmark, M. and J. Rockström. 2008. Building resilience to drought in desertification-prone savannas in Sub-Saharan Africa: The water perspective. Nat. Resour. Forum 32:93-102.
Foley, J., R. DeFries, G. Asner, C. Barford, G. Bonan, S. Carpenter, F. Chapin, M. Coe, G. Daily, and H. Gibbs. 2005. Global consequences of land use. Science 309:570-574.
Geist, H. and E. Lambin. 2002. Proximate causes and underlying driving forces of tropical deforestation. BioScience 52:143-150.
Hutyra, L., J. Munger, C. Nobre, S. Saleska, S. Vieira, and S. Wofsy. 2005. Climatic variability and vegetation vulnerability in Amazonia. Geophys Res Lett 32:L24712.
Laurance, W. and G. Williamson. 2001. Positive feedbacks among forest fragmentation, drought, and climate change in the Amazon. Conservation biology 15:1529-1535.
Los, S. O., G. P. Weedon, P. R. J. North, J. D. Kaduk, C. M. Taylor, and P. M. Cox. 2006. An observation-based estimate of the strength of rainfall-vegetation interactions in the Sahel. Geophys Res Lett 33:L16402.
Millennium Ecosystem Assessment. 2005. Ecosystems and human well-being: synthesis?.137.
Nobre, P., M. Malagutti, D. F. Urbano, R. A. F. De Almeida, and E. Giarolla. 2009. Amazon Deforestation and Climate Change in a Coupled Model Simulation. J Climate 22:5686.
Oyama, M. and C. Nobre. 2003. A new climate-vegetation equilibrium state for tropical South America. Geophys Res Lett 30:2199.
Oyama, M. and C. Nobre. 2004. Climatic consequences of a large-scale desertification in northeast Brazil: A GCM simulation study. J Climate 17:3203-3213.
Pinto, E., Y. Shin, S. A. Cowling, and C. D. Jones. 2009. Past, present and future vegetation-cloud feedbacks in the Amazon Basin. Clim Dynam 32:741-751.
Reynolds, J., D. Smith, E. Lambin, T. Ii, B L, M. Mortimore, S. Batterbury, T. Downing, H. Dowlatabadi, R. Fernandez, J. Herrick, E. Huber-Sannwald, H. Jiang, R. Leemans, T. Lynam, F. Maestre, M. Ayarza, and B. Walker. 2007. Global Desertification: Building a Science for Dryland Development. Science 316:847.
Rietkerk, M., S. Dekker, P. de Ruiter, and J. van de Koppel. 2004. Self-organized patchiness and catastrophic shifts in ecosystems. Science 305:1926-1929.
Saleska, S., K. Didan, A. Huete, and H. da Rocha. 2007. Amazon forests green-up during 2005 drought. Science 318:612-612.
Scheffer, M. 2009. Critical transitions in nature and society.
Sternberg, L. 2001. Savanna-forest hysteresis in the tropics. Global Ecology and Biogeography:369-378.