Impacts on Biomass
Biomass is defined as the mass of living organisms in an ecosystem. It is obvious that clearing forests causes significant decreases in biomass. The real question is what the differences are between forest biomass and different land uses after burning.
Fearnside (1995) estimated that degraded grasslands take 13-14 years to become secondary forests from a biomass perspective. Fearnside estimated that the average biomass of secondary forests is about 28.5 t/ha while the biomass of primary forests is about 463.5 t/ha. He notes that his estimates of regenerated biomass are higher than those of other studies in the 1990’s, because they take into account the fact that not all land becomes degraded pasture. However, they still show that the replacement biomass of secondary forests is only about 6% of that of primary forests. Secondary forests are estimated to take at least another 100 years to reach primary forest status from a biomass perspective (Fearnside, 1995).
In a recent study by Loarie, Asner, and Field (2009), the authors discovered that deforestation is slowly encroaching on more fertile, biomass-rich parts of the Amazon. Most deforestation occurred around the southeastern edge of the Amazon. As it proceeds to the denser northwest, more biomass will be lost per hectare of land that is cleared (Loarie et. al., 2009).
Impacts on Species Diversity
One of the impacts of this decrease in overall biomass is a decrease in species diversity in the cleared forests. Various studies of differing regions suggest this decrease in species richness. Repeated slash and burn agriculture led to a 29% decrease in woody species in a wooden deciduous forest studied by Miller and Kauffman (1997). Mamede and Araujo (2008) noted an 80% decrease in seed density and a 44% decrease in seed species richness in a caatinga* region in Northern Brazil. Rossi et. al. (2009) found that conversion of forest to pasture leads to a decrease in the diversity and activity of soil invertebrates that are responsible for incorporating organic matter into soil and building pores through which plants obtain nutrients. A significant loss of soil invertebrates would make recovery of forests or maintenance of pasture unsustainable (Rossi et. al., 2009; Hauser & Norgrove, 2001). It is possible for many plant species to return with sufficient fallows and secondary forest regeneration. However, it is also important to note that even a 10% decrease in the number of species in a given ecosystem is considered significant (Fujisaka et. al., 1998).
*Caatinga is a type of vegetation that is thorny and shrubby. The word also refers to ecological regions that consist of caatinga vegetation.
Impacts on Local Weather
The decreases in biomass that result from deforestation also affect forest albedo. Tinker, Ingram, and Struwe (1996) found that deforestation leads to an increase from the forest albedo of .11-.13 to a grassland albedo of up to .2. This decrease in biomass combined with an increase in albedo leads to less evapotranspiration. Evapotranspiration refers to the sum of water vapor from evaporation and plant transpiration. Plant transpiration is the release of water from plant stomata. With fewer plants, the rate of transpiration inevitably goes down. Forest biomass also functions to intercept rain in its descent, allowing the water to evaporate quickly before sinking into the ground. With less forest to intercept rain and less radiation being absorbed due to a higher albedo, evaporation occurs less frequently. In areas like the Amazon that depend on evaporated water for rainfall, the authors estimated a 30% decrease in rainfall. Areas that depended more on the ocean for rainfall like Thailand were only estimated to have a 10% decrease. Since the rainfall in places like Thailand is estimated to be relatively constant, the infiltration rate of the replacement vegetation needs to match that of the forest in order to avoid floods and excess run-off of rainwater. The authors do make clear that these local weather problems are more caused by how the land is used after deforestation than deforestation itself. A cattle pasture will have different features from a secondary forest. They also point out that more studies are needed to draw any conclusions about the different climate impacts of these clearings (Tinker et. al., 1996).
