The tremendous growth of oil palm production is a major cause of environmental concern and has become a matter of public debate since the 1990s.
Deforestation and biodiversity losses
In the past decades, millions of hectares of tropical rainforest in South-East Asia, which provides the highest biodiversity of any terrestrial ecosystem, have been converted to oil palm production. In 2010, Malaysia and Indonesia grew oil palm on 4 and 5 million ha respectively, a two-fold and seven-fold increase since 1990 (FAO, 2012). This high rate of deforestation has resulted in the loss of habitat for a number of key species, such as the orang-utan (Wakker, 2005; Wakker, 2000).
Land clearing practices
Land clearing for oil palm cultivation used to be done by burning felled biomass, which resulted in large emissions of greenhouse gases. A catastrophic forest fire in Indonesia in 1997, which was partly blamed on plantation owners, caused massive air pollution in South-East Asia. This led to a ban on burning in Asian countries with the adoption of zero-burning, which has resulted in shredding palm trunks and stacking them in the interrows (Mohd Noor, 2003; Mohd Hashim et al., 1993). Zero-burning allows recycling of large quantities of nutrients through the two-year decomposition process, improvements in the physical properties of the soil and reductions of 20-30% in the requirement for inorganic fertilizers during the first 4 years of growth (Prabhakaran Nair, 2010). An objection to zero-burning management is that unburned biomass promotes the breeding of diseases and pests such as the rhinoceros beetle (Oryctes rhinoceros). Integrated pest management systems involving a mix of cultural, physical, chemical and biological control have been developed in order to decrease the use of chemicals (Mohd Noor, 2003; Teoh Cheng Hai, 2002; Kamarudin et al., 1997; Chopra et al., 2005).
The soil is particularly susceptible to erosion during the clearance period, which may turn into an acute problem in upland regions (Wakker, 2005). In some cases, a leguminous cover crop is established to limit erosion, but this requires the preventive use of weed killers (Mohd Noor, 2003).
Approximately 4% of palm tree plantations in Indonesia and Malaysia are on peat soils, which store large quantities of carbon. When these soils are drained for palm plantation, the increased aeration results in oxidation and in the release of carbon dioxide. Estimations for carbon emission in tropical peat soils vary between 10 and 20 t/ha/year (Schmidt, 2007).
Pesticides
Large scale oil palm cultivation may require up to 25 different pesticides, including Paraquat, a common herbicide. The use of Paraquat in palm oil production is controversial as this product is toxic to humans and could be dangerous for plantation workers (it has been prohibited in the European Union since 2007) (Wakker, 2005).
Waste management
Palm oil processing yields large quantities of solid and liquid wastes. The more fibrous wastes are generally used for energy production within the mill plant or may be valuable as mulch or as mushroom substrate (Chavalparit et al., 2006). Palm kernel meal is recycled in animal feeding. The most environmentally problematic palm oil waste is palm oil mill effluent: it was once estimated that a processing plant with a capacity of 10 tons fresh fruit per hour would require a water treatment plant comparable to that required by a population of half a million inhabitants (Brezing, 1986). Most mills have outdoor basins in which the effluent is stored and partly detoxified (by adding oxygen) but these basins easily overflow during heavy rain or intensive production. Some companies allow the liquid to flow directly into the rivers. Because of its high Biological Oxygen Demand (BOD), palm oil mill effluent is highly polluting to waterways and has significant negative effects on aquatic life downriver (Wakker, 2005).
Positive impacts
Oil palm is sometimes used for rehabilitating degraded areas. In Sumatra, it has been established successfully on abandoned farmlands taken over by Imperata cylindrica. Its shading effect is lighter than that of other plantation crops such as rubber or cocoa and it is suitable for undergrowth that can be grazed by livestock (Orwa et al., 2009). Cattle in oil palm plantations participate in weed control: in Malaysia, reduction in weeding costs varied from 32 to 73%/ha/yr (Sharun et al., 2003). Raising livestock in oil palm plantations is a subsidiary source of income (Orwa et al., 2009).