Case Study: Pines

Plantation forestry in South Africa contributes R1.8 billion to the GDP and supplies over 100,000 jobs. Downstream industries, which are based on forestry, produce export timber products that are worth an additional R10 billion. Forestry is thus an important part of the South African economy, but forestry practices are causing significant ecological damage. To increase productivity, commercial foresters have imported non-indigenous trees, such as various species of pine. Compared to indigenous tree species, pines are easier to manage, grow faster, and are more readily established in marginal lands. Introduced to South Africa from North America, pines now occupy over 3 million hectares of South African mountain catchments, forest fringes, grasslands, and fynbos.

As pine stands replace grasslands, they negatively impact biodiversity, ecosystem functioning, and catchment hydrology. A stand of invading trees uses approximately the same amount of water as a grassland in the same area during the rainy season, but during the dry season, when grasslands become dormant, the trees continue to transpire. Depending on the species, pines can develop very deep root systems. For example, Pinus radiata roots can reach a depth of 2.6 meters within four years of germination. Root depths such as these allow invading trees to tap much further into underlying water tables than native grasslands can. This depletes water tables and diminishes their ability to replenish during the rainy season, which can effectively dry up area streams. Twenty-three years after planting pines, the Western Cape experienced a 55 percent reduction in streamflow while the KwaZulu-Natal Drakensberg experienced an 82 percent reduction. Six years after replacing grasslands in Mpumalanga and the Northern Province, invading pines had completely dried up the local streams. It is estimated that invasive plants use almost 7% of South Africa’s run-off, greatly reducing the ability of existing water supply schemes to deliver sufficient water to meet human needs.

The pine problem is not limited to South Africa, however. Pines have invaded and established self-perpetuating populations in Australia, Madagascar, Malawi, New Zealand, and Uruguay, often causing the same negative impacts on local biodiversity and hydrology. Effective control of pines requires physical removal. The most successful of these removal projects is South Africa’s Working for Water program. After clearing pines 30 meters on either side of a stream (just 10% of the catchment) in the Western Cape, streamflow was increased by 44 percent, bringing in 11,000 cubic meters of water for each cleared hectare. Successful recovery such as this provides hope, but invasion continues at a much faster pace than removal.

-- Richardson, D. M., "Invasive Alien Trees: the Price of Forestry," IUCN World Conservation, April 1997, p. 14-15

-- CSIR Division of Water, Environment and Forestry Technology, The Environmental Impacts of Invading Alien Plants in South Africa (Pretoria, SA: Dept. of Water Affairs and Forestry). p. 4-11