In the 1930s, South Africa planted pine and eucalyptus across native grasslands to become self-sufficient in timber; 23 years later, streamflow had fallen 55% in one region and 82% in another

South Africa began expanding commercial tree plantations in the 1930s as the government was trying to make the country self-sufficient in timber. Pine, eucalyptus and wattle were planted on land that had been dominated by grasslands and other native vegetation earlier.The policy created a large commercial forestry sector, but the trees also changed how water moved through some of the country's mountain catchments. A U.S. State Department case study on invasive pines reported that, 23 years after planting, streamflow had fallen by 55% in the Western Cape and by 82% in the KwaZulu-Natal Drakensberg.The plantations were meant to solve a timber shortageThe South African government's account of the forestry sector says extensive commercial plantations were established during the 1930s to make the country self-sufficient in timber and provide employment. Private companies subsequently planted large areas with exotic species, especially pine, eucalyptus and wattle.Commercial species such as pine and eucalyptus grow rapidly and remain active throughout much of the year, unlike many native grasslands that are dormant according to the season, as their roots can also reach deeper into the soil. That difference became important in areas where plantations occupied catchments that supplied streams.Pine trees began taking more water from the catchmentsThe State Department case study cited research showing that some pine species develop deep root systems. Pinus radiata, for example, can reach roots about 2.6 meters deep within four years of germination, which allow them to draw water from parts of the soil profile that native grassland vegetation may not use to the same extent. The result was a reduction in the amount of water reaching streams.Streamflow was reported to have fallen 55% 23 years after pine planting in the Western Cape, and in the KwaZulu-Natal Drakensberg, the reported reduction reached 82% over the same period. The same case study said that local streams had dried up completely six years after grasslands were replaced by invading pines in parts of Mpumalanga and the Northern Province.A 2008 study in Water Resources Research examined two long-running paired-catchment experiments in South Africa that involved pine and eucalyptus plantations. The researchers found that planting fast-growing exotic trees produced large reductions in streamflow and that the reductions increased as the trees aged. They also found that water use could decline as plantations matured, and streamflows recovered to some degree later in the rotation.Grasslands and plantations used water differentlySouth Africa's native grasslands have a seasonal cycle, and a lot of the vegetation becomes dormant during the winter. Evergreen plantation trees continue transpiring during that period, and their deeper roots also give them access to water farther below the surface. A South African government forest policy document notes that commercial plantations largely replaced low, seasonally dormant grassland, open woodland or fynbos, and estimated that mature pines and eucalyptus could use the equivalent of roughly 300 to 600 millimeters more rainfall each year than the natural vegetation they replaced, with mean yearly runoff reductions of about 200 to 300 millimeters over a plantation rotation.South Africa eventually introduced controls on afforestation in catchments where reductions in streamflow could affect water supplies. Government records describe plantations as supplying most of the country's demand for wood, while modern forestry operations continue to use pine and eucalyptus for products that range from sawlogs to pulp and paper.The change could be measured not only in hectares of plantation but in the amount of water still reaching the stream, in catchments where native grasslands had been replaced by water-hungry exotic trees. The 55% and 82% reductions documented after 23 years came from two specific South African regions, but they became part of a much larger body of research that shows why forestry in naturally treeless water-producing catchments requires different management from forestry in areas where trees are native.Catch the latest World News and Live updates. Download the TOI app.