South Africa, in the mid-1800s, planted millions of Australian acacias as a dune fix; 180 years later, that decision is costing the country its rivers and ecosystem

Sometimes, a fix to a problem quietly becomes a much bigger problem decades later. This is exactly what happened along South Africa’s coastline, where a tree brought in to solve a sand problem is now blamed for stealing water from entire river systems.

A 19th-century fix for a very real problem
According to a study, ‘A review of coastal dune stabilization in the Cape Province of South Africa,’ published in Landscape and Urban Planning, organized dune stabilization in the western Cape began in 1845, when Australian Acacia species were introduced because they were considered the most effective plants available for holding down shifting coastal sand. Colonial authorities attempted to anchor the drifting dunes that were consuming farmland and other infrastructure in the area with fast-growing, hardy wattles from Australia. The plantings continued for the rest of the 19th century. For the narrow purpose for which they were planted, they worked: the dunes stopped moving.

The paper notes that the first phase relied on direct seeding of Port Jackson Wattle onto bare sand, but by 1875, managers were spreading city refuse over the dunes first to create a temporary crust before sowing. It adds that a French foredune method introduced in 1896 used wooden poles or Marram Grass barriers, followed by brushwood packing and grass seeding, and says successful stabilization typically took at least five years.


Fast forward to today: the same trees are draining rivers
The same acacia species, along with pines and eucalypts introduced for similar practical reasons, have spread far beyond their original planting sites and are now major water users in South African river catchments, according to a 2002 study, titled ‘Invasive alien trees and water resources in South Africa: case studies of the costs and benefits of management,’ published in the journal Forest Ecology and Management. Acacia species alone account for some 37 percent of all water lost to invasive trees in the Upper Wilge catchment. Black wattle, or Acacia mearnsii, accounts for about 20 percent of the water lost to invaders in the Sonderend catchment and about 21 percent in the Keurbooms catchment.

In the study’s four catchments, Sonderend, Keurbooms, Upper Wilge and Sabie-Sand, the authors found that invasions had already occupied 44%, 54%, 2% and 23% of the areas to some degree, with the corresponding flow reductions estimated at 7.2%, 22.1%, 6.0% and 9.4%. They also warn that if control is delayed until areas are fully invaded, clearing costs would rise sharply, to as much as US$278 million in the Upper Wilge catchment.

Already, at the time of the research, all four catchments studied had lost natural river flow to invading trees, with reductions ranging from about 6 percent in the Upper Wilge catchment to about 22 percent in the Keurbooms catchment. In the absence of management, the researchers projected that more than 40 percent of the flow could be lost in the Sonderend catchment and more than 95 percent in the Keurbooms catchment.

It is not just about water anymore
Acacias do not just take up more water than the native vegetation they replace. According to a study published in the journal Restoration Ecology by Yelenik and team, one of the wattle species introduced to South Africa, Acacia saligna, is a nitrogen-fixing plant, which means it changes the chemistry of the soil it grows in. Even when the acacia is physically removed, changed soil chemistry can favor weedy grasses rather than the native fynbos plants that should reestablish themselves. This is important because fynbos is not a normal shrubland. The same 2002 study noted that the fynbos flora includes some 8,754 plant species, more than 68 percent of which are found nowhere else on earth. That habitat lost to a dense monoculture of acacia is not a minor side effect.

The researchers tested how clear-cutting altered soil microclimate and nitrogen cycling, then tracked how those changes affected a weedy grass species. They report that the invaded sites had elevated soil N and that the grass grew better on soils from cleared acacia stands than on soils from uninvaded fynbos, underscoring how the invasion can lock ecosystems into a more grass-dominated state.

The price of reversing a 180-year-old decision
Clearing those trees back out will not be a quick or cheap process. The 2002 study estimated that the cost of clearing all four catchments to the level of invasion that existed at that time would be somewhere between 4 million and 13 million US dollars. Acacia clearing alone accounted for approximately 36% of the total clearing budget in the Keurbooms catchment and approximately 40% in the Upper Wilge catchment, underlining how widespread the invasion had become. The longer you wait, the higher the bill will be. In the Upper Wilge catchment, delaying control work for 35 years would have increased total clearing costs by more than 25 times.

The takeaway
None of this makes the initial decision to plant acacias irrational. 19th-century authorities were facing a real and urgent problem with the best tools they had at the time. But the story is a reminder that ecological choices are rarely contained to the reason for which they were made. A tree planted to keep sand from burying farmland can, more than a century and a half later, quietly drain the rivers on which whole towns depend and push out plants found nowhere else on the planet.

South Africa has been trying to reverse the consequences for decades through its Working for Water program, which combines the physical labor of clearing invasive trees with job creation in rural communities. It's slow, unglamorous work. Yet each hectare cleared is measured in liters of water returned to a river, and in indigenous fynbos allowed to grow again where it once stood.