For various reasons much of the South African veld is largely treeless. Take a patch of bald Albany suurveld
For various reasons much of the South African veld is largely treeless. Take a patch of bald Albany suurveld
and find its equivalent climate on Australia’s New South Wales coastal escarpment, and what do we find? Trees, trees and more trees, mainly wattles and gums. This means that when these Australian plants were introduced to the Albany region by the colonial authorities in the mid 1800s, and farmers were encouraged to plant them, they began invading the veld. Today invasive alien trees – species of wattle, pine, mesquite, hakea and many more – cover millions of hectares of South Africa. Initially, these plants spread quite slowly and no one seemed to bother. Indeed, many farmers saw them as a good thing; they supplied shelter from heat and cold for livestock, and plenty of timber.
However, since the ’40s, the conservation community has been concerned. They observed the negative effects of pines and wattles on the extremely rich flora of the Cape fynbos, and started lobbying the authorities to initiate clearing operations.
The authorities – specifically the Department of Agriculture – were not especially interested: they were still handing out bags of black wattle seeds to farmers as late at the ’80s.
In the ’40s, Prof CL Wicht of the University of Stellenbosch’s Forestry Faculty initiated experiments in the nearby Jonkershoek Valley to investigate the impact of afforestation with pines.
He arranged for the establishment of weirs on several streams and the afforestation of catchments of half the streams while leaving the other catchments clothed with indigenous fynbos vegetation.
Then streamflow was monitored over several decades. What he found with hindsight was not unexpected: afforested catchments used, on average, far more water than the fynbos-covered areas.
This means that the growing of trees incurs a cost in terms of water delivery. However, the country needed timber products (as it still does).
In order to manage the tradeoff between timber needs and water delivery, the Department of Forestry instituted a permit system for afforestation that restricted the extent to which any catchment could be planted.
But what about the invasive trees that establish outside of plantations? They too have an impact on water delivery, a service provided by nature’s pristine ecosystems.
Alien invasive trees are estimated to use 6.7% of the estimated mean annual runoff of water over all of South Africa and up to 16% in the Western Cape where large areas of fynbos are invaded.
Riverside aliens – species such as black wattle, eucalyptus and poplar use approximately 9% of the water in our rivers, or about as much as the total annual flow in the Vaal River system.
Supporting these scientific studies are the anecdotes of numerous landowners recounting how the removal of alien plants has increased streamflow and revitalised dormant springs.
In a waterscarce country such as ours, we cannot afford to tolerate this unnecessary drain on our most precious of resources.
Given that alien invasives spread at a rate of about 5% per year, the medium-term prospects for water resources are alarming.
Alien trees also impact on other services provided by nature. For one, they change the characteristics of wildfires.
Both grassland and fynbos (the predominant veld types in the Albany suurveld) support relatively low intensity fires compared with a dense stand of pine and long-leafed wattle.
Very intense fires are difficult to control. Moreover, the burnt stumps of alien trees can smoulder for weeks, thus providing a source of ignition for new wildfires whenever the weather conditions are right.
Finally, intense fires supported by aliens can scald the soil to the extent that it becomes biologically inert and incapable of supporting a dense cover of vegetation.
The result: erosion. Alien trees occupy space that formerly supported grazing for livestock and game, thereby reducing the value of a property.
Indeed, aliens suppress most of the indigenous fauna and flora and can render rare species totally extinct. A hill slope covered in a tatty mess of wattle and pine is pretty much good-for-nothing.
This is why the government-initiated Albany Working for Water Programme (implemented by the Gamtoos Irrigation Board) is committed to eradicating alien plants.
For it to be successful, this programme requires the co-operation of all landowners in alien-affected areas.
We need to ensure that nature’s services are again delivered efficiently and effectively by eliminating alien trees from the Albany suurveld.
