Tue, 13 Apr 2021

Mapping alien trees to protect our forests

03 Mar 2021, 23:11 GMT+10

The theme of this year's World Wildlife Day taking place on 3 March is "Forests and Livelihoods: Sustaining People and Planet".

In South Africa, indigenous forests are recognised as one of a few ecosystems that provide disproportionate benefits to society relative to their size. These benefits include food production, medicines, recreation, water purification and carbon storage.

South Africa's temperate forests, called Afrotemperate Forests, have some of the highest number of species in the world. There are many different types of forests across the country, with our national vegetation map recognising 12 forest groups, which are further subdivided into 26 types.

However, indigenous forest covers less than 0.56% of the country's area, and ranges from tiny patches (1 ha), with few greater than 1 km², with larger forests only occurring on the Garden Route and Lowveld Escarpment.

There is a misconception in South Africa that forests are one of our most threatened ecosystems. Many endeavour to "plant trees to save the earth". These often misguided initiatives can be extremely problematic. What has been called "the long shadow of colonial forestry" is a real threat to some of our other South African ecosystems, like savannas, shrublands and grasslands.

However, this doesn't mean that our indigenous forests do not face significant threats. In the past, some forests have been removed for the establishment of exotic plantations. Other forests are severely impacted by wood harvesting for fuel and building materials, bark harvesting for medicinal use, habitat loss from harbour development and water abstraction.

Alien plant invaders

Another serious threat to forests is that of alien plant invaders. A major invader of forests is Blackwood ( as well as Bugweed (), and Lantana ().

Invasive aliens cost the South African economy in the order of tens of billions of Rand each year. Invasive alien trees in particular cause a huge amount of damage, and have been described as 'fire lighters', "enemies of biodiversity" and "water guzzlers".

Indigenous forests in South Africa rarely burn, unlike other local fire-adapted ecosystems like shrublands (fynbos), savannas and grasslands. In some cases this is due to climate (high humidity); however often forests are confined to fire-safe habitats, like ravines, scarps and screes. Invasive alien plants can invade up to, or into, the forest edge, softening this fire-protective barrier, and allowing fires to enter and damage old forest systems.

Alien trees are also no friends of biodiversity.

Many studies have found that plantations or alien tree infestations negatively affect small mammals, invertebrates, and plant species. Lastly, invasive alien trees guzzle water.

According to one study, invasive alien trees reduce our mean annual runoff nationally by 1.4 billion m3 (3%).

READ | Could alien tree clearing near dams be Cape Town's solution to saving water

To make this easy to picture, this has been described as the equivalent of 577 600 Olympic sized swimming pools.

Therefore, it is important to clear invasive alien trees, to protect biodiversity and our water resources, but also to reduce fire risk and damage.

One of the major issues in invasive alien tree management in South Africa, is the lack of adequate, up-to-date maps of invasive alien trees for prioritisation and costing to support clearing efforts.

This is what a study in the Conservation Ecology & Entomology Department at Stellenbosch University tried to address.

We know that we can map invasive alien trees in the Fynbos and Grassland Biomes, but what about the Afrotemperate Forests? Is it possible to tell native forest trees and alien invasives apart in these types of forests?

The study used a fusion of 10 bands of optical Sentinel-2 satellite imagery and 39 indices, as well as explored fusion with radar (Sentinel-1) to map invasive alien trees in the fynbos and forests in the Garden Route Dam Catchment, George.

This catchment was chosen because it is severely affected by invasive alien trees, which set the scene for extremely dangerous wildfires in 2018. We also used drone footage, with field trips into the mountains, to help collect training data to run the classification.

We wanted to produce an up-to-date alien tree map for the George Catchment using a process which is repeatable, and which will be made freely available to stakeholders. We took a trans-disciplinary approach so that we could involve relevant stakeholders in the prioritisation of alien tree classes to map, collection of training data, and validation of results.

Clear distinctions

The results of the stakeholder survey confirmed that a map was desperately needed because the current most up-to-date map available for the area is 12 years old. Stakeholders identified the key alien tree classes to be mapped were: wattle, gum and pine.

We had expected that it would be difficult to discriminate invasive alien trees from indigenous forest in the Forest Biome.

Instead, we found that these invasive alien tree categories could be distinguished from other indigenous vegetation (including native forests) with an 89% accuracy. We could even tell alien tree categories apart with 87% accuracy. This is useful to help clearing invasive alien trees more effectively.

So, what can be done about invasive alien plants on World Wildlife Day and beyond? The public has a very important role to play. Perhaps in South Africa, we shouldn't be buying into a European agenda of tree planting, but protecting our African ecosystems by clearing alien trees.


Source: News24

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