Tokai Park's Eye in the Sky
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Tracking alien trees using satellite imagery at Tokai Park
By Nicholas Coertze and Alanna Rebelo
Invasive alien trees are a major issue at Tokai Park, part of the Table Mountain National Park World Heritage Site. Invasive alien species, along with habitat transformation, threaten biodiversity and push species to extinction. Invasive alien trees are a particular problem, a legacy of plantation forestry that has now exited the region.
Tokai Park is a particularly precious place, even within the great Table Mountain National Park, because it forms a peninsula of fynbos jutting into the suburban matrix. This is important because it is one of the only lowland areas of the park connecting the mountain to the sandy flats, home to Cape Flats Sand Fynbos. Cape Flats Sand Fynbos is a very rich vegetation type, but is extremely threatened, with less than 1% conserved in the world.
The Friends of Tokai Park are custodians of Tokai Park, and volunteers are actively involved in clearing invasive alien trees. Lower Tokai Park is largely alien free, but the upper parts of the park are battling after the 2015 fires stimulated the considerable seedbanks of dormant invasive alien Wattles and Gums. After fire, these seedlings enter an arms race with fynbos and, because they grow so fast, they quickly overshadow the fynbos, out-competing it. Without intervention, we would lose the fynbos.
This isn’t just about biodiversity, although saving species is critical. Invasive alien trees also increase fuel loads and guzzle water. The threat to homes and lives posed to those living near these infestations is significant.
Therefore, we need a tool to monitor spread of invasive alien trees, as well as clearing efforts. We have produced a living map of invasive alien trees for Tokai Park, as well as greater Table Mountain National Park.
We know from experience that invasive alien trees are often difficult to tell apart from indigenous Afromontane Forest, as well as from each other, using Sentinel-2 imagery alone. Sentinel-2 is an Earth observation mission from the Copernicus Programme, made freely available, funded by the European Space Agency.
High return frequency has to be traded off against spatial resolution, amongst others, and therefore the data are only available at a 20-metre spatial resolution for most bands – so we expected some challenges in mapping invasive alien species at Tokai Park.
What did we find?
We found that invasive alien trees could be distinguished from other indigenous vegetation with an impressive 98% accuracy. We could even tell alien tree types, such as Gum (Eucalyptus Genus), Wattle (Acacia Genus) and Pine (Pinus Genus) apart with 97% accuracy.
Our classification suggests that out of the 8.37 km² reserve at Tokai Park, 1.97 km² (23.5%) was alien tree invasion in 2018. In 2020, invasive alien trees have proliferated and are now estimated to cover 3.22 km² (38.4%). That is a worrying 14.9% increase in invasive alien tree cover.
In terms of the different alien tree categories, gums dominated the alien invasion. We found 1.03 km² (32.0%) of wattle, 0.33 km² (10.4%) of pine and 1.85 km² (57.6%) of gum within the park boundaries in 2020.
Friends of Tokai Park Map of Invasive Alien Trees within Table Mountain National Park
This alien tree map for Tokai Park can be updated as regularly as every 15 days, which is the time it takes for the Sentinel-2 satellite to pass over and obtain an image of sufficient quality. The results of the classification are freely available to all in the app that we have made available on our website.
Working for Water, with SANParks, and Friends of Tokai Park volunteers are working hard to clear the invasive alien trees. However this work is never finished. The cleared alien stands will require follow-up alien clearing to make sure new alien seedlings don’t simply take over again. We need to be in this for the long haul.
What is exciting about the alien tree clearing initiatives at Tokai Park is that it creates an opportunity for the fynbos to recover, reproduce and set seed. This will, in turn, promote the recovery of the indigenous fynbos seedbank.
We now know that we live alongside fynbos, which needs fire to survive. However, alien trees can increase the fuel load by as much as 60% and result in far more dangerous wildfires. These invasive-alien, tree-fueled wildfires are far more volatile and potentially damaging than those which naturally occur in the fynbos. The management of invasive alien trees will therefore lower the risk these fires pose to residential areas and firefighting services.
This article is the second of two. The first is Nicholas Coertze’s Road to Recovery – Monitoring invasive alien trees in Tokai Park with advanced remote sensing methods.
This project would not be possible without funding from the Izele Grant
