The Road to Recovery

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Monitoring invasive alien trees in Tokai Park with advanced remote sensing methods

By Nicholas Coertze

Tokai restoration area — Roads crisscross the damaged but recovering fynbos at Tokai Park, dividing the park up into management blocks. Some blocks have been recently cleared of invasive alien trees by Working for Water (the brown/red blocks) and others have recovering fynbos (green-brown blocks), while others remain infested with wattle and gums (bright green blocks), the sinister legacy of plantation-forestry that has exited the region. **Photo** Byron-Mahieu van der Linde

My first visit to Tokai Park in mid-summer 2021 was guided by local ecological expert Dr Tony Rebelo. I was astonished by the task force at work on the other side of the park fence. Multiple Working for Water teams, averaging 15 people each, were hard at work tackling dense and sparse stands of invasive alien trees on every tier of the park.

The Cape Floristic Region of South Africa is recognized as having the richest temperate flora on Earth and is considered a biodiversity hotspot (1 of 25 around the world). A biodiversity hotspot is a biogeographic region containing a significant reservoir of biodiversity threatened with destruction.

Tokai Park is a relatively small 600ha piece of land adjacent to Table Mountain National Park located on the Cape Peninsula which is exceptionally species rich even by Cape Floral standards. This land was previously leased by MTO Forestry company which leaves behind a legacy of invasive alien plant seeds amongst the natural seedbank. Tokai Park is a site of international conservation importance due to its high biodiversity, endemism, and a worryingly high biodiversity loss.

This land is currently undergoing fynbos restoration and conservation which has already created the opportunity for more than 550 species to reestablish in the lower portion of Tokai Park alone. More than 150 species within this park are threatened with extinction or are extinct in the wild due to anthropogenic development, degradation and habitat loss, defining it as one of Earth’s “quintessential biodiversity hotspots”.

On the hike I was shown areas that were previously under dense pine plantation, that have been fully restored to fynbos. These areas have been maintained by Friends of Tokai Park volunteers who perform regular hacking meets which aim to keep these cleared areas alien free or maintain a low density of aliens. The consistent effort of the volunteers is evident – their work is important as it promotes the recovery of the indigenous fynbos.

Invasive alien trees cause a great deal of damage, threatening our economy, costing billions of rands a year. This is why their removal is a priority. The Working for Water task force aims to completely clear invasive alien trees from Upper Tokai Park this year. But this needs to be monitored, because the seedbanks of these alien trees – a legacy of forestry – still survive under the ground, and these trees will be back.

There is a focus on clearing invasive alien trees because they tend to proliferate, growing tall quickly, outcompeting and overshadowing the fynbos, thus killing it. The strategy is to care for the fynbos by clearing the trees and, in turn, the fynbos should be able to compete with the alien shrubs and grasses itself. Scientists call this “passive” restoration.

This year I have been working as an intern with the Friends of Tokai Park, under the supervision of Dr Alanna Rebelo, funded by the Izele Small Conservation Grant. The aim of my work is to produce a living map of invasive alien trees, to monitor the restoration of Lower Tokai Park (Cape Flats Sand Fynbos) which is relatively alien free, and Upper Tokai Park (Peninsula Granite Fynbos, and Afromontane forest) which is overrun with aliens.

How it is being done?

We are using a method developed by Dr Alanna Rebelo and Dr Petra Holden which uses data of landcover types collected in the field, which are used to train a powerful classifier. This classifier is applied to freely available satellite imagery, specifically the Sentinel 2 satellite, on a free platform: Google Earth Engine. This makes these methods relatively easily repeatable and updatable. We are also exploring automated change detection methods, like that developed by Glenn Moncrieff.

Expected outcomes?

We will produce a map for the last few years (2018 to 2020), which will be embedded into a map of management blocks at Tokai Park, with information like number of species, restoration history amongst others. This will be the living map which our members, the general public and researchers can interact with to find out more about the park.

What we will do with the data?

We will use this living map to monitor the spread and clearing of alien trees in Tokai Park. This is useful both to keep track of restoration efforts, but also to prioritize intervention, and to detect invasions early, for early detection and control. Perhaps most importantly is that this map can be used to engage stakeholders, and increase buy-in and interest in alien clearing.

This monitoring is crucial as the park undergoes large scale passive clearing, herbicide application and stacking of invasive aliens. These dense stands of aliens will indefinitely regrow and outcompete fynbos within a year without an active restoration management strategy. Active restoration management strategies such as “hot” prescribed burns in summer and regular hacks to maintain aliens at low densities are required to prevent proliferation and to give fynbos the chance to compete.

We now know that we live alongside fynbos which needs fire to survive. Fire promotes seed germination in many species. Due to past practices however, invasive alien trees like pines, wattles and gum trees are widespread within the park. These trees can increase the fuel load by 60% and result in far more dangerous wildfires of higher intensity, just like the fires experienced in 2015.

These invasive alien tree-fueled wildfires are far more volatile than those which naturally occur in the fynbos. With greater fuel loads, these fires burn hotter than fynbos fires, scorching the earth, killing native seeds and promoting invasive alien tree establishment. This creates a vicious cycle of invasive alien tree stand growth that threatens the biodiverse, species-rich ecosystem within Tokai Park and, simultaneously, people’s homes.

Invasive alien trees, especially in rivers, are thirsty and have been found to reduce our mean annual runoff nationally by 1.4 billion m3. With climate change predicting further drying in our country, we really cannot afford to waste a single drop. Furthermore, human development, such as vineyards, is encroaching on the already critically endangered Cape Flats Sand Fynbos and Peninsula Granite Fynbos. The implication of this is that we really cannot afford to allow invasive alien trees or human development to encroach on a single hectare of the remaining 11% and 43% respectively of these precious vegetation types that are home to so many species.