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Tree health experts from around the country are collaborating in efforts to save four iconic native tree species found in the Cape that are under threat: the Silver trees of Table Mountain, southern Cape tree ferns, milkwoods and the Clanwilliam cedar.

Silver trees
Kirstenbosch National Botanical Garden, in collaboration with the Botanical Society of South Africa, Table Mountain National Park (TMNP), FABI (Forestry and Agricultural Biotechnology Institute) and the South African National Biodiversity Institute (SANBI), has launched an innovative project to save the endangered Silver tree (Leucodendron argenteum) which only occurs naturally in a tiny area of the Cape Peninsula.

The Silver tree, also known as the Silver Leaf Tree, Witteboom, or Silwerboom, is a protected evergreen tree which is part of the Protea family. This enormous silver Protea is naturally confined to a tiny area in and around the city of Cape Town – with its main population growing on the slopes of Table Mountain, notably the Lion's Head area, above Rhodes Memorial and the mountain slopes above Kirstenbosch National Botanical Garden.

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Historically, the Silver tree was widespread on Table Mountain, covering much of its slopes in shimmering silver forests. However, early demand for timber led to much of these forests being felled and now the Silver tree is a rare and threatened species - in danger of becoming extinct in the wild in the next 50 years.

Unfortunately the Silver tree is susceptible to infection from the fungus Phytophthora cinnamomi, a soil borne pathogen that causes root and collar rot. Phytophthora feeds on living plant roots and stems, reducing the plants’ ability to transport water and nutrients – often leading to the death of the host.

This introduced pathogen is present in wild Silver trees as well as in the population found in Kirstenbosch National Botanical Garden, where it is causing the death of many of these iconic trees.

In a bid to save the Silver trees, Kirstenbosch staff, in collaboration with FABI, have launched an innovative treatment trial.

“We are testing the application of phosphite, a biodegradable fungicide which can protect plants against Phytophthora die-back. Phosphite will not eradicate Phytophthora from the soil, but it can protect plants from infection, and can help them recover if they are already infected,” explained Dr Trudy Paap of FABI.

“We have sprayed some of the Silver tree seedlings and injected bigger trees with phosphite. We have also left other plants untreated, as controls, and are monitoring the survival rates of the treated and non-treated trees.”

Phosphite is used in South African agriculture and horticulture settings to guard against Phytophthora root and crown rot of several important commercial crops. However, it has never been applied to indigenous South African flora before.

When the FABI research team re-visited Kirstenbosch in May they found good evidence that the spray applications that were done in 2020 have been effective in reducing the impact of the disease on young plants.

A number of bigger silver trees were also treated with phosphite during this visit and the results will be evaluated later this year.

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Milkwood trees
Another natural tree on the radar of the FABI team is the milkwood tree (Sideroxylon inerme) that has been dying in the Hermanus area. The problem was reported to the FABI team by well-known tree surgeon, Leon Visser.

A tree health survey in May enabled the team, including Prof. Mike Wingfield, Prof. Brenda Wingfield, Prof. Francois Roets (Stellenbosch University) and Dr Trudy Paap, together with ecologist Dr Casper Crous to join Leon to inspect the trees. These trees had symptoms of various stages of decline with many having already died.

Roots and above ground parts of the trees were inspected and samples were collected for further study. Although opportunistic pathogens may be involved in the problem, the group was generally of the opinion that an environmental factor is likely to be the initial cause. Mike Wingfield says the team suspects water issues, which could have resulted from the opening up of the land for housing construction. He says that the trees that are dying appear to be close to the houses, and there is no obvious indication of a pathogen problem.

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Tree ferns
Iconic tree ferns Alsophila (Cyathea) dregii have been observed dying in southern Cape forests for some years. These plants appear to die very rapidly and populations of these tree ferns - such as those found in the Knysna forests - have collapsed dramatically.

A recent inspection of the trees by the research team noted that tree ferns that were only slightly affected during a previous visit have now died, with some areas having almost no living plants left. The team took samples of these trees and it is hoped that laboratory research will lead to a better understanding of this dramatic decline.

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Clanwilliam cedar
FABI initiated a research project in 2020 focused on the fungi associated with bark beetles infesting the Clanwilliam Cedar Widdringtonia (cedarbergensis) wallichii. This work stemmed from earlier research during the 1980s, to consider the possible role of insects and fungal pathogens in the demise of these iconic trees.

The project is being led by Prof. Mike Wingfield, Prof. Brenda Wingfield and Prof. Francois Roets (Stellenbosch University) and supported by MSc student Handré Basson.

During their recent Cape visit the research team undertook a survey of beetles infesting Widdringtonia nodiflora, a species that is more widespread than W. wallichii. Beetles infesting these trees were found and collected for further laboratory work. This will make possible a comparison of the beetles and their fungal associates on the two tree species and provide further insight into their role in tree decline.

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The Clanwilliam cedar, which is endemic to the Cederberg and is protected within the Cederberg wilderness area, is considered critically endangered. This species, which once grew in abundance in the Cederberg, was heavily logged for timber by the early European settlers who arrived there in the late 18th century, as there were few other trees growing naturally in these mountains.

In 1879 alone more than 7 000 trees were cut down for use as telephone poles. This overexploitation caused a significant reduction in the Clanwilliam cedar, and the species never properly recovered.

Researchers have suggested that the ongoing decline over the past century could be due to a harmful fire regime. Monitoring of permanent plots set up 29 to 35 years ago distributed across this species range indicates that a 94% decline of mature individuals has taken place in less than one generation (66 - 200 years).

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Champion tree
Fabi tree health researchers were recently alerted to a concerning die-back affecting introduced Araucaria trees in Stellenbosch. Dr Trudy Paap joined the well-known arborist and tree climber Leon Visser, who assisted her in climbing some of these trees to sample their dying tops. This included a 200 year-old ‘Champion Tree’ Araucaria heterophylla (Norfolk Island pine). At 45m, this is the tallest tree in town.

The trees had symptoms of a top-down die-back, and with Leon’s help, Trudy was able to sample the trunks at the disease interface. It is suspected that the extreme drought that occurred from 2015-2017 played a significant role in the die-back, and that opportunistic pathogens may also be involved. Being able to collect samples right at the point of interest in the canopy, however, presented an opportunity to confirm the identity of biotic agents. It is hoped that the knowledge gained from this unique sampling effort will help guide a strategy to limit the decline and restore tree vigour.

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Related article: Indigenous trees of Southern Africa (Part 3) – Cape Ash