Brouhaha over tree felling in Helderberg

The City of Cape Town programme aimed at containing the outbreak of polyphagous shothole borer (PSHB) in Somerset West has raised the hackles of local residents and at least one tree expert, Stellenbosch-based arborist, Riaan van Zyl.

Bolander first wrote about PSHB in 2018, when a Somerset West resident raised the alarm about a possible PSHB infestation in a yellow wood tree.

According to Mr Van Zyl, the City is not following the prescripts of its own protocol, as set out in “City of Cape Town, Invasive Species Unit, Version 1: 28 May 2019”, a copy of which Bolander has read.

Mr Van Zyl’s engagement with the City came about when private clients alerted him to the felling of PSHB infested trees in Somerset West.

The back and forth between Mr Van Zyl and various City officials over this issue, has endured since at least late September, with Mr Van Zyl averring that, after personal inspection of trees that have been identified by City monitors for felling due to PSHB infestation, the trees have:

• No evidence of decline due to PSHB and are completely functional in the urban forest;

• Some trees have no evidence of PSHB infestation of any kind, but marked for felling; and

• Most trees with PSHB gallery holes have only an average three to five entry points.

The City’s protocol document notes that the decision threshold for whether or not to fell an infested tree is, among other considerations, 50 or more PSHB entry points.

Mr Van Zyl also told Bolander that, in his opinion, the monitors who complete reports on the condition of affected trees are not competent to make the fell/not fell decision.

He added that this amounts to a failure in oversight, and that each report completed by a monitor should be physically reviewed before a decision is made whether or not to fell a particular tree.

Aside from the degree of infestation, Mr Van Zyl also said: “(a tree with) 50 or more (entry points) for sure needs attention – but we still need to consider how (the) tree has been affected in terms of its condition and structural integrity, and its contribution in landscape value, environmental functionality and considering its significance, which looks at size, age and historical value, amongst others. We (are) working with an urban forest and people.”

Mr Van Zyl said that rather than removing heavily infested trees, chemical treatments should be considered.

Bolander subsequently emailed three City officials, raising Mr Van Zyl’s concerns, but at the time of going to print, Bolander had received no official comment on the matter, nor had any of the three officials responded to Bolander’s request for comment.

Ward 15 councillor, Greg Peck, in whose ward a number of trees have already been felled, with more identified for felling, told Bolander: “I’m fully aware of residents’ concerns about the removal of PSHB infested trees in my ward.

“I’ve approached the responsible officials and the Mayco member concerned, and requested that oversight is exercised over the tree monitors’ reports on tree condition, in the form of a second physical inspection, before a decision is taken to remove an infested tree.”

Bolander also spoke to Professor Francois Roets of Stellenbosch University.

Professor Roets, a leading expert in the field of PSHB, trained the City’s monitors to identify trees infested by PSHB. (Bolander also understands that all monitors have either a nature conservation or environmental diploma or degree.)

Speaking in his capacity as a member of the Forestry and Agricultural Biotechnology Institute, University of Pretoria (FABI), Professor Roets said: “From a scientific view point the removal of a tree is justified in two cases:

• When the beetle infestation area is small and your objective is to try and to contain/eradicate the pest; and

• When a tree individual represents a major contributor to the local population of beetles.

“For the first year of infestation in Somerset West I believe that the aim was guided by principle 1,” said Professor Roets.

“However, after major expansion of the pest at the end of last year, I believe that the (City) started looking at option 2, based on resources and where to spend these most effectively.

“The situation in Somerset West currently seems to be somewhere between 1 and 2, in the sense that the focus is first on the heavily infested breeding host individuals, and after this the removal of less affected individuals.

Professor Roets added: “For case number 2, a tree individual should be defined as a ‘heavily infested breeding host’. These tree individuals are seen as ‘amplifiers’ for the local beetle population.

“With an increase in beetle populations the chances of transport to new (and unaffected) areas increase, but also, the chances of these attacking and impacting not only other breeding hosts, but also non-reproductive hosts are amplified. This does not seem to be the case with low PSHB population numbers,” he observed.

“Unfortunately there is no strong data for when a breeding host can be considered an amplifier individual and therefore the cut-off of 50 holes, although arbitrary, is usually applied here and elsewhere around the world.

“When left, tree individuals with currently only few holes will likely become amplifier trees, especially in the case of extremely susceptible hosts such as is unfortunately the case for the English oak.

“If infected areas of trees can be removed that would be a good option, but in my experience, PSHB seems to preferentially move into the trunks of trees in South Africa.

“This limits this possibility to a few cases only, and has not been an effective strategy in heavily impacted areas such as the oak trees in George.

“These criteria, and the decision to remove a specific tree individual, are independent of the tree’s historic, intrinsic and its contribution to the urban forest (even though quantifiable) as it is based only on reducing further infestations and possible long-distance spread to other areas,” he added.

“I agree that this is very unfortunate, but the entire process is aimed at trying to minimise further impacts on very valuable trees, not only in the urban settings but also in our natural vegetation.

Until a viable chemical or biological treatment becomes available (or any other treatment) this is considered the most plausible way to manage the current pest outbreak.

Please note that no effective chemicals have been registered in South Africa specifically, for the management of PSHB yet,” said Professor Roets.