Sycamore tree spread a rapidly growing problem

Sycamore is one of the worst threats to Dunedin’s native vegetation, writes Matt Thomson.

Autumn is here and leaf-fall comes around again. The 20m-tall Sycamore's stark grey trunks and branches are exposed, and the deciduous tree's leaves will block drains, cover parks, make footpaths and train tracks slippery.

They quietly wait for another spring to come when a multitude of seedlings will spring forth and adult trees will continue to quietly shade out hundreds more native trees across Dunedin.

I feel it's time to comment again on just how bad this plant is and move towards what I believe is the only option for it. A position of no tolerance.

Whatever you think of exotic trees and woody-weeds in general, my position is that sycamore is one of the worst threats to Dunedin's native vegetation, period. Nothing else compares. Seedling recruitment near parent trees can be in the hundreds per square metre.

Trees can gain 1m-2m in height per year, they ``sucker'' from roots, they coppice (re-sprout from cut stumps), their ``whirly-bird'' winged seeds travel phenomenal distances and they produce prolific amounts of them. Sycamore provide little to no benefit for native birds, no co-evolved nectar producing flowers for our bellbird, tui and wax eye.

No fruit for our kereru. They actively out-compete native vegetation to the point where it is shaded out and killed. Mature stands of sycamore completely smother everything underneath with a heavy leaf fall each autumn. Their seeds are a valuable food for rats and mice and trees will live more than 200 years. You get the picture. They're just plain bad.

Sycamore is a death by a thousand cuts threat in my opinion. Just 10 years ago, the problem seemed much more manageable. However, I estimate the total biomass of Dunedin's sycamore would have doubled in that time.

In Dunedin there are areas you would have to say will take a significant effort to retrieve back to a predominantly indigenous state: Signal Hill, Port Chalmers, Caversham, Kaikorai, Northeast Valley and Leith Valley to name a few. Sycamore will take a botanically diverse area with many dozens of species of plants, and reduce it to include occasionally only itself. Literally taking on all-comers.

What happens when sycamore creep up to Leith Saddle and compete with mountain cedar? Or run up through Bethune's Gully and over Mt Cargill? Or take out the last remaining kowhai stands in Leith Valley?

Multiple scenarios will eventuate if we don't start to knock back the problem at a decent pace from now on - until they're gone. I talk about a need for eventual eradication, as really that's the only way to control something so virulent.

But there is good news. For roughly the past 10 years, stoic individuals such as Steve Walker, chairman of the Port Chalmers Community Board, have been petitioning the DCC for an action plan on sycamore, eventually getting a trial removal and replanting a section of about 400m undertaken at Ravensbourne in late 2015 by Up-Front Environmental.

About 700 native plants followed in autumn 2016. According to the DCC Parks and Reserves team, positive public feedback received by them was the most significant they had ever received for any project. A second 700m continuation of this coastal section through to Maia was undertaken by local arborist contractors in 2016.

It seems now there will also be a stage 3 sometime soon. Monowai Ecological has undertaken areas of control sponsored by Ravensdown Fertiliser Co-op, Blackhead Quarries at Logan Point, and Dunedin Railways in Port Chalmers. Contractors Up-Front Environmental have been controlling sycamore in the Dunedin Town Belt and elsewhere for about 10 years and have made significant impact.

Last year, a further section of sycamore was taken out below the Robert Scott monument above Port Chalmers. Subsequent poisoning trials are happening before winter in the Port Chalmers town belt.

A small but merry group of concerned West Harbour inhabitants has been pushing for a management plan for the control of sycamore initially across West Harbour. It is envisaged West Harbour could be controlled to a manageable eradication level within 10 years.

Plans for other areas of Dunedin need to be instigated by community groups and individuals with a view towards a cohesive plan for greater Dunedin City. To date, companies and landowners seem more interested in sponsoring control work than management plans.

However, a plan is vital if ultimate outcomes are to be achieved and maintained. Recognition and funding for control of this weed by the ORC would also be desirous.

In the past five years, a new product has become available in the war against sycamore, X-Tree Basal. Previously, contractors and others experimented with this product under development by Doc in Dunedin and Queenstown in the fight against wilding pines.

It is an environmentally safe product which is applied directly to the trunk of a mature tree of almost any size. It is deadly to sycamore. The product breaks down to a simple acid and is non-persistent in the environment. Being able to simply, safely and cheaply treat trees standing enables, for the first time, a viable shot at controlling sycamore at a landscape scale.

-Matt Thomson is director of Monowai Ecological, a vegetation control, ecological restoration and conservation company.