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Wayward Willows

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Our series on Trees in the Wrong Place wouldn’t be complete without visiting the Willow family (Salix spp.).

Originally from China, there are now 32 different species, subspecies and hybrids invading riverbanks and wetlands in temperate Australia.

Nearly all have become naturalised and are listed as Weeds of National Significance. A few species including Weeping Willow (S. babylonica) are not listed but are of great concern because they hybridise with other species flowering at the same time such as Pussy Willow (Salix cinerea) and Crack Willow (Salix fragilis var. fragilis). The resulting hybrids become very invasive and are practically impossible to identify.

Willows have had great economic and environmental impacts in south eastern Australia and millions of dollars are spent each year controlling infestations.

In the severe droughts of 1981, government departments actually planted willows to rehabilitate riparian zones on grazing properties where soil salination and erosion were an issue. Bundles of Crack Willow were simply poked in holes five metres apart along creek banks. Years later, the same teams were back removing the trees, which proved to be an almost impossible task.

Willows spread their roots into creek beds, slowing the flow of water and reducing aeration. They also form thickets which divert water outside the main channel causing erosion on vulnerable creek banks. In autumn, fallen leaves create a flush of organic matter further reducing water quality and available oxygen and directly threatening aquatic plants and animals. Gradually, native vegetation like River Red Gum disappears as do native animals depending on them for food and nesting hollows.

Planting Trees For The Future

With climate change effects and rising temperatures governments are under pressure to provide research for better landscape solutions to avoid mistakes of the past.

Victoria’s Royal Botanic Gardens has designed a world first Landscape Succession Plan to help people plant the right kind of trees in the right places to ensure the trees are long-lived. It uses modelling to make predictions on what the climate will be like in the next 20 years.

Director RBG Professor Tim Entwistle said the Gardens are not just about aesthetics, there is a strong focus on climate change, water sustainability and biosecurity. He said Botanic Gardens is responding and contributing by using its collections, doing research in the garden, and through education and learning.

There are two million visitors to the Melbourne Botanic Gardens each year including 35,000 school children.

“As a race, we may be able to bring down those levels and mitigate some of those impacts,” he said.

“We have 8,000 species and manage plant collections scientifically, understanding where they grow, the type of soil and so on. Our goal is to be able to transfer a landscape and collection and position to one that’s more resilient in accordance with climate predictions.

“People are shocked when they find out that Melbourne’s landscape will be similar to that of Dubbo by the year 2090. We can’t now plant for the climate that’s been here in the last few decades, we have to plan for what’s coming,” he said.

“Willows have had great economic and environmental impacts in south eastern Australia and millions of dollars are spent each year controlling infestations.”

“We can look at where a plant grows naturally and predict what will grow in the future. That’s what we are looking into at the moment to guide us in what to collect and this can be done across the world.

“We’ve always tried to grow plants outside their natural habitat. That’s what we do best in botanical gardens and we can use that knowledge to help others.”

Professor Entwistle said certain plants will have a weed potential with an increase in temperature but every new plant is run through a model.

Manager Horticulture at Cranbourne Gardens John Arnott said there are several Australian plants that have become weedy after being planted away from their endemic range.

“One of the most interesting would be Sweet Pittosporum undulatum – a native of Eastern Victoria with the common name of Native Daphne due to its exquisite perfume. “It was widely planted outside its natural range in the 1970’s and within a decade it had escaped from our gardens through birds dispersing the fleshy seeds. By 2000 our Native Daphne had become so widespread that very few areas in Victoria with rainfall above 650mm are devoid of this plant,” he said.

A study carried out at the Gardens in conjunction with economists at a local university analysed where and how carbon is stored in various trees. Different species were found to absorb different amounts of carbon; the fast growing eucalypts proved to be the quickest at carbon storing, and the older the tree the more storage. About half the carbon was stored underground in roots and in all kinds of bacteria and living organic matter.

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