Growing pains

Victoria Clark, a Masters student at the Australian National University, has been researching tree density and habitat quality at Tarcutta Hills Reserve in New South Wales.

The forestry industry has long used thinning techniques to manipulate tree growth and height but thinning as a conservation practice is a relatively new idea that could help restore some disturbed areas.

Southern boobook (Ninox novaeseelandiae), flying to nest at night. Copyright Michael Maconachie/AUSCAPE. All rights reserved.

Grassy white box woodlands were once prolific along the western slopes of the Great Dividing Range. More than 90% have been cleared for crops and grazing and they now exist in small, fragmented and often degraded patches.

Bush Heritage’s Tarcutta Hills Reserve contains one of the largest remaining remnants of grassy white box woodland and provides important habitat for threatened species such as swift parrots and diamond firetails.

When cleared forests and woodlands regenerate naturally, saplings often grow back in high densities, locked in competition for light, water and nutrients. With limited room, the saplings struggle to develop into mature trees, remaining thin and unable to form large branches. It can take over a century to naturally develop the structural diversity, fallen timber and tree hollows so important for habitat.

When Bush Heritage purchased Tarcutta Hills Reserve in the 1990s around a third of the property had been cleared.

Brush tailed phascogales like to use a number of tree hollows over consecutive nights. Copyright Frank Woerle / Auscape. All rights reserved.

The regenerating woodlands and forests have sections with very dense tree regrowth. My research looked at whether thinning could improve habitat quality in densely treed native forests and woodlands – Tarcutta was an ideal testing ground.

I visited regularly over a three-month period, assessing habitat on 24 sites, and comparing it with the needs of local native species. I used straightforward methods so volunteers could continue the study and collect long-term data. In each plot I counted the trees and hollows, measured tree diameters, vegetation cover, and the amount of fallen timber.

The shedding of large branches is important for creating hollows, and in eucalypts this can take 120 years to begin. Fallen branches (and trunks) also provide habitat for numerous species of fungi and invertebrates as well as hollows for ground-dwelling mammals.

Natural understory, Tarcutta Hills Reserve, NSW. Photo Wayne Lawler / Ecopix.

Tree hollows in a variety of sizes are important. For example brush-tailed phascogales use a number of different hollows (around 4cm in diameter) over consecutive nights whereas barking owls prefer hollows with an average diameter of 28cm – only found in large, mature trees. Nesting boxes can help some species over the short-term, but can’t meet the needs of all animals.

“The shedding of large branches is important for creating hollows, and in eucalypts this can take 120 years to begin.”

My results showed that the regenerating areas on Tarcutta had much higher densities of small-diameter trees and very few medium or large ones compared with undisturbed areas. Competition between these thinner trees appears to be slowing their rate of growth, which will affect time taken to form large hollows.

This is a relatively new area of conservation research and the next step would be to conduct tree thinning trials in regenerating areas. These could tell us whether we can speed up the growth of the remaining trees to increase the structural diversity and habitat quality of these woodlands.

Generous support for the acquisition of this property was provided by the Australian Government under the National Reserve System Program.