'Gardening' the Forest

Forest and Bird, Issue 299, 1 February 2001, Page 45

'Gardening' the Forest

KEN MASON.

Pruning and thinning can speed native

forest regeneration, says

he time taken to build the structure of a native forest, with plantings or natural regeneration, can be at least halved by helpful shaping of trees to bring more light into the forest. By intervening in the natural process, trees can be helped to gain height more quickly, while a broader range of forest floor species can be established. Over more than 30 years’ involvement in forest restoration, I have developed a number of techniques which I'm now applying to seven ecological restoration projects about Dunedin. In essence, given space, both regeneration and plantings tend to spread overground in bush form. These can be pruned to a leader or two, and thinned below so they grow upward, rather than outward. This promotes the upward growth of a forest canopy, and allows in more light and rain, so other species can establish and not be smothered. These restoration techniques are based on observations over many years, but a number of examples stand out in my mind. As a schoolboy in the early 1960s I took part in a Dunedin City Council planting at Craigieburn Reserve. The plantings then had as much space to grow outward towards the light as upward, resulting in subcanopy trees growing as multistemmed giant bushes rather than as trees. Nearly 40 years on, the forest floor was dark and lacked any _ regeneration. Slower-growing, planted species were either ‘out-competed’ or stunted dwarves. Only where a large ‘bush’ had died, leaving a lightwell, were than any ‘second phase’ tall saplings or forest floor species. My second area of observations involved natural forest regeneration, a prime example being the Society's Moore Bush

in Leith Valley, Dunedin. Formerly a dairy farm, it was purchased by Ellie and Percy Moore in 1946 to actively encourage bush regeneration. Many of the podocarps were by then reduced to standing alone in pasture. When the property was bequeathed to the Society in 1973, natural regeneration either side of some of the tracks formed a dense physical and visual barrier. By the mid-1980s fierce competition for light had reduced the number of trackside stems to the point where there were now glimpses through them to the big rimu and miro trees. By the mid 1990s the tall regenerating stems had so reduced themselves in number that the tracks had been totally absorbed into the forest. There were now largely unobstructed views of the big trees above the subcanopy, and a healthy forestfloor fern cover. To replant as densely as in natural regeneration would need a large number of trees and acceptance of considerable casualties. There would also be the problem of a lingering, dark, forest floor. So at this point observations of plantation forestry practices came into

play. In exotic forestry, sidepruning helps form a single clear trunk, and pushes the tree upward while allowing more light to reach the ground. Often the number of trees is further reduced to allow the remaining stems to grow to a larger diameter. Something similar must happen in nature: ever notice how relatively few stems support an enclosed canopy. Pruning and thinning of native

vegetation is a way to accelerate its upward growth and encourage greater biodiversity in the form of plants which would otherwise by smothered. At Kanuka Bush, a small Dunedin City gully, I applied these observations in the restoration process. In 1985, the big old kanuka were reduced in number and the remainder were fighting for their survival against large sycamore trees and domestic goats. By 1987 both the sycamores and goats were gone, revealing largely bare ground. Regeneration, both from nearby relict seed sources and plantings, was subsequently managed. Young trees are side pruned to one or two stems to help them push upward. As the process continues, whole trees are removed by cutting them into small lengths and leaving them on the ground as humus. At the more advanced stages, stems are further thinned to select the final canopy trees. This is done by shallow ringbarking, and poisoning, as at this stage felling would damage the developing forest structure.

This process also allows light to reach the ground and enables ferns, shrubs and second-phase trees to develop. The outward-reaching branches of fast-growing trees near the bush edge are also trimmed back to a main stem. This creates room for further edge planting which also helps form a dense shelter against the wind.

Some reintroduced trees need their neighbours trimmed back to give them a chance to enter the canopy and become seed sources. Often they are planted in clumps for ease of looking after and increase their chances of establishing a niche. To enhance their growth, plantings are helped with added compost, water crystals and slow-release fertilisers. On really dry sites water penetrants are added to the soil. Micro-organisms, fungi and invertebrates are established by bringing in nearby soil and leaf

litter. Missing local ferns, tree ferns, astelias, bush grasses, perchers, mosses, scramblers and shrub species are reintroduced. It is most satisfying to see these plants spread once there is a seed or spore source. After 14 years, the Kanuka Bush sub-canopy is now up to 10 metres under open-spaced, large, emergent kanuka. This is very good growth for this southern latitude. Suitable micro-climates have formed for the re-establishment of delicate species such as filmy ferns. The formerly muddy ruin of a creek is now a delight, restocked with freshwater crayfish enjoying a shady bush environment. Over the past five years, methods trialled at Kanuka Bush have been applied to a number of much-larger restoration projects about Dunedin, including replantings on Quarantine Island in Dunedin Harbour, and in an area of silver beech. The methods outlined are not necessarily applied to the whole of a revegetation project. Better growth and moister areas are targeted first, along with sections along the visitor tracks where visual enhancement can add another dimension to restoration. The increased biodiversity created can then be left to spread into adjoining, untreated areas. If feeling a little hesitant to try these methods, because they involve pruning or removing native trees, consider these points from nature. The race for light and resources in the bush is not very pretty. Losers die. That nice little fern will throw its fronds over the adjacent seedling to kill it, for the alternative is a tree and a dead fern. The methods outlined are just speeding up what nature is doing, but more within our lifetimes. — KEN MASON is vice-chair-man of Dunedin Forest and Bird, co-ordinator of Dunedin Teen Conservation, and an adviser to the New Zealand Ecological Restoration Network.