Five summer months of strong salt-laden winds have taken their toll on shelter-belts. Scientist, Bob Hathaway has been studying the damage the winds have wrecked, particularly on poplar and willow shelter-belts in the Manawatu.
Strong summer winds normally last only a month or so, but last summer’s abnormal conditions has taught scientists several lessons.
One lesson is that there are serious limitations as to where these particular species can be planted. Mr Hathaway says eight kilometres from the coast is a safe distance. The willows and poplars have suffered leaf-burn and die-back of tree leaders which means the trees are actually smaller than when they were planted, he says.
The trees can be cut back to re-shoot from live material but the damage has set them back at least a year. However, those planted in less exposed areas have proved “quite satisfactory”.
The casuarina (she-oak) species used in shelter-belt tests has also been found to be inadequate in exposed coastal conditions.
This species has also been burnt by the salt winds, but to a lesser degree than the willows and poplars. But, although the Casuarina glauca planted was thought to be quite tolerant, it has not stood up to severe coastal conditions. This has shown mainly through burning and poor growth rates.
“It will be necessary to use more resistant species such as Pinus radiata and, possibly, Leyland cypress for exposed western boundaries,” he says. Pinus radiata is very tolerant to salt winds but has not been favoured because of its large eventual size.
Management of the trees is essential. Side-trimming is needed to keep them dense at the base and prevent them opening up. As they grow they will need topping to prevent shade and competition with crops.
Leyland cypress is not as tolerant as Pinus radiata but can probably be used in slightly less exposed situations. Although it is slightly susceptible to cypress canker, recent experiments in the Manawatu have shown the species can live with the disease. “They’re vigorous enough to overcome it, but it will always be there.”
The MWD plant materials centre at Aokautere is co-operating with the Ministry of Agriculture and Fisheries in selecting suitable shelterbelt species for land exposed to strong salt-laden winds. The MAF trials on its Manawatu land deal with irrigation and fertiliser requirements to maximise growth rates. The MWD centre’s role is advisory in selection and management of species for these difficult sites.
Bob Hathaway . . . studying the damage the summer’s salt-laden winds have made on poplar and willow shelter belts. The shape of the tree-top in the background might tell us something about prevailing winds.
– from Growing Today, July 1983 MWD, Aokautere: Ministry of Works and Development. Gone, like evermore of the taxpayers’ most valuable assets into the hands of profiteers.