Farm Forestry New Zealand

Instability in lusitanica - a learning curve

Ian Brown, New Zealand Tree Grower May 2005.

Toughing out in the Far North

In 1983 we planted 300 Cupressus lusitanica on a difficult slope near a boundary on our Northland property. The site is elevated, steep and exposed to prevailing south west winds as well as to periodic north east gales which are often accompanied by heavy rains. The soils are rocky, and infertile, and much of the planting was done by crowbar with little attention to root orientation. The trees were essentially fillers, simply occupying an unwanted space. We planted at 200 stems per hectare, pruned to four metres and I anticipated trouble with wind throw. To my surprise we had no problems, and after a slow start the trees thrived. They have remained stable, and are looking quite impressive at 22 years.

I assume that because they had to tough it out at the start, they were forced to adjust to the winds. The low site fertility ensured a limited sail area and encouraged the roots to forage among the rocks for water and nutrients.

A better site at Pirongia, Waikato

In 1997 we planted over 3,000 lusitanica on a much better site which had been recently farmed. It is relatively flat, fertile, and with free-draining volcanic soil. It is elevated, in parts exposed to winds, and has high rainfall.

Again I anticipated some wind throw and planted the trees in groups, partly for mutual protection and for other reasons about which I have some reservations. On this occasion we were able to plant carefully, with the pull-up method that ensured even root spread. For protection from wind we inter-planted with pines and released the cypress by spray over two summers.

Starting to topple

At about 18 months things started to go wrong. The trees grew vigorously outpacing the pines for the first three years, but in the autumn preceding the second winter they began to topple in strong winds. I then spent some time pruning, taking the best two trees in each group to half their height, and followed through with an annual prune. It worked, and losses from wind throw were relatively slight, but it meant a lot of work and was a close run thing.

By year three the trees stabilised, with an occasional tree blown down. I expect that on exposed patches occasional losses will continue.

I surmise that the main cause of instability in this location, and the chief point of distinction from the Northland site, is the higher rainfall and soil fertility, which encouraged vigorous foliage growth and discouraged root development. I did not help by releasing the trees in the second summer. This was clearly a mistake, allowing pugging after heavy rain and reducing root competition.

On an adjacent area we planted small blocks of Leyland cypresss and C. ovensii in pasture and at final spacing, and with one release spray. The Leylands were predictably stable, but surprisingly the C.ovensii, which have a reputation for instability were equally stable.

Pruning trial

At 18 months I set up a small trial to assess the effectiveness of pruning in reducing instability. I selected 360 trees in an area that was particularly exposed to wind, and allotted the trees in each group to three treatments. The trees selected for pruning were treated annually for four years.

The control group was unpruned, a second group was clearwood pruned to half the height, and the third group had branches shortened to half the height, removing over half the length of the branches.

At age seven I assessed 100 groups – 300 trees – measuring diameter and checking for wind throw. With regard to diameter there was no difference between any of the three groups. Both pruning methods were equally effective in reducing wind throw, limiting it to one third of the incidence in the unpruned trees in which 19 trees were affected.

It was clear that on this site neither pruning method had any adverse effect on diameter growth, but both resulted in a significant reduction in the incidence of wind throw.

Branch shortening

It is clearly preferable to prune branches once rather than in two stages. However there are occasions where branch shortening can be useful.

The first of these is in an emergency when toppling is imminent and there is little time. Branches can be sheared quickly with a powered hedge trimmer. With lusitanica planted on fertile sites, the time of particular risk is during the autumn winds after the second summer. The shortened branches can later be removed at leisure.

Secondly it is useful to shorten any large branches that remain in the crown after completing a pruning lift. These branches are generally located at levels in the crown where growth had resumed in spring after a period of winter quiescence, or in response to damage to the terminal shoot.

Inter-planting with radiata

We planted pines in the gaps between the groups of cypress. The aim was partly to reduce branch size to make easier pruning and to limit the defect core. In this it has been effective. However with regard to reducing toppling I am still undecided.

The pines, which were planted at the same time as the cypress, were too small to act as protection at the most vulnerable time in the second winter. To do so they would have had to be planted one or two years in advance.

Toppling reduced

After three years the incidence of toppling dropped sharply. Whether this was due the protective influence of the pines, or the stabilizing influence of maturity in the cypress was not clear – I suspect it was a bit of each.

In these fertile conditions the cypress outpaced the pines for the first three years. The pines caught up by year four and were clearly dominant by year six. The period of time in which they might have been contributing to stability was therefore limited. By year six they were potentially threatening.

The pines can be disposed of by either felling them or slow death by ring-barking. Our previous experience had shown that on this type of site the pines must be felled by year six, before the final and most difficult stages of cypress pruning have been completed. The resultant pine debris makes access to them difficult. The cypress are then exposed to wind, with resultant losses for at least a year until they stabilise. In contrast, slow death by ring-barking has the advantage of allowing continuing access for two years for completion of silvicultural work. It gives time for the cypress to adjust to exposure as the pines gradually lose their foliage.

Ring-barking protecting trees

Ring-barking is not difficult if done correctly, and I have learned a couple of tricks from Geoff Brann.

Ring-barking must be done in spring, when the bark can be stripped off easily. This allows a generous segment of bark to be removed. It is important to use the right tool – an old-fashioned carpenter’s draw knife, which is like a giant spoke shave, is very effective. It takes me about a minute to deal with each tree.

Ring-barking must be done before the tree is too big. In older trees when the bark is thick and corrugated it is hard work and difficult to remove long segments. It is easy to leave strips of cambium which re-establish a vascular connection and keep the tree alive.

As ring-barking works indirectly by depriving the roots of nutrients, it takes about 18 months for the tree to die. I dealt with my trees at year six, at which time some of them were getting uncomfortably large. I should have attacked them a year earlier.

The dead trees should not be left to fall naturally. Apart from the safety risks imposed by hung trees, they can destroy adjacent trees as they fall. When the cypresses have had time to adjust to wind exposure, the pines should be directionally felled.

Too much protection

Clearly cypress benefit from protection from strong winds. At the same time there is a risk in overprotection. They should be allowed limited exposure, especially when young, to toughen them up so that they can resist more severe wind events later. This became apparent when I used tree shelters to protect a group of lusitanica against damage by hares. Because of stress-shielding most of the trees toppled when the shelters were removed. In addition, some of the surviving trees toppled several years later.

This suggests that the development and orientation of their roots may be determined very early, that this process might be responsive to wind stresses, and when established could be irreversible.

Summarising the solutions

It has been clearly shown the most important cause of toppling in radiata pine is root distortion caused by poor planting technique. The same is true of cypress. However the story does not end there. You cannot always blame the tree planter as lusitanica is inherently unstable in certain conditions. It is useful to have some awareness of site factors that predispose to instability, and of some strategies that will minimise it.

These site factors include areas of high risk such as fertile North Island farm sites when periodically high winds are accompanied by heavy rain. As with radiata, the time of greatest risk is the second winter after planting. It is not easy to predict sites at risk. They are not confined to areas exposed to prevailing winds. For example fertile valley bottoms, where the trees are protected from prevailing winds but exposed to infrequent wind storms from an unusual quarter can be troublesome.

On sites at risk it does not pay to be too kind. Do not encourage the crown by feeding the tree with fertilisers. Encourage the roots to forage for nutrients. Do not release them more than once, do not over protect from early wind exposure and do not use tree shelters.

With regard to pruning on fertile sites, be prepared to start as early as the second summer, lightening the crown by either removing branches or shortening them. Mixed planting may be helpful, but is not easily managed, and unless done correctly can make matters worse. The big mistake is to delay removing the nurse species.