Official website of the New Zealand Farm Forestry Association

NZFFA logo: green conifer tree on white background

Join NZFFA >>

 Blackwoods and the nurse

New Zealand Tree Grower August 2006
Ian Brown

Trees as people

One of the risks in growing trees is a tendency to attribute human characteristics to them. This is the case when we employ the term nurse to describe trees that are used to protect or to modify the form of a target species. My wife is a nurse, so at this point I am beginning to feel uncomfortable, but for convenience I will go along with it.

The term may be used as metaphor, but metaphors can be misleading. In this context we are encouraged to take sides in one of the fundamental debates in forest ecology. Is the relationship that exists between trees essentially one of cooperation, or one of competition?

Trees as good mates, or natural born killers?

In nature, trees form part of an interlocking web of relationships and it is easy to find examples of cooperative arrangements that involve them. Trees form associations with fungi – mycorrhizae, bacteria – nitrogen fixation, and insects and birds – pollination. However it is difficult to see their association with other trees as anything other than a lethal contest.

Some trees belong to shade-tolerant species that have adopted a secondary role in forest succession. When young they may benefit by protection from wind, frost, and strong light, and by chance and over time, some of them will emerge through the canopy and find their place in the sun. However blackwood is a different category. They are intolerant of shade, and form part of the group of pioneer species that occupy disturbed sites. The response that it makes to its neighbours is not a cooperative arrangement, but a defensive strategy based on a perception of threat.

In its natural environment, blackwood makes its way in an ecological war zone in the shifting boundaries between eucalypts and rainforest species. Here its survival is supported by flexibility in its growth response to site and competition. On open sites, where fire is a constant threat, the growth habit of the tree is determined by the need for early maturation and seed production. The result is the familiar open grown blackwood, with a short bole and spreading branches.

On moist and fertile sites, survival is threatened by other trees which jostle together in competition for light. Blackwood adjusts to this by making rapid height growth, at the expense of branch production. Here the blackwood is not benefiting from the attentions of a kindly nurse, or being drawn up to the light by a supportive companion – it is responding to a lethal threat. The aim of mixed planting is to capture and control this response. It can be done, but it is not easy and there are pitfalls.

Lessons from nature

As eucalypts have been widely used as a nurse species, I will use them as an example.

One of the cliches in forest management is that silviculture is applied ecology. It is useful to know, for example, that radiata pine grows naturally in temperate climates, prefers free draining soils, and grows on the Californian coast as a monoculture on soils exposed by fire. This tells us that attempts to conform to politically correct theory by incorporating them in uneven-aged mixtures are liable to fail.

During milling operations in natural Australian forests, blackwoods are often extracted from coupes where eucalypts form the main crop. At 70 years the blackwoods have modest diameter but often good stem, a feature that has been attributed to their association with the eucalypts. It is now known that it is not the eucalypts, but a dense community of short lived shrubs that germinated at the same time that were responsible for the influence on stem form. To plant with eucalypts alone does not therefore replicate the natural ecology of blackwood. It simply exposes them to an aggressive competitor.

To introduce eucalypts into a plantation of another species is rather like inviting a group of skinheads to a family party, with a number of predictable outcomes.

Eucalypts as skinheads at the family party

They will come at a price specifically, the cost of seedlings, planting, and subsequent felling.

They cannot be trusted.  This is a dynamic system, and will need regular supervision to ensure that trees do not get crowded. If managed correctly, branching is reduced, but form pruning is still necessary. This has been increasingly recognised by Australian researchers who have extensive experience with eucalypts in mixed planting.

They will take all the food and drink. On fertile sites, the loss of nutrients may not be an issue. However, competition for water is a different matter. The invasive roots of eucalypts are superbly adapted to a dry climate, and on any day their leaves will continue to photosynthesise and shed water vapour until they eventually stop, gasping with thirst – or more correctly they tolerate a low water potential. I have no doubt that it is this unseen competition for water that is the main threat imposed by eucalypts.

They will turn out the lights. This is not so important as eucalypt crowns are relatively open, but shading is likely to further inhibit growth and influence crown architecture.

They will dislodge the residents. Blackwoods should be evenly spaced, to allow symmetrical crown development. Correct spacing is often compromised in order to accommodate the nurse, resulting in trees jammed into wide spaced rows.

They will block up the doorways. After the eucalypts are felled, access is difficult for further silvicultural work.

They will rough up the guests. Blackwoods can be damaged when the eucalypts are felled, especially if this is delayed, and they are then exposed to wind stresses. Whether decaying eucalypt foliage chemically inhibits plant growth in the field is undecided.

They will trash the house with their litter. This is a mix of bark, leaves and branches, which is highly flammable.

You will never get rid of them. The problem is that you may not want to get rid of them. After about six years, when the eucalypts have served their purpose and are ready to be sacrificed they look great and are clearly dominant. The temptation to retain them can be irresistible, they become increasingly dominant, and what started as a nurse then becomes the final crop.

They may set fire to the house. Not only the house, the whole neighbourhood is at risk. This is not a major threat in our climate, but worth considering. Eucalypt ecology is based on fire, and you do not have to travel far in the Australian countryside to find hills blackened by fire in eucalypt forests.

Eucalypts as pets

In case any reader feels the urge to give one of their pet eucalypts a comforting hug, let me outline the nature of the pet. The eucalypt is a creature, born into a large family, which spends its childhood suffocating its siblings and then consuming their bodies. It develops into the school bully, terrorising its classmates and stealing their milk rations. It regularly sheds its skin and amputates its own limbs. Finally it expires in a fire, for which it has carefully prepared the fuel, and in its death throes gives birth to the next generation.

I should point out that I have nothing against eucalypts, and grow them. They deserve much respect for their great timbers, their survival skills in a harsh environment, and for what one writer described as the strange melancholy that they bring to the Australian landscape. It is just that for slower growing species, struggling to find their space in a crowded suburb, the eucalypts do not make good neighbours.

Are pines any better?

Pines are not better, but different. If the eucalypts are the lords of the playground, then big dumb lumbering radiata also deserves respect.

Whereas the main problem with eucalypts is competition for water, with radiata it is light. Blackwood is intolerant of heavy shading, which it will encounter under a pine canopy. As a result growth is suppressed. The pines will provide lateral shading from about year four to year six, but then become dominant. The blackwood crowns are forced up, and the small high crowns that result condemn the stem to slow diameter growth. If thinning is delayed, the blackwood are exposed to wind stresses, and the felled trees make access difficult.

As with eucalypts, if pines are to be used successfully as a nurse, they must be felled as soon as they have served their purpose, which is as soon as the five or six metre clear blackwood stem is in place, and its crown begins to emerge above it.

Planting in scrub

Some good plantations have been established by planting in regenerating scrub. The nurse is in place, it is not too vigorous, and its height is restricted. To avoid getting lost, or missing trees in subsequent visits, it is best to cut lines in the scrub rather than plant in gaps.

After planting, the work is not over. Regular visits are still needed, to maintain gaps and carry out form pruning.

Having your cake and eating it

Many growers have been attracted to the idea of retaining the nurse, and eventually milling both species. Good luck to them. To my knowledge no-one has succeeded in doing so in a plantation, and there have been some impressive failures.

Commonly the management of both species is compromised. I do not believe it is possible to grow pines to millable size and also extract a blackwood crop. It might be possible to retain eucalypts and extract them for pulp at about age 15, but I am sceptical. On the few sites where I have seen either pine or eucalypts used successfully as a nurse, they have been sacrificed at an early stage.

The alternative – form pruning

Australian researchers have persevered with nurse crops, principally eucalypts, but now accept that form pruning is still necessary, although its frequency is reduced. In New Zealand we have come increasingly to abandon mixed planting, and replace it by form pruning. This has proved to be effective, and avoids the complications of nurse crop management.


Farm Forestry - Headlines