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Notes on West coast blackwoods

Sunday, March 26, 2017, Ian Brown's Blog

On an evening in Southern Chile a few years back I was sitting at a long table in a German brewery with a group of New Zealand friends. We were sampling a range of local beers, a rite of passage in those parts. Across the table were Catherine Van Paasen and Glen Crickett from Hokitika. As time passed I noticed that Catherine was looking at me with an expression of increasing distaste, when she announced "Ï cannot stand the sight of you a moment longer".

Not a good start to a 10 day forestry trip. Catherine then reached across, removed my spectacles, studied them critically, then reached into her bag and extracted from it an Optometrists First Aid Kit ( Catherine is the resident optometrist in Hokitika). What followed was the equivalent of an archaeological excavation as she removed consecutive layers of detritus accumulated over time. When I replaced them the world was transformed. I could now read the labels on the row of bottles, which in order to avoid offending our hosts, we felt obliged to empty.

If Catherine has a hobby of rehabilitating distressed spectacles, Glen has taken a similar approach to their 17 Ha lifestyle farm near Hokitika, where we looked at his blackwood plantations at the FFA Conference last year. In striking contrast in scale and management, the blackwoods in the Ianthe Forest which we visited later form part of a group of blackwood plantations in South Westland, comprising in total 1600 ha., that dwarf any other blackwood plantations in New Zealand or Australia. According to Gordon Bradbury, the largest blackwood plantation in Australia is just 25 hectares. (The naturally regenerated forests are of course much bigger).

Although greatly different in scale, both presented a similar challenge: to establish an exotic plantation on sites that are common in South Westland. These have shallow and fragile soils formed after the last glacial retreat, and rendered infertile over time as a consequence of relentless rainfall.

Ianthe Forest. A replacement for rimu.

The history of the West Coast blackwoods has been expertly covered by Ross Jackson in articles in the August 2006 Tree Grower, and in the second edition of the Blackwood Manual.

When the rimu forests of South Westland were brought into the conservation estate, with a consequent loss of jobs, the government of the day cut a deal with the local community. In compensation, a plantation forest of 10,000 ha. comprising specialty species would be established in cutover forest. It was matter of determining the most suitable species.

In the 1960s, a range of trials covering a large number of species had been established by Forest Research in Central and Northern Westland. When these were later assessed, the majority of species tested had died, or had failed to thrive. The shortlist rapidly narrowed to two main contenders: blackwood, and cypress.

Blackwoods had been planted in a trial at Ianthe in 1979, in lines cut in scrub regenerating after rimu extraction , and initial growth was promising.

Moreover as the main component of the swamplands in North-West Tasmania, blackwood had good credentials for tolerating the hostile soil conditions that support rimu-dominated forests. Blackwood and rimu are both tough critters with a range of site tolerance, and although they perform best on mesic soils, they can survive and outcompete most of their broadleaf rivals on waterlogged sites.

Extensive plantings were carried out in the '80s, slowing down in the '90s as certain problems emerged. Like much of Westland, the forests had a mosaic of soil conditions, with good and bad sites. After a promising start there was a decline in growth and vigour, especially on poorer soils. Insect pests, chiefly psyllids and leaf miners, became a problem. In addition it was becoming less environmentally acceptable to plant an exotic species in naturally regenerating forests. Planting stopped well short of the initial target of 10,000 ha., but at 1600 ha. it is still a substantial resource.

In response to the experience gained from previous plantings, Timberlands West Coast, the most recent of a series of entities that have managed the forests, later modified the regime:

  • planting was now carried out on a more modest scale, and limited to good quality sites.
  • a suitable nurse crop or scrub species had to be present. Lines were prepared by bulldozer, with as little soil disturbance as possible.
  • early and repeated form pruning was found to be essential. Form pruning had been carried out at about 6 to 7 years. This was found to be too late to limit malformation, and resulted in a high percentage of short butt logs.

The outcome is reported as a mixed bag. There are some very good trees, but others of poor quality. There are 240 ha. in which the trees have a pruned stem of over 4 metres, and in addition a substantial number are pruned to over 3 metres.

Viaduct Creek

At Viaduct Creek Glen has converted a wilderness into high quality pasture. This has been an achievement, but he has also managed to grow quality trees on a site that most of us would find intimidating. The property had been infested with weeds. The topsoil was thin, waterlogged and acidic, with drainage impeded by an impenetrable iron pan. As we walked on a path from the oldest the most recent plantings we followed the trajectory of a learning curve. Glen soon found that methods in use on benign northern sites do not necessarily apply here, and has had to adapt his management to local conditions.

In contrast to Ianthe, the existing vegetation at Viaduct Creek was not suitable as a nurse. Glen started by planting blackwoods in a mixture with E. nitens, and soon found that this was a mistake. I had the same experience in Northland in a 1982 planting. You can become seduced by the eucalypts and reluctant to sacrifice them when they have served their purpose. Before you know it the nurse has become a predator, raiding food and light. The nitens are thriving.

Glen then shifted to open planting combined with form pruning. For this to work well you should have a good site supporting rapid early growth. Here the trees struggled at first, and early pruning was unhelpful. Glen resolved this by delaying pruning, and eventually settled for a system combining leader training and gauge pruning, as suggested in the Growers Manual, and the benefits are apparent in his more recent plantings.

At first, financial constraints were an obstacle to improving drainage by soil preparation, and when this was undertaken prior to later plantings there was a strong growth response.

On free-draining soils, blackwoods have an expansive root system and are very stable. However on this site, with a high water table the roots are shallow, with a risk of windthrow in high pruned trees. To reduce the risk Glen limits the pruned stem to 4 metres rather than pushing it to 6 metres.

He uses group planting, rather than planting in rows. I have used this system for years, using the following prescription: plant 3 or 4 trees per group, 1 to 2 metres between trees in the group, and 7 to 8 metres between groups. Form prune, and thin to one tree per group when the selected tree in each group has a 2 metre crown above the pruned butt of 6 (or here 4) metres.

Advantages in group planting for blackwoods include:

  • a high selection ratio. All the trees planted are available for final crop selection.
  • final crop blackwoods should be evenly spaced, to allow a symmetrical crown to develop. This avoids the problems caused by an asymmetrical crown - curvature and tension effects in the stem. Final crop trees will be spaced at 7 to 8 metres apart. With row planting therefore, many good trees will have to be sacrificed, not because they are growing poorly, but because they happen to be growing in the wrong place.
  • I think what follows is probably true: the trees within the group act as mutual nurse crops and interact with each other, eliciting a shade avoidance response. I have discussed this in a previous blog. This should result in faster height growth, delayed conversion from the juvenile to adult growth, and smaller and more orderly branches. (Someone might like to trial it, using single open-grown trees as a control).

The outcome of all of Glen's efforts is an impressive plantation, with trees looking as I think they should: well spaced, straight pruned butts, and symmetrical crowns. As the crowns expand into open space over time there should be a strong growth response in diameter.

Growth Performance

When looking at growth performance, how does the West Coast compare with northern locations? I will comment on some of the site factors linked to growth rates on blackwood, and on what are probably the best of the data available.

Site factors. Four of these are linked to blackwood performance:

  • temperature. A study in Tasmania has shown that warm temperatures are strongly associated with blackwood growth. In New Zealand it might be expected that blackwoods grow faster on warm northern sites than in the cooler south. This seems to be supported in the study that I will refer to below. The mean annual temperature in Hokitika is not very different (about half a degree cooler) from the blackwood swamp territory in North West Tasmania, the home of the best of their blackwoods. It is about a degree cooler than Rotorua, where most of our trial work has been carried out, and where there are some fine trees. In the balmy north, Kaitaia is about 4 degrees warmer.
  • shelter. In a study in 1979, Ian Nicholas showed that good shelter is strongly linked to blackwood growth performance. The West coast is breezy, but probably no more so than many parts of the country, and shelter is tied to local factors including topography rather than regional ones.
  • moisture. I havent heard any complaints of a lack of moisture on the West Coast. In Hokitika rainfall averages about 3000 mm per year. By comparison the annual rainfall in the blackwood swamps in Tasmania is about 1100 mm, not very impressive, and less than we get in Hamilton or Auckland. In the wet sclerophyll forest in North West Tasmania it is about 1500 mm. The blackwood swamps are dry underfoot for much of the year, but seriously wet in winter. They are floodplains, collecting flowing water from the local rivers, and drainage is slow. Blackwoods dominate the territory because they can tolerate those conditions, while most of their competitors cannot.
  • soil. Blackwood is a tolerant species, and will survive, even if it does not flourish, across a range of soil types. In the blackwood swamps it can cope with saturated soils provided the water is flowing and oxygenated, but if the flow is obstructed, for example by road construction, and the ground water become anoxic, they will die. To grow well, blackwoods need decent soil, moist but not permanently saturated, well drained, and of adequate fertility. I think they should therefore be regarded as site-specific. Timberlands became aware of that, and have for some years confined their planting to selected sites that have those characteristics.

Blackwood growth models.

The best data that I am aware of on relative growth and yields for blackwood were published in a paper in 2007, ''Preliminary Growth and Yield Models for even-aged Acacia Melanoxylon Plantations in New Zealand '', by Pascal Berrill, Ian Nicholas, and Ham Gifford. This was based on 1722 plot measurements from 229 sample plots, a formidable amount of work from Ian and Ham. Pascal, a bright young gun at Forest Research, who moved on to foreign pastures, contributed some impressive mathematics.

The key word is "provisional". The authors accept some unavoidable limitations in the study. Only a limited number of plots were approaching a millable stage - the mean age of the plots was just 9 years. Silvicultural treatment differed between plots, and there was a strong influence of site and stocking. However this is the best information that we have, and bearing in mind the withdrawal of funding from blackwood research, may be all that we will get.

The mean MAI values for different districts were as follows:

Northland 12 plots, 7.14 cu.m./ha. 
Auckland 51 plots, 5.57 cu.m./ha. 
Waikato 37 plots, 5.79 cu.m./ha. 
Bay of Plenty 101 plots, 4.91 cu.m./ha. 
Gisborne 12 plots, 4.14 cu.m./ha. 
Taranaki 5 plots, 4.59 cu.m./ha. 
Westland S.I. 11 plots,  3.87 cu.m./ha. 

These data show a decline in productivity from north to south, suggesting an influence of temperature. However I am not convinced that productivity in Westland is as bad as shown here. Most of the northern plots would have been located on sites that were selected as suitable (valley slopes for example), of modest size, and would have received more intensive silviculture. Some of the Westland plots would have been on less suitable sites, and because of the scale and cost of the operation would have had less attentive treatment.

The growth model came up with the following conclusion: for an average site, thinned to 200 stems per ha. by age 10, you might expect a Mean Top Diameter of 49cm by age 35. For an above average site this rises to 52 cm. This is some way short of 60 cm, which is achievable only on high quality sites (top !0%), and at a lower stocking of 100 trees per ha.

How does Ianthe fit in here? On our visit we were given some plot data from several plots. Only one of these had been thinned to an acceptable level. When measured last in 2007, at age 28 and at 190 sph, the MTD was 43 cm . This is located on the growth model curve for an average NZ site at that age, which predicts a likely figure of 49cm MTD at 35 years. Notwithstanding the MAI values referred to above, I would suggest that based on these figures and on the trees that we looked at, the performance of blackwoods on decent soils in Westland is not likely to be very different from average sites in the North Island.

However 49 cm is some way short of 60 cm. The surface area of a 60 cm diameter stem is 50% higher than one of 49 cm and in addition will have a higher conversion value.

So is 60cm at 200 sph at 35 years achievable? I think it is but it needs a good site, and attention to one or two silvicultural details. In the Blackwood Manual there is a growth curve for a good North Island site that has been well managed. At 200 sph on that site the MTD is predicted to reach 60 cm by age 35.

I will refer to some personal experience. I have a plot on a site in the Far North which we planted in 1980. It was part of an abandoned farm, and has some good features. It is located on a north-east facing slope in a sheltered valley. It is close to a range of hills, and rainfall is about 2000mm, although it can be dry in summer. The soil is clay loam derived from ancient volcanics, sticky in winter and prone to crack in summer. The original vegetation was dominated by puriri. In Northland, as a site indicator, puriri is good news, kauri is bad news. We planted in groups and interplanted with pines, which we felled at age 6, and form pruned the blackwoods annually from the first year (these were probably the first blackwoods to get annual form pruning), and thinned to 240 sph by year 8. When last measured at age 31at 230 sph the MTD was 57 cm, well on the way to reach 60 cm at or before 35 years.

In the Waikato I have blackwoods growing in a sheltered valley on the lower slopes of Mt. Pirongia. It is a few degrees cooler than Northland, but the free-draining volcanic ash soil is better, and the rainfall about 2500 mm. In 1992 I planted blackwoods in groups, carried out annual form pruning, and all the pruning and thinning was completed by year 8. Ian and Ham and I put in several sample plots. At 24 years they lie on the same growth curve as the Northland plot, and should reach 60 cm dbh by 35 years.

I have looked at many well managed North Island sites where the growth rate is similar, and a few where the trees are better than mine. So on good sites and with one or two silvicultural tricks a 60 cm stem is achievable by 35. However on average sites it will need more time, or lower stocking. But what is so special about 60 cm, or 35 years?

The Target Tree

It is useful to have an objective when growing trees, and a commonly quoted formula for blackwood is "a 6 metre log, at 60 cm dbh, at 200 sph, by age 35 years". I am not sure where this originated, but as it appears in the Growers Manual I have to share some responsibility for promoting it.

However there are two questions that might be asked:

Is this achievable? The short answer is yes, but as I have suggested, only on very good sites, and with appropriate silviculture. On average sites you will need to wait beyond 40 years to reach 60 cm., and without timely thinning that could be extended to 70 years.

Is this necessary? I will take apart the four elements in the formula, bearing in mind that they are interlocked.

  1. The 6 metre stem. This appears to have been derived from radiata pine management. Blackwoods are different, with quite distinct end uses. If you send off a 6 metre blackwood log the miller will be grateful, but it is more than likely that he wont need 6 metres and will promptly cut it in half. It is a simple matter to prune blackwoods to 4 metres - the first three metres can be dealt with from the ground. To extend from 4 to 6 metres requires more time and effort, and in the real world both time and effort are often rationed. In addition there might be special circumstances, such as the possibility of windthrow on shallow soil, as on Glen's property, where a longer stem puts the tree at risk. So although 6 metres might be ideal, I don't think that anything less should be regarded as a failure. Where conditions are tough, 4 metres could be a more realistic target.
  2. 60 cm diameter. From the perspective of a miller, the diameter of a blackwood log is more important than its length. That is how it is seen in the marketplace. In South Africa for example, blackwood is sold by auction, and a premium is paid for large diameter logs: a cubic metre from a fat log is worth more than a cubic metre from a longer thin one. People I have spoken to who have milled blackwood have indicated a clear preference for large diameter. Unless there are time constraints why stop at 60 cm? Why not aim for 70 or 80 cm? At 200 sph that would mean a long wait, but at lower spacing it could be done. Given time and space blackwoods can reach a diameter of over a metre. The biggest I have seen was a tree in a reserve in North West Tasmania, with a diameter of well over a metre, and it was over 15 metres to the first branch. There has been concern that fast growth might compromise wood quality. That has been resolved by two good studies, one by Gordon Bradbury in Tasmania as a theme for his Ph.D, and the other by Ian Nicholas. Both came to the same conclusion: the wood from fast growing blackwood is as good as from slow growing trees, and better in some aspects.
  3. 200 stems per ha. Blackwoods growing in natural stands tend to stabilise at about that number over time. In blackwood plantations, this seems to be the generally accepted figure, as a compromise likely to provide trees of acceptable dimensions (60 cm) at an acceptable age (35). To satisfy both values, on average sites the number might have to be lower if you want to harvest within a reasonable time frame. Certainly the number should not be higher than 200, or not far above it. As I will suggest, what is important is not simply the final number, but also the timing at which thinning is done.
  4. 35 Years. Don't expect to harvest trees of 60 cm dbh at age 35 unless you have a very good site. On an average site it will probably mean waiting to 40+. If 35 years is still the target, you will have to thin to lower numbers, perhaps 100 sph, or accept a smaller diameter. In compensation, if you are prepared to wait beyond 35 years, you are likely to hit an improved market value, and there is anecdotal evidence that the wood darkens and develops more character with age.

A few years ago I asked Don Britton what he considered to be an ideal blackwood log. Don could be considered an expert. He ran Brittons mill at Smithton in North West Tasmania, which processed a third of all the blackwood milled in Australia. The logs they get, taken from naturally regenerated forests come in all shapes and sizes, long and short, bent and straight, fat and thin. He has had to deal with whatever arrived on the trucks. But for an ideal log his prescription was simple: length is not important - longer is better, but anything over 3 or 4 metres will do. They should be straight and knot free (I know that knots provide some interesting grain patterns, but the millers prefer clear grain, and our job is to give them what they want rather than what we think they should have). And the fatter they are the better.

To satisfy the millers, the silvicultural prescription for blackwood is simple: for logs that are straight and knot-free, they need form pruning, with or without a nurse. And for large diameter, they need space for the crowns to expand, which means timely thinning.

Stem Management

Over the last 30 years the trend in blackwood silviculture in New Zealand has been to shift from the use of nurse crops to reliance on form pruning. Nurse crops, in practise eucalypts or pines, can cause problems of their own, and my views on that are covered in the Growers Manual.

However if a suitable nurse is already present in the form of regenerating scrub, it makes good sense to use it. At Ianthe, scrub was regenerating for 10 years after milling the original forest, and this proved to be an effective nurse. When using a nurse, the frequency of form pruning is reduced, but is still necessary if you want a straight stem of adequate length. Delay in form pruning has been a problem at Ianthe.

The principles and methods of form pruning are well understood, but I will make a few comments related to form pruning applied to trees grown in the open, without the support of a nurse.

Which trees are suitable? A normal prerequisite is a good site that encourages vigorous early growth. On a good site, with weed control and good planting technique, a percentage of seedlings should reach 2 metres or more in the first year. Where these conditions are not met, and the trees are struggling, reliance on form pruning without a nurse becomes much more difficult.

When do you start form pruning? I normally wait until the trees have reached at least shoulder height, which might mean late in the first summer , certainly by the second year. For annual pruning there is evidence that late spring is the best time.

Which method is best? The basic principle is simple: visit the trees annually, reduce competing leaders to one shoot, and remove or shorten any large vertical branches. How the branches are selected is a matter of choice. It can be done visually, which is quick and easy, and with extended pruners restricts the need for a ladder. Or you can use a 3 cm caliper as a surrogate, a method that eliminates the need for judgement, and designed for people with no prior experience. Or both can be combined, and this worked for Glen at Viaduct Creek. I trialled both methods on several hundred trees at Pirongia, and when I look at them 20 years later there is no difference between them. Both methods work well.

How much foliage can you remove? When the trees are growing strongly they are quite forgiving. My daughter Nicola did a trial at Pirongia as a component of her science degree, which involved pruning blackwoods at different intensities. With the most aggressive regime there was no effect on height, and a minimal impact on diameter. Libby Pinkard, a Tasmanian forest scientist, studied the effect of different intensities of pruning in a blackwood trial, and found that removing 30 % of the foliage in one hit had no effect on either height or diameter. At 50% there was some effect on diameter, but none on height. So as a general rule, provided the trees are growing well you could remove up to a third of the foliage in one season without compromising growth.

Crown Management.

A blackwood crown is unlikely to have any commercial value other than as a source of firewood. The wood turners will descend on it like vultures, and can be encouraged as they will give the timber some market profile, but dont expect to get paid for it. The crown should be regarded simply as a photosynthetic factory, and its value lies in the contribution it makes to the diameter of the stem. For that it needs space to expand, and it should be allowed to do so while retaining live branches near the base of the crown.

That can only be achieved by thinning, and by thinning as soon as practicable. I think the thinning to final crop spacing should be complete as soon as the crown has extended 2 or 3 metres above the top of the stem. This can be as soon as early as 6 to 8 years on a good site. That might seem radical, but a delay in thinning can exact a heavy price, which is loss of diameter in the stem.

When thinning is delayed, the crowns interact, and can retreat at disconcerting speed, and the outcome is the typical forest blackwood with a small high crown perched on one or more long slender stems. The tree is now condemned to slow diameter expansion and a long rotation.

The blackwoods at Viaduct Creek bear little resemblance to the tall and elegant examples found in natural forests, but this is not a beauty contest. They look more like the unmanaged broad-crowned blackwoods scattered at random across farmland in North West Tasmania. The difference is that unlike the short-stemmed Tasmanian trees, Glen's trees have stems of millable length.

I am hesitant to comment on the trees at Ianthe because we entered only a small section of the forest. So I might be wrong, but this is how I would interpret it. I think the same principle related to the importance of early thinning might apply to the trees we looked at. They were about 35 years old, Most of them were tall and slender with high crowns and in close proximity.

Scattered at wide spacing among them were trees that looked quite different. These had straight clear stems over 4 metres, presumably from pruning, and most strikingly they had much greater diameters. The best of them was a fine tree with a 60 cm diameter. What appeared to distinguish these from the rest was apparent when you looked up into their crowns. These had expanded above the main stem. To do so they must have been provided with space at an early age. These trees will soon be ready for milling. The rest will take a lot longer.

Insect Pests

Acacia psyllids are ubiquitous in blackwood plantations in New Zealand. I do not think they should be held responsible for branching and malformation. This is a consequence of shoot tip dieback, which is a feature of the periodic growth habit of blackwoods when they emerge from the juvenile to adult phase in the second or third summer. However they play a secondary role in contributing to malformation, and they also cause a reduction in extension growth. Although they are a nuisance rather than a problem on northern sites, we would be better off without them. Fortunately they do not seem to be a problem in the established canopies of older trees.

However in the Westland forests they appear to have been a much more significant problem, and have been linked to a decline in the vigour of the trees after a promising start. The reason for this is not clear, but a number of suggestions have been offered. One is that the trees have been stressed by their environment, in particular the soil conditions. Commenting from the sidelines rather than the field of play, this seems to me a likely explanation. When blackwoods enter their adult growth phase in their second or third year, they become susceptible to pysllid predation. Psyllids attack shoots that are beginning to wilt prior to aborting their tips when they terminate a growth period. Juvenile foliage, where growth is continuous rather than intermittent, is of no interest to them. However they will attack juvenile foliage if this is stressed experimentally, as I have described in a previous blog.

Another factor might be the exceptional size of the Westland forests. This might have contributed by causing a critical mass effect which encouraged an explosion in insect populations .

Whatever the explanation, psyllids are likely to remain a problem in future planting, and will need some strategies for control. The current approach in Westland, where blackwoods have been more recently planted on selected sites where they are separated by other species, should produce healthier trees and reduce population densities.

Then there is the possibility of genetic resistance among the natural blackwood provenances. In a detailed study of acacia psyllids, Clive Appleton looked at 6 different provenances and found that all were susceptible. On my property at Pirongia we have a trial of over 60 Australian provenances, extending from southern Tasmania to the Atherton tablelands in Queensland. This was set up by Ian Nicholas and colleagues at Scion. I provided some slave labour. Although it was not part of the agenda, I took the opportunity to look at all the trees in their second and third summers, when the psyllids were very active in the hope of finding some that might be resistant. It was disappointing. All the trees were hit.

Well, not quite all. There was one of the NSW provenances where the psyllids were clearly not interested. I had my doubts about it. The trees were a little different from the rest. Their phyllodes are long and thin. This is generally true for provenances from the north, and from dry inland sites, but these were different. The flowering time was also a little different from the rest. I wondered if an intruder might have infiltrated the party, and sent a sample of foliage to Scion. A rather huffy reply came back:"Of course it is blackwood". I still have a lingering doubt, but for the record the provenance is Lawlers Creek near Stroud in NSW. The trees are vigorous, although like the other NSW provenances, it is very branchy (so much for psyllids as the cause of branching and malformation). As it happens, Stroud is located just north of the Hunter river, which is a location of interest in blackwood genetics. A study of blackwood provenances has shown a genetic disjunction corresponding to the Hunter river, in which blackwoods north of that location appear to be separating from those in the south. A distinct flowering time might suggest that the Lawler's Creek provenance is forming a distinct subcategory. Or I might have got it completely wrong.

A few years ago I was travelling through Tasmania with a small group, including the Davies-Colleys. Richard and I had a shared interest in a little beast, the Southern Ladybird, Cleobora mellyi. This dines on acacia psyllids as its primary food source, but will extend its menu to some of the eucalypt predators when given the opportunity. We had both been involved on behalf of the eucalypt and blackwood groups in projects supported by the Sustainable Farming Fund, and set up by Dean Satchell to locate Southern Ladybirds in the Marlborough Sounds, and propagate and then distribute them on suitable sites. We had a look at every blackwood we located where there was accessible foliage, from southern to northern Tasmania, and found the ladybirds on every tree we looked at. So in their natural environment they must have a role in keeping psyllid populations under control.

A few years ago. I spent several days with Dean Satchell in the Marlborough Sounds hunting down Southern ladybirds. They had survived there following an introduction in 1975. We brought back enough to initiate a breeding program and eventual distribution to about 15 sites. I am not sure I contributed much to the project. Dean, with help from colleagues at Scion did all the work, and deserve credit for it. Cleobora are now well established on my property at Pirongia, and I find the occasional example sunning itself on a warm day on a fence post, presumably digesting a meal of psyllids. To my knowledge, they have not been released in Westland, where they may otherwise help control psyllid numbers.

Another insect predator that has been of concern in Westland is the leaf miner. This seems to have been a lesser problem, and is reported to be under control by two species of eulophyd wasps.

Future management.

It seems to me that on Glen's property, all the essential work has been completed. The chief concern will now be to keep Glen out of the crowns, where he has some ideas of carrying out further "corrective" surgery. I'm not convinced that an attempt to extract segments of straight timber of adequate diameter from the crowns would be economically viable, and any pruning that might reduce crown volume would compromise volume growth in the main log. However there is one problem in a blackwood crown that needs correction, although it is better dealt with by prevention. That is a crown that starts with a fork at the base. These have an unfortunate habit of splitting apart in strong winds, and can peel open to the base of the tree. It can be prevented by early recognition of a double leader. If recognised too late, one of these is better shortened rather than removed at the base, as the remaining leader can then peel off, leaving a solitary stem. I have done this more than once, and it usually manages to attract the interest of someone with a camera. A fork higher in the crown does not matter. It is much less likely to split, and if it does the damage is likely to be confined.

After some excellent work Glen is now entitled to relax a bit, and spend some time admiring his handiwork. However knowing the guy, I woudn't hold my breath.

Having visited one small segment of the Ianthe forest, I am in no position to comment on future management, but for the sake of argument let us assume that the pattern we saw there is replicated through much of the rest. First, some general points:

  • it contains a number of trees of good form and decent diameter, of over 50 cm. These are approaching a time for milling ( unless there is an alternative market, perhaps best delayed until the last of the windfall rimu from cyclone Ita have been extracted). These appear to be the product of pruning and early thinning. Selective extraction of these trees should not be a problem to West Coast foresters, who have had great experience in selective harvesting in native forests.
  • these are mixed with a larger population of tall thin trees with small high crowns. These are well short of milling. Experience in Tasmania is that unthinned blackwoods take about 70 years to reach millable size, and even them can be marginal.
  • after more than 30 years, the soil under the trees will contain a large seed bank of viable seeds . A study in Tasmania, based on the pattern of blackwood regeneration in sclerophyll forest and rainforest at different intervals after fire, has shown that blackwood seeds can remain viable in the soil for 200 to 300 years, ready to germinate after soil disturbance. Whatever land use is planned for the future, blackwoods once established will remain a continued presence. So there is a case for accepting that fact, and making the best use of them. If a decision is made to allow regeneration to natural forests, blackwoods will inevitably form part of them. Fortunately in New Zealand, in contrast to South Africa, where birds are vectors, it has been found that blackwoods will not spread outside sites where they have been planted, or into existing native forest.
  • after milling blackwoods, it could be expected that the disturbed site would be rapidly occupied by blackwoods growing from ground-stored seeds, and from damaged roots. There would be no need for further planting.

After selective harvesting, the next question will be how to manage the remaining trees, and the gaps created.

Managing the gaps. The gaps created by milling are likely to be rapidly occupied by a cluster of young blackwoods growing from ground-stored seeds and damaged roots. The aim should be to reduce these after a few years to one tree per space with a straight stem and a spreading crown. I would expect that with uninterrupted growth, competition for space, and modified light from above, that these would show better growth performance than the originally planted trees. My approach would be to allow them to compete, carrying out light form pruning on the best of them, and delay thinning until they are over 6 metres. Then thin in two stages to one tree per gap, allowing its crown to occupy the available space. They should be ready for harvest in 35 years or so.

Managing the rest. These should also be ready for harvest in another 35 years or so, allowing the possibility of a clearfell, which is the best and simplest way of managing blackwoods. It might be asked should they be thinned now? I would have some reservations. I think any growth response would be very limited. The crowns of these trees are very high, and would have a very limited capacity for expansion, and this would be at the risk of instability. The crowns are interlocked, and thinning could be technically difficult. Any felled trees would obstruct entry for ongoing silvicultural work on the regenerating trees. Finally, the newly exposed trees would be susceptible to windthrow.

This is all theory of course. The Westland forests are under new ownership with Ngai Tahu, and their future management will no doubt be carefully considered, and will be based on not only economic, but also on cultural values. I wish them well. They have a substantial resource, with long term potential in an international market. 

One post

Post from Brian Finch on April 21, 2017 at 4:56PM

Hi Ian, thanks for such a comprehensive and fascinating account - you've given so much valuable information. I'm out to measure my blackwoods soon!  My question is about the market for 35 year old or 60cm diameter logs. Is there a pool system for selling logs through or mills that buy such logs? Keep up the blackwood blogs,  Brian

Disclaimer: Personal views expressed in this blog are those of the writers and do not necessarily represent those of the NZ Farm Forestry Association.

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