NZFFA Guide Sheet No. 3: Eucalypts for Timber
Introduction
Eucalypt species
Eucalypt Establishment
Eucalypt Silviculture
Introduction
The eucalypts (gum trees) offer some excellent hardwood timbers that
have properties distinctly different from, and complimentary to,
radiata pine. In particular eucalypt timbers are:
- stronger
- harder
- heavier
- more durable
- attractive in appearance
However, there are over 600 species of Eucalyptus. Many are poor
timbers, and even amongst the good timbers there can be problems in the
sawing, drying and processing.
An understanding of the timbers behaviour is essential for successful
sawmilling. Leaning trees are prone to splitting and twisting due to
growth stresses.
The different species and provenances (regional seed sources) vary
markedly in:
- site requirements and site tolerances
- growth rates
- form
- timber properties
- ease of sawing and processing
- susceptibility to insect pests and diseases
- coppicing ability
By comparison, radiata pine seems relatively even and tolerant of a
wide range of site and climatic conditions.
In considering timber properties, one group of eucalypts is clearly
superior. The Stringybark Eucalypts offer several advantages:
- They are the easiest eucalypts to saw and process with only
limited growth stresses and few problems in drying.
- They have hard, strong, durable and attractive timbers that are
inter-changeable and useful for a wide range of uses from fence posts
to structural timbers, flooring and furniture. For “on-farm” uses they
are about 70% stronger and more durable than C. macrocarpa.
- They grow well on low fertility, dry sites.
- They have few insect pests.
Some may be able to be milled on shorter rotations .i.e. milled at
smaller diameters than many other eucalypts.
On the other hand, they do have some disadvantages:
- Most of them require fairly warm sites (northern slopes) and many
are frost tender.
- They prefer free draining soils and are prone to disease on wet
sites.
- Form can be variable with a tendency to throw double leaders.
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Eucalypt species
E. muelleriana.
This produces probably the best timber of the group. It is a vigorous
tree growing to a large size, but the form can vary with some trees
forked. It has performed well on a wide range of mainly warmer sites
throughout the North Island including heavy Northland clays and coastal
sand dunes and turns up quite often in some of the early eucalypt
plantings. However it is considered quite frost tender, but grows in
cool areas where there is sufficient air drainage. There are some
markedly better provenances round, though without hard research data to
support them. Check with local farm foresters.
E. pilularis.
Though not strictly a stringybark, it has almost identical timber to E. muelleriana. Another vigorous
tree of variable form that grows to large sizes. Similar site
requirements to E. muelleriana,
frost tender and prefers coastal sites.
E.
globoidea/eugenioides.
Two
closely related, easily confused species, generally smaller and less
vigorous trees than E. muelleriana
with somewhat different site preferences. To describe them as
tolerating harder sites may be an over-simplification. Frost tolerance
and form varies with provenance, but at best they are impressive trees.
E. laevopinea
has not been
widely grown in New Zealand to date but appears to combine vigour,
fairly good form, large size, good wood properties and much better
frost tolerance than the above species.
There are a number of other promising stringybarks such as E. agglomerata, E. baxteri
and the cold tolerant E. youmanii and E. caliginosa.
These, along with a
few more species have been included in research trials commenced in
2003.
The Ash Eucalypts are species from cooler, higher rainfall and often
higher altitude areas of south eastern Australia. They were so named on
account of a perceived similarity between their timber and European
ash. They are common and grow well over
much of New Zealand favouring moderate to well drained sites with
adequate rainfall and often cooler temperatures. Their timber
properties are not as good as the stringybarks, in particular there can
be problems with collapse during drying, and internal checking, but
they will grow on cooler sites. Generally they have few insect problems
but can suffer root rots and fungal leaf infections. The common species
are:
E. fastigata
has the best timber of the ash eucalypts, being the easiest to process.
It is a fast growing, large tree that seems to be able to carry larger
volumes and basal areas per hectare than any other eucalypt. Form can
be variable when young, but this is related to seed source. There are
few insect problems, but leafspot fungi can appear under warm, humid
conditions. Frost tolerance varies with provenance from about -10°C to
about -14°C in some cases.
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E. obliqua
is probably the second choice amongst the ashes. It is a variable
species with a wide geographical spread. The best provenances are fast
growing, very large trees of good form. It is amongst the most site
tolerant of the ash group. The timber can be subject to gum veins.
E. fraxinoides
is vigorous but of poorer form, better known as a shelter species but
it mills well. It needs good drainage and will not tolerate wet feet.
E. regnans/E
delegatensis are other common ash eucalypts with merit. They are
fast growing but need cooler, inland sites with good drainage. E. regnans looks magnificent and E. delegatensis
is very frost hardy, but both have considerable defects in solid wood
during drying. Veneer is certainly a favoured option. Both species have
health problems (leaf spot) on humid sites.
E. nitens
is the most site tolerant of the timber eucalypts handling wet and
moderately dry sites, severe frosts and wind while maintaining very
fast growth rates and excellent form. These attributes combined with
its good pulping properties have made it
the most widely grown eucalypt in New Zealand. However, it does not
like warm, humid conditions and does have a lot of insect problems. For
solid wood uses are limited because of collapse and high shrinkages,
but young plantation material is being used for framing in Australia.
For decorative end-uses it requires quartersawing, and it does produce
good veneer that can dramatically improve the performance of radiata
pine based laminated veneer lumber.
Although many eucalypts related to E.
nitens
have been grown over the years, including a host of ornamentals, none
have really gained favour as timber species. However, recently there
has been recognition of the merits of E.
maidenii.
This species, planted quite widely up to about the 1950s, produces a
very high density, high strength timber that deserves more attention.
The Eastern Blue Gums produce very attractive, red timbers well suited
for flooring, furniture, decorative and structural uses as well as
having good ground durability.
They can suffer serious growth stresses making them difficult to saw,
but they dry and process well. Unfortunately, they suffer from a wide
range of insect pests with three or four serious pests having arrived
over the last 20 years. They cannot really be recommended until these
are under control or resistant strains have been identified. The two
main species are:
E. botryoides
varies with provenance from a scrubby coastal species to a tall forest
tree of moderate form. It tolerates saline winds but is prone to crown
breakage. On good sites with moderate or better fertility it can be
very vigorous and it will stand quite wet sites.
E. saligna
is a taller, better form tree but needs moderate to good fertility and
does not like saline winds. The provenances vary considerably. Other
species in this group include the swamp species E. robusta and E. grandis, which is a large tree
of good form widely grown in South America, but has been varible in
growth and form in NZ.
Other Species
There are numerous other timber species that have been grown
successfully in New Zealand and some notable options are:
E. microcorys
needs warm, frost free sites, but is rated the best quality hardwood of
eastern Australia. It produces an extremely durable, hard, strong,
attractive, light brown coloured timber that mills very well. It is
slower growing but will handle heavy Northland clays through to coastal
sand dunes.
Corymbia maculata
(was E. maculata) is a good
milling species for warmer sites on a range of soil types.
E. bosistoana
produces one of the heaviest, hardest, most durable timbers imaginable.
It will handle seasonally wet sites and significant frosting. E.
melliodora is the dry site equivalent. Both are excellent nectar trees.
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Eucalypt Establishment
In different areas and environments, eucalypts have been successfully
established almost every month of the year under a variety of regimes.
However, we would recommend the following for stringybarks:
- Site selection, plant on best site available, preferably with
shelter and good air drainage.
- Cultivation, this is critical to successful establishment,
minimum treatment is spade cultivation, but it is better if the soil
can be broken up.
- Releasing of eucalypts from weed competition is vital, but needs
great care. They are sensitive to many of the commonly used chemicals,
especially when used at higher rates. Gardoprim, Gallant and Versatill
have all been used successfully, but species, temperature and soil type
all affect sensitivity. Avoid getting chemical on the trees, some cover
with shields. At higher temperatures (above 18°C ) and on light or
sandy soils, soil active herbicides are likely to affect eucalypts.
Some growers use glyphosate (Roundup), but this needs great care, calm
conditions, low nozzle pressure and preferably a fan nozzle. Releasing
young trees from weeds by hand can be successful, but requires good
timing and repeating before weeds recolonise and smother trees. root
competition means slower growth rates than with chemical released
trees.
- Use side-slot container grown stock 20-30 high.
- Cut off the bottom 20-25% of the root bundle if the container had
a solid bottom.
- Plant with slow release fertiliser tabs or put a trowel of high
nitrogen fertiliser such as DAP or urea in a spade slit 10-20 cm from
the seedling, about one month after planting.
- Spring planting is probably safest starting in August in the
north through September on warmer, drier sites to October, November and
even December oncooler, moister sites. Other eucalypts can be
established as above but many growers prefer to plant the more frost
hardy species as bare rooted stock in July or August.
- Interplanting with pines is cheaper than solid eucalypt planting.
Alternating rows of eucalypts and pines 4-5 metres apart with a total
initial stocking of 1000-1200 per hectare may work. Faster growing
eucalypts (i.e. most species on appropriate sites) should be planted a
year after the pines. Just remember which species you are trying to
grow and be committed to eucalypt management. Managing a mixture gets
more difficult as the trees get older. However, the more eucalypts that
are planted, the more good eucalypts there are to select crop trees
from. High stocking, between 1600 and 2000 trees per hectare is much
more expensive but allows the best selection of only the best crop
trees. However, timely thinning is essential to bring the stocking down
to an appropriate final crop.
- Avoid grazing with stock until trees are tall enough to damage to
the leader, eg sheep, 2-3m and cattle 3-4m.
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Eucalypt Silviculture
Many eucalypts will shed their smaller branches naturally, but pruning
from about age 2 to 3 years is recommended. Pruning should be done in
the driest part of the year, mid to late summer, and preferably
before the branches exceed 2-3 cm in diameter. Pruning gauges can be
useful. Double leaders need to be removed to a single leader, the
earlier the better. Annual visits are recommended.
Thinning is critical and should be done progressively as crowns touch
until the trees are 12-15 metres high and pruned to 6-7 metres. At this
stage thinning can be aimed at getting a final crop of 100-350 trees
per hectare, depending on the target log diameter and rotation length,
with trees reasonably evenly spaced. Production thinning can produce
low quality sawlogs. At this stage check your trees. Often there
will be only 3-5 branches in the next 2 or 3 metres of trunk and
pruning to 8-9 metres is an easy option. If there is a double leader
within a couple of metres of the pruned trunk, it may help to top one
leader, if this can be done safely.
If you are using contractors for silviculture, you may choose to follow
a structured regime:
Year 3 - form prune 300-400 potential crop trees per ha.
Year 5 - prune the best 200 trees per ha. to three metres and thin to
about 300-400 trees per ha.
Year 8 - prune 150 crop trees to six metres.
Year 12 - prune to nine metres when mean crop height reaches 25 metres
and thin to 150 trees per ha.
On a good site eucalypts should get to 70cm diameter within 30-35
years. It might be noted that in South America, eucalypts are grown for
solid wood on rotations as short as 16 years. Shorter rotations may be
possible, and stringybarks may suit a 25 year rotation.
Every locality is different and there is no substitute for local
experience. We recommend that you join and actively participate in your
local branch of the New Zealand Farm Forestry Association P O Box
1122, Wellington to find out what has succeeded
and what has failed in your area. Join the NZ Farm Forestry Association
Eucalypt
Action Group.
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