No. 24 Establishing native hardwood trees for timber
NEW ZEALAND FARM FORESTRY ASSOCIATION INFORMATION LEAFLET
There is in New Zealand a constant demand for high quality wood
products made from native species, including hardwoods, principally for
joinery, furniture and the tourism markets. As greater restriction is
placed on extracting timber from the dwindling natural forest estate,
the most logical long-term solution is to establish new hardwood
plantations managed for, amongst other values, timber production.
A survey of native tree plantations by Forest Research in the mid-1980s
(Pardy et al. 1992) identified species that showed promise for growing
or managing for timber production in plantations. The two most commonly
planted and best performing hardwoods were the predominantly North
Island
species, puriri (Vitex
lucens) and rewarewa (Knightia
excelsa). However, other hardwoods with well-regarded
timber qualities (visual and working) include kohekohe (Dysoxylum
spectabile) and mangeao (Litsea
calicaris).
Despite being healthy, having high survival and good growth, stands
that were established without a sheltering nurse crop often had little
usable timber due to poor form. In contrast, stands growing within or
surrounded by a tall cover of vegetation produced straight boles
despite the frequent lack of
tending.
Therefore a research programme was initiated to investigate methods to
improve on what had been observed during the survey.
Sites
with existing natural scrub cover
Artificial nurse crops
Recommendations
Conclusions
Sites
with existing natural scrub cover
(“If you’ve got it – use it”)
Rather than removing existing high vegetation before planting, where
possible and practical such cover should be regarded as a nurse crop
and an asset for the development of new plantings. These nurse crops
may comprise single or multiple species native cover, frequently manuka
(Leptospermum scoparium) and kanuka (Kunzea ericoides), or mixtures
with exotic species e.g. gorse (Ulex europaeus). Forest Research has
established a number of trials on the East Coast of the North Island
where hardwood seedlings have been planted beneath existing vegetation
using different methods of establishment including lane and gap
planting.
Lane planting:
Lanes have been cut in scrub to allow planting of a
range of indigenous species and to compare different lane widths and
their influence on survival, growth and stem form. Lanes vary between 1
and 6m wide, with widths based on the height and vigour of the
surrounding vegetation and
the individual requirements of the species being planted. Initially
lanes exceeding 3m increase the incidence of frost damage to young
seedlings, although over time lane widths narrow as further growth of
the scrub occurs. An
undisturbed dense canopy suppresses or slows seedling growth and/or
results in damage or hindrance to emerging leaders.
Although stocking rates have not been tested thoroughly at this stage,
it is unlikely that there is a need to exceed 400 to 500 stems per
hectare of the desired hardwood tree species. Lane centres 8m apart and
seedling spacing of 3 to 6m will result in stockings of 200 to 400
stems/ha. In one trial
using this method of establishment, puriri, titoki (Alectryon excelsus)
and rewarewa planted in 3m wide lanes are up to 6m tall after seven
years, with predominantly single leaders. Estimates of costs in 1997 to
cut 3m wide lanes in scrub with minimal understorey over 1ha of
scrubland using 8m lane centres are $500 @ 44c/m.
Gap planting:
An alternative to lane planting is to establish parallel
access tracks through scrub and at regular intervals cut circular gaps
4 to 6m in diameter within which 3 to 4 seedlings are planted. A
planting pattern with access lanes at 10m intervals, gap centres at 15m
intervals along lanes and 3 to 4 seedlings per gap equates to a
stocking regime of 180 to 240 stems/ha. In a trial in the Auckland
Region, puriri planted in gaps in 4 to 6m manuka cover were 2 to 3m
tall after 4 years, but did not show as strong a tendency for single
leaders as those planted in lanes. Canopy gaps exceeding 6 metres in
diameter appear better suited to light-demanding species, but will
place frost-tender hardwoods at risk.
(top)
Artificial
nurse crops
A number of trials have been initiated to establish nurse crops on open
pastureland prior to the eventual planting of hardwood timber species.
The aim of these trials is to determine appropriate species and
planting patterns for nurse crops, and to establish woodlots of
hardwood timber species for
future research.
After almost 10 years the three most successful nurse crop species are
manuka, kanuka and tree lucerne (Chamaecytisus
palmensis), although akeake (Dodonaea
viscosa)
has also shown good potential. Nurse crops were established as
single-species blocks, and to date a number of stocking
regimes have been tested ranging from 800 to 2500 stems/ha (3.5 x 3.5m
– 2 x 2m).
The advantages of higher stocking rates have been better weed
suppression and the development of a sheltering cover for the final
crop species sooner than for lower stocking regimes. Advantages of
lower stocking rates revolve around the cost of establishment as fewer
seedlings are required but releasing and weed control costs are higher.
At the highest stocking rates, frost tender hardwood species were
planted within 3 years of the establishment of the tree lucerne and 4
years for the manuka/kanuka nurse crops. At the lower stocking rates,
the establishment of frost tender hardwoods was delayed until 5 years
after planting. For
all planted nurse crops canopy closure occurred within 4 years of
planting with most species having attained a minimum of 3m in height.
Although not tested in these trials, other species known to be suitable
for use as nurse crops, either in mixtures or as pure species cover
include the Pittosporum
and Coprosma
species.
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| 90-year-old totara at Puhipuhi in Northland. |
Removing grazing from pastureland and establishing a nurse crop
resulted in an immediate up-rush or re-growth of pasture and minor weed
species.
While initially this is unsightly, observations suggest that the humus
produced by decaying grass and root activity by the new seedlings has
resulted in a substantial improvement in soil quality and worm activity
2 to 3 years after planting. By age three most plantings were
suppressing grass growth beneath the developing canopy, and birds
(native and exotic) were using the sites for nesting. Within 3 to 4
years of planting, viable seed was produced by all nurse crop species,
and natural regeneration was seen within but not outside the trial
areas.
Beneath these nurse crops, a number of native timber species (hardwoods
and conifers) were planted in small single-species blocks.
Experimental stocking rates for hardwoods have been between 250 to 2500
stems/ha although initial stocking rates around 400 stems/ha (e.g. 4 x
6m spacing) will allow for some selection further in the rotation once
suitable bole lengths have been formed. The use of the very high
stocking rates would assist with form development as young trees emerge
from the canopy, but would require substantial future thinning which in
these plantings is probably wasteful of expensive seedlings.
Height increments exceeding 1m/yr for individual puriri beneath various
canopy types have been recorded in recent trials, while the original
survey identified mean annual height increment for puriri and rewarewa
of 32 to 49cm for the first 20 years. After four years puriri and
rewarewa planted in these new trials average 3.6m and 3.1m in height,
with growth exceeding 60cm/year. The majority of these young, emerging
trees have single leaders, and are slender and lightly branched.
If this order of height increment is maintained a merchantable bole
length of 5 to 7m will be in place by age 10, although merchantable
diameters will take considerably longer to be achieved.
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| Forty-year-old puriri planted in a canopy gap created
in tall vegetation at Te Puia Springs. Photo: John Barran |
A number of reports (e.g. Herbert et
al. 1997, Pardy et
al. 1992)
suggest that, for these faster growing species, diameters
exceeding 50cm at breast height will take between 60 and 80 years to
achieve, although well-tended plantations on good sites could be
expected to improve on these figures. However, it is unknown how much
heartwood there would be in comparatively young fast-grown hardwoods at
this size. Larger diameter
stems (60 to 70cm) may therefore need to be achieved before harvesting
to ensure adequate heartwood content.
(top)
Recommendations
The early growth and survival of planted native hardwood trees will be
improved with attention to the following:
- Control of browsing animals, including fencing to exclude
domestic
livestock, before
any planting;
- Provision of shelter or use of a suitable existing nurse
crop;
- Use of high quality nursery-raised seedlings and good
planting
practices. Seedlings should be tall (preferably 50 to 80cm) especially
for sites where competing regrowth is expected. Container-grown
seedlings are preferred over bare-rooted seedlings for most hardwood
species;
- Good site preparation – pre-plant spraying of grass with
herbicides,
and where appropriate, cutting lanes or gaps in sheltering vegetation;
- Regular weed control, especially during the first years
after
planting.
Conclusions
There is now sufficient research to show that native hardwoods timber
species can be successfully established where the aim is long-term
specialist timber production. Many of these species will grow
successfully outside of their natural range with appropriate site
selection and establishment practices while
their ecology makes them suitable for sustainably managing as either
single or mixed species in even or uneven-aged planted stands where
future harvesting will involve low-impact selection logging techniques.
Although the management of native hardwoods for timber production will
be on longer rotations than exotic conifer species, other values
including amenity, landscape and environmental are often important
additional objectives incorporated into plans for planting. This may
tip the balance for
potential growers in favour of planting native hardwood timber species
for high value timber production on selected good quality sites.
(top)
References:
Herbert, J.W.; Bergin, D.O. and Steward G.A. 1997: Native trees for
production forestry. What’s New in
Forest Research No. 243. New Zealand Forest Research
Institute Limited.
Pardy, G.F.; Bergin, D.O. and Kimberley, M.O. 1992: Survey of
Native
Tree Plantations. Forest Research Institute
Bulletin No. 175.
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