Continuous cover forestry: Management practice
Ian Barton
New Zealand Tree
Grower May 2006
This is the second and final part of the article on continuous cover
forestry.
The first part was published in the November
2005 issue of the Tree
Grower.
See also Continuous
Cover Forestry: A Handbook for the Management of New Zealand Forests
by Ian Barton.
Establishing the continuous cover forest
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| Blackwood age 26 as nurse for 23-year-old
kauri with other native
species naturally regenerating on forest floor. |
New forest on bare ground
This requires a similar approach to the planting of a single species,
even-aged forest. But more attention must be paid to choosing species
best suited to the site and planting appropriately. For example species
like totara are better suited to drier sites while rimu and kahikatea
will succeed
better on moist sites. Kauri will establish on either but grows much
faster where moisture is readily available.
All grow better on soils of good quality and poor soils need
amelioration by cultivation and fertilising. Because management
involves following ecological principles, the siting of seedlings
should result in a mixture of species – often planted in small groups.
If planting in the open, nurse plants will probably be required. These
can be either indigenous or exotic and preference should be given to
choosing the most effective nurse.
Conversion of a mature,
even-aged stand in full production
The best approach is to divide the block into more or less equal areas
or coupes which will range in size from a few square metres to about a
fifth of a hectare. The actual size depends upon the site requirements
of the species being introduced. Coupes will be felled over a
predetermined time and planted or regenerated with the appropriate
mixture of species for continuous cover forestry. If the original
species is to be completely replaced with others then it must be
prevented from regenerating.
An example concerned the conversion of a Douglas fir plantation, 10
hectares in area and 35 years old. The conversion process envisaged
regenerating small groups every five years for 35 years. If the average
group size were set at 1,500 square metres then eight of these groups
would be felled every five years.
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| Black beech small coupe harvest with pruned
and thinned regeneration
aged 17. |
Conversion of young
even-aged stands
This is essentially the same as the previous option but initially the
trees will not be large enough for economic harvest. Examples include
situations where several species are regenerating through an indigenous
or exotic scrub canopy or where several species have been deliberately
planted under such a canopy.
Single tree selection, group selection or irregular shelter wood
systems can be used for management, depending on the relative shade
tolerance of the species concerned. The aim is to develop greater
structural diversity within the stand by encouraging the growth of
better quality trees chosen because they have greater root stability,
good form and vigour, and are appropriately spaced. At the point where
these trees emerge through the scrub canopy there should be up to 300
stems per hectare. Higher stocking will reduce the quantity and quality
of following regeneration and long term productivity.
As the stand ages, natural regeneration, begins to colonise the under
storey. From the beginning progressive thinning the canopy of nurse or
non crop species will open up gaps and encourage rapid growth of
regenerating seedlings. Because of differing light levels within the
stand, these seedlings will have variable height growth as more
favoured trees grow towards the canopy establishing the two to four
tiered canopy structure. Up to 50 years may be required to reach this
point and from now management is aimed at developing the ‘reverse-J’
structure traditionally identified with classical selection
systems.
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| Conversion of scrub hardwood stand by
planting with rimu. |
Conversion of a mature
but degraded forest
In New Zealand such sites will mostly be indigenous cutover forest but
could be a failed exotic stand. The process here takes notice of the
condition of the existing forest and the inventory will be more complex
than that for converting an even-aged plantation. This is because it
may have pockets of quite high quality forest or areas with no
desirable species at all.
The stand is mapped identifying those areas which, on the basis of
current quality, will improve if carefully tended. These are treated
appropriately carrying out whatever thinning, pruning or releasing may
be required.
Remaining areas are scheduled for replacement over a predetermined time
period. For example if the total area of forest to be upgraded is 10
hectares and it is desired to do the work on an annual basis over a 15
year period, then 0.67 hectares should be treated each year.
Sizes of individual coupes should be similar and not greater than 2000
square metres. Poor trees are felled with any useable wood removed,
followed by re-planting or regeneration of these areas. Small groups of
good existing regeneration are retained and thinned if
required. The occasional pole or small tree with potential is also left.
An example of the type of forest which might be managed in this way is
old kauri cutover which has scattered regeneration, from
seedlings to small pole size, usually on ridges. Similar sized tanekaha
will be found throughout and there will be scattered rimu and totara.
Gullies generally contain only scrub hardwoods and tree ferns and can
be converted to timber species by careful management.
Conversion of young,
even-aged stands of scrub species
The site will usually be dominated by one species such as kanuka or
gorse. There may be some early regeneration of timber species.
The choice of silvicultural system will depend on the appearance and
pattern of any natural regeneration and what additional
planting is required. If regeneration is present then the distribution
of this is used as a guide for supplementary planting. If there is no
regeneration, planting should be carried out using ecological knowledge
of the species concerned. Once the site is fully stocked with
seedlings, releasing must be done as frequently as required.
Silviculture
Successful continuous cover forestry requires a higher standard of
silviculture than that practiced with even-aged, clear fell management
systems. Inventory is essential, for in each part of a continuous cover
forest there must be a full range of age classes with, continual
additions to the growing stock by natural regeneration or planting.
Inventory builds up knowledge of the species and size classes present
in the forest and is vital to the process.
Silvicultural activities, based upon the ecology of the forest, are not
confined to specific parts of the forest but take place in many stands
at any one time. Harvesting by single tree or group selection depends
on the shade tolerance and seedling regeneration
characteristics of the species. Harvesting is not confined to the
removal of large trees. Some of these may be left to grow to their
fullest potential, occasional poor trees are left for habitat purposes,
and smaller trees may be removed to give more vigorous neighbours
greater space. The main selection criteria are to remove trees with
reduced increment, impaired health and vigour, those which are
malformed or damaged and thinning to reduce density. The objective is
to allow the best of the dominant trees to put on as much volume as
possible while allowing subdominants to increase their growth rate when
their over story is removed.
Continuous cover forests are managed for production so there will be
some diversion from natural processes. For example they may be single
species forests, mixtures of native and exotic tree species, or
infertile species where planting of replacement stock will be required.
Ecological requirements of different species, growing in the same
stand, will need to be compatible and a good deal of trial and error
will be needed to devise a suitable silvicultural processes.
Following harvesting, growth at all levels in the continuous cover
forest must be encouraged. New tree establishment is obtained by
encouraging seed germination and coppice regrowth or, as a last resort,
planting. Existing seedlings and saplings need to be released from
competition and excessive numbers thinned. Repeat releasings are likely
during the first few years to free newly planted or germinated
seedlings from competition. Encouraging regeneration requires control
of seed supply, light levels, competition and predation of both seed
and small seedlings.
|
J curve graph shows natural distribution of
size classes compared with
desired distribution.
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Yield control and
inventory
Yield regulation in the continuous cover forest is done by dividing the
forest into stands of similar species and size classes and making
inventories of the growing stock every six to 10 years. This enables
determination of the relationship between increment and
growing stock, the determination of yield for the next period and
planning felling in order to maintain a normal distribution of size
classes.
The basic rules are –
- Maintain various tree age and size classes in the correct
proportion,
the number of plants in each size class should be about double that of
the class above it.
- With a mixture of tree species, try and maintain a consistent
percentage mix of species in each age class.
- Free young seedlings and saplings from suppression
- Remove defective stems whenever they hamper better ones, unless
they
are important for habitat.
The focus is on size rather than age and target diameters should be
adopted for the forest. In Europe these can be as high as 60 to 70 cm
but have not been determined for New Zealand. Other parameters are
residual basal area, stem size distribution and shape of the reverse J
curve. Here the objective is to bring the seedling and tree size
distribution curve of stand close to the theoretical one over time.
Rotation
Crop rotation, as applied to plantation forestry, has no place in
continuous cover forestry where the important criterion is the cycle of
individual trees from establishment to felling. A fully productive
continuous cover forest could be composed of several species, each with
a different growth cycle. If the forest is well managed, the individual
cycle lengths will be as short as needed to produce maximum volumes of
high quality timber.
Management plans
Good management is controlling yield by keeping the annual harvest
volume at or below the annual increment. It is based on knowledge of
the dynamics of the forest and sound ecological principles.
A good operating plan is essential. For example, where ground hauling
methods are to be used, a well planned network of skidder tracks spaced
at about 40 metre intervals is needed. All main ridge and valley
systems will need to be accurately mapped and contour plans
drawn.
Initially the forest should be mapped by forest type. This is normally
dictated by the dominant species and is required whether the forest
natural or planted. This is the first step in making an inventory of
the forest and as more data is obtained the more refined the
forest plan becomes. The completed management plan should have
individual stands mapped by GPS.
Monitoring and reserve areas
Monitoring is essential. If the inventory is not in place and accurate
control of the yield is not maintained, it is very easy for the forest
to become degraded due to –
- Uncontrolled felling, especially removing only the best trees
- Decline in value of the growing stock because of market changes
- Severe damage by fire, wind or disease
- Uncontrolled grazing by domestic or feral animals
- Agricultural practices like litter removal.
All management plans must have good monitoring systems in place. With
indigenous forest it is important to have reserves of non-harvest
forest to serve as reference points. As alternatives to setting aside a
specific part of the forest, these could be areas of protected forest
on neighbouring land or non-harvest riparian areas.
|
| Single tree harvesting from regenerating
kauri stand using helicopter. |
Harvesting
Several harvest methods are available including one, or a combination
of –
- Ground based, such as tractor or rubber tyred skidder
- Cable system
- Aerial extraction by helicopter.
Issues to consider when choosing a harvesting system include –
- Selection method of single tree, small group or coupe of
specified
size. Whichever is chosen, and all could be used in the same area,
depends more upon the ecology of the species being grown than any other
aspect.
- Steepness of site
- Environmental considerations such as the potential for soil and
water
damage.
Combined with the above are the requirements of the Resource Management
Act, especially the requirements of regional and district plans and
consideration of extraction method, roading and skid track patterns and
costs.
Environmental impact
There are two issues involved, the impact on the residual vegetation
and the damage to soil and water values. Damage is reduced by choosing
the best method for the job and using well trained operators.
Helicopter logging extraction usually causes the least damage and
tracked or wheeled skidders the most.
Land use consents
Prior to construction of roads and the clearing of vegetation from an
area greater than a certain size, a land use consent may be required
from the local regional council. Because continuous cover forestry only
requires the felling of trees on very small areas, a clearing permit
may not be needed.
When granting consent, regional councils must take notice of Section 17
of the Resource Management Act which states the duty to avoid, remedy
or mitigate adverse effects on the environment.
Roading
Continuous cover forests on easy country generally require a denser
roading network than forests which are clear felled. Ground harvest
methods involve extracting logs by hauling through the trees to the
road, either by small skidder, agricultural tractor, mini
skidder or horse.
Roads must be built to a standard high enough to easily extract logs
while protecting the soil from erosion. Unlike clear felling, where
roads may only be heavily used once every 25 or 30 years, continuous
cover forestry use will be lighter but more or less continuous.
Harvest methods
On reasonably flat country with less than 15 degree average slope,
ground extraction is less expensive. On steeper country helicopter
extraction, although more costly, is better. Small haulers may have a
role in some places.
Ground harvest methods using horse and mini skidder which have low
impact on the environment also have low production potential. They are
applicable in small scale operations where the value of the timber is
moderately high. Hauler systems have high productivity, but
the greater impact on the environment restricts their use in continuous
cover forestry. Skidders, which can also have negative environmental
impacts, are only applicable on flat and easy country. On balance
helicopter extraction is best, not only because of minimal damage to
other plants and the soil, but also because the need for road
construction is greatly reduced. However timber value must be
reasonably high to permit helicopters to be economic.
This outline is intended to give a brief overview of the principles and
practice of continuous cover forestry. Much more information will be
needed by those who wish to investigate the subject further. In part,
this will be met by a manual being prepared by Tane’s Tree Trust with
the assistance of a Sustainable Farming Farm grant. However the only
way to understand the system is to begin practising it.
It is hoped that over the next few years many will take up the
challenge. To this end Tane’s Tree Trust would like to hear from anyone
who has comments on these continuous cover forestry articles, because
discussion now can only make the proposed manual better.
Further reading
Benecke U 1996 – Ecological silviculture: The application of age-old
methods. New
Zealand Journal of
Forestry.
Benecke U 1996 Near natural sustainable forestry from an international
perspective. In Native
Trees for the Future. Eds. W Silvester, R McGowan.
Proceedings of a Forum held at the University of Waikato.
Helliwell R 1999 Continuous
cover forestry. Published by author.
Mason B, Kerr G & Simpson J 1999 What is continuous cover
forestry? Information Note FCIN 29. Forestry Commission.
Matthews J D 1999 Silvicultural
Systems. Clarendon Press.
Ian Barton is a
registered forestry consultant
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