Coast redwood silviculture in New Zealand
|
| A 35-year-old pruned stand of redwood
growing on the East Coast |
New Zealand Tree
Grower February 2007
Mark Dean
Californian coastal redwood, Sequoia
sempervirens,
has proved to grow well in New Zealand when correct siting and
establishment practices are attended to. On good quality sites – warm,
sheltered and with regular rainfall such as Taranaki and the Bay of
Plenty – growth rates can equal or better those of radiata pine. Mean
annual increments over 30 cubic metres per hectare a year are
achievable.
Redwoods are more shade tolerant than most conifer species so will
survive in very low light situations. However they do require full
sunlight to grow rapidly. Perhaps because of these attributes redwoods
grow readily from coppice and stump or root sprouts. These
physiological attributes potentially allow some innovative regimes not
commonly practised with more light demanding conifer species.
Redwood markets differ from pine
Although not commonly used in New Zealand, primarily due to supply
constraints, there is a strong traditional demand for quality redwood
timber in California. Here it is looked upon as an iconic species in
much the same way as rimu or kauri in New Zealand.
As access to natural stands in the USA is increasingly restricted, the
potential for an export market to develop for New Zealand plantation
grown redwood is considerable.
However the redwood timber market differs from the radiata model we are
used to. In California, clear heart grades attract a large premium over
those with sapwood content and unpruned tight knot grades, which in
turn are more valued than timber with bark-encased knots. This grade
and price differential, based on branch condition as well as size,
provides an important distinction from the radiata timber market that
most New Zealand forest growers are used to, and will provide some
challenges in managing coast redwood silviculture.
Establishment
While redwoods are shade tolerant, once established they must have good
weed control for the first one or two years. Suppression from grasses
can hold back early growth dramatically. Retrospective ring analysis on
several stands planted in the 1970s found that on average trees were
four years old before reaching breast height of 1.4 metres.
Stands planted more recently with good spot release weed control
commonly achieve 1.4 metres in the first or second year.
Another consideration to take into account when choosing sites for
coast redwood is that of out of season frost. In its natural range
redwoods grow from sea level to over 900 metres altitude. However out
of season frosts, particularly on the volcanic plateau, are believed to
have been responsible for the failure of many early redwood plantings.
Pruning and thinning
While New Zealand growth rates allow for rotation lengths of around 35
years, careful attention will need to be paid to designing regimes
which will capture most of the potential value in these stands. The
target will be to have large pruned butt logs with upper unpruned logs
with green or moribund branches less than 50 mm diameter. Although
there have been several hundred hectares of redwoods grown and
harvested in New Zealand, most have been non-intensively managed so the
effect of various pruning, thinning and initial and final crop stocking
combinations has not yet been quantified.
Ensis trials
Over the past few years Ensis, with assistance from New Zealand
Forestry Ltd, The New Zealand Redwood Company and the Plantation
Management Cooperative have initiated a number of silviculture trials.
A series of final crop stocking trials have been installed in existing
stands located in the Central North Island, East Coast, Hawkes Bay and
Otago. Within four or five years there will be sufficient data to
produce a thinning function to aid the basic growth functions
implemented in a growth model being developed by Rob Webster. This will
allow managers to predict the results of various thinning and final
crop stocking regimes in terms of tree growth.
Pruning is a favoured option, given current market preferences. It may
be worth pruning redwoods to a small DOS as has been done with
cypresses in some instances. Two pruning trials, at Tutira and
Gisborne, have been installed by the Plantation Management Cooperative
in partnership with The New Zealand Redwood Company. Pruning redwood is
not difficult as the wood is relatively soft and branches in plantation
grown trees are generally small and well spaced. However, although
redwood bark is thick and fibrous on
mature trees, with young trees the bark is thin and easily damaged, so
care must be taken to not damage the bark and cambium around branch
collars when pruning. Although still in progress, interim results
indicate that both stocking and pruning severity a effect tree growth.
|
| Epicormics sprouting from the branch collar
and cambium of a young pruned tree |
Pruning stimulates epicormics
Of much greater interest is the impact of pruning severity on the
incidence of epicormics. These originate from buds in the cambium and
are stimulated by the combination of stress induced by removing the
green crown and light on the stem. When early severe pruning treatments
are applied the stems can be covered with epicormics which has the
effect of negating the pruning effort. However by delaying pruning
until the bark is thicker, and by leaving more green crown, much of
this problem may be negated.
There has been some anecdotal evidence that pruning can allow entry of
a native longhorn borer or huhu beetles whose tunnels severely
downgrade the value of the sawn timber. This problem has been more
commonly reported from warmer area such as Northland and the Bay of
Plenty, but the extent of and severity of this risk has not yet been
quantified.
Maintaining lower branches
|
| The epicormic growth on the lower portion
of these trees is only 12 months old |
Another silvicultural challenge is to maintain live or moribund
branches in the lower unpruned logs, typically from 6 to 15 metres on
mature stems. Currently our understanding of crown height
recession with increasing stocking and age is fairly basic. There is a
need to quantify this and to understand the time delay between branches
dieing and the formation of bark encased loose knots.
As for managing redwood stands to minimise the incidence of bark
encased knots in sawlogs, there are a number of possible methods which
have yet to be tried. For example, regimes involving multiple thinnings
or possibly ultra high pruning may go some way to achieving the desired
goal. There have recently been sales of small logs, down to 20 cm small
end diameter, which can be sawn for utility uses. This may make it
possible to carry out production
thinning on some sites.
Log quality important
Log quality is of paramount importance in order to maximise the returns
from coast redwood. To date very little is understood about the timber
grade and value recovery from various log types. Until this
understanding is significantly advanced, it is difficult to produce
realistic economic analyses required to attract investors and set fair
market values for logs.
Currently Ensis and New Zealand Forestry Ltd are trying to bring
together interested parties to fund a sawing study on one of very few
mature pruned stands of redwood in New Zealand.
Because redwood is not core business of most of the large forestry
companies, organisations such as the NZFFA need to have a greater role
to play in supporting this vital research. Any readers interested in
contributing to this research in any way should contact Graham West at
Ensis.
Continuous cover management
|
| Insect attack, possibly pin hole borer, on
an 8-year-old pruned redwood |
Because of its ability to grow from coppice and tolerate low light
situations redwood is an ideal candidate for growing in uneven aged
stands which are harvested by a series of thinnings.
Pascal Berrill, from Ensis who is currently studying at Berkley
University, has been developing a multiple age management model which
predicts growth of Californian redwood stands under this system. There
are also a few examples of successful multi-aged stands of redwood in
Whaka and Kaingaroa forests. Most thinned redwood stands
including all of the recently installed thinning trials have coppicing
stumps. At some sites the growth of these are being measured by Ensis
researchers but a more structured effort is required to identify and
answer the key ‘how to’ questions of uneven aged stand management. It
is my belief that we will see more examples of actively managed mixed
age redwood stands particularly in sensitive environments where
aesthetics or soil and water values require continuous vegetation cover.
(top)
Mark Dean is currently a silviculture
analyst for Timberlands Ltd. Prior to that he spent over twenty years
with Ensis (formally FRI) researching radiata silviculture and over the
past five years has initiated much of the current redwood silviculture
and modelling research.
|
|
