Douglas fir – Grows well and reproduces well
Nick Ledgard
New Zealand Tree
Grower August 2007
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| Douglas fir cone litter under 20-year old
trees in the Mackenzie Basin |
Over the last decade, southern New Zealand has seen more new land
planting of Douglas fir than any other tree species – including radiata
pine. The reason is because it grows well in the inland parts of the
South Island high country where world record growth rates have been
measured. In such areas, Douglas fir not only performs well, but it
reproduces well. Many readers will have seen the ankle-deep carpets of
cones beneath trees, which can produce thousands of viable seeds almost
annually. The seed is winged and light, and if produced in an exposed
site can be distributed by the wind for many kilometres.
Vigorous spread
The above reasons contribute to the fact that Douglas fir is now
recognised as a species capable of vigorous wilding spread.
Surprisingly, this was not the case 20 years ago. It is suspected that
the Douglas fir cone litter under 20-year old trees in the Mackenzie
Basin increasing numbers of fast-growing wildings seen today are
associated with the presence of mycorrhiza, symbiotic fungi associated
with their root systems, which appear to have only become widespread
comparatively recently.
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| Behind Queenstown Douglas fir has moved a
considerable way up the slopes of Bowen Peak in between 1986 and 2003 |
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| Douglas fir can establish under forest gaps |
Shade tolerant
Douglas fir is more shade tolerant than the common pines and will
invade shrublands. Small seedlings can also be found within forests,
but contrary to popular belief, they will not establish readily under
high canopies. Research has determined that the species needs 40% of
full light to achieve the consistent growth needed to reach canopy
level.
Cooler inland hills
The best growth for Douglas fir is often found in cooler, inland hill
and high country areas, where the surrounding vegetation cover and
grazing levels can be light. Such circumstances can lead to a high risk
of wilding spread. Assessment forms are available to assess this risk,
available from the author, and there are a number of strategies which
can prevent or reduce it.
Grazing and improved pastures are the most obvious – one never sees a
wilding alongside the many-species shelterbelts growing on developed
farms. Fertilising alone will increase competition from resident
vegetation and thereby reduce wilding emergence. But the increased
fertility may also promote exotic weeds which can submerge slower
growing native species. Another option is not to plant on exposed
take-off sites, or to plant a less spread-prone species, such as
ponderosa and radiata pine, around stand margins as much of the seed
leading to wildings comes from edge trees. However sometimes, despite
there being excellent growth opportunities, the risk of spread can be
just so great that Douglas fir should not be planted at all.
Douglas fir research co-operative
The author is secretary of the New Zealand Douglas fir research
co-operative, and also currently manages a three year
wilding project
funded by MAF’s Sustainable Farming Fund. It involves all the major
stakeholders, and aims to make wilding prevention and control an
accepted component of everyday farm management.
Nick Ledgard works for Ensis in
Christchurch
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| Douglas fir can also invade shrublands |
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