Pests and diseases of forestry in New Zealand



 Swiss needle-cast of douglas fir in New Zealand


SWISS NEEDLE CAST DISEASE OF
DOUGLAS-FIR  -  IMPACT ON GROWTH
 



Swiss needle cast page

From Forest Health News 114, December 2001

Phaeocryptopus gaeumannii, the cause of Swiss needle
cast disease of Douglas-fir, was first found in New
Zealand near Taupo in 1959. A survey throughout the
country revealed that at that time this needle parasite
was present only in the Rotorua-Bay of Plenty region,
and in parts of the Waikato and Hawkes Bay. In the
1960s and 1970s, there was much concern about the
unthrifty and chlorotic appearance of many older
Douglas-fir stands in the central North Island,
particularly in Kaingaroa Forest. Considerable debate
ensued about the cause of the disorder, particularly
when it became apparent that annual growth production
was substantially below that predicted in yield tables.
The future of Douglas-fir became the subject of a New
Zealand Forest Service Symposium in September, 1974.
It gradually became evident from the accumulated
results of a number of field and glasshouse studies that
the loss in growth increment in Kaingaroa Forest was
associated with the introduction and spread of
P.
gaeumannii. However, until recently, information had
been lacking for stands in other parts of New Zealand.


Following its initial discovery, careful records were kept
of the distribution of the causal fungus based on
laboratory examination of samples collected by
surveillance officers. It took approximately three
decades for
P. gaeumannii to spread through most of the
country, and it is known with some precision when
infection first reached many forests. It was decided to
make use of this information to investigate the impact of
the fungus by examining data from the 1500 permanent
sample plots in New Zealand Douglas-fir stands, in
relation to the onset of infection. Measurement dates
were standardised by converting them to numbers of
years since or prior to the first observance of the fungus
in each forest. The data were then analysed as two
discrete sets for the significance in growth difference
between records before and after infection, respectively.
This was done by calculating the Site Basal Area
Potential (SBAP) representing the basal area increment
of a fully stocked stand at around 30 years of age. The
SBAP can be thought of as a growth potential factor
analogous to site index but applicable to basal area
instead of height. It is independent of stocking density.
The results, summarised in the graph below, show that a
clear reduction in site basal area growth potential
occurred in different forests following the appearance of
the Swiss needle cast disease fungus. The gradual
decline to the lower plateau over a period of several
years is consistent with the slow build up of infection
that is known to occur as the fungus invades new
forests.



Further analyses were undertaken to determine whether
there were regional differences in the growth reduction.
Some of the initial results are presented in the table
below which shows four forests containing significant
numbers of plots with measurements taken both before
and after the introduction of the disease. The tabulated
percentage change in mean SBAP is derived from data
collected before the appearance of the disease in each
forest, and from that taken during the period from 10
years after.




The average reduction in SBAP across the country
following the appearance of the disease was 20%, which
would translate into a similar decline in volume growth
per ha. However, because smaller diameter logs have a
lower value per m3, the decrease in log value would be
somewhat greater. STANDPAK simulations indicate
that a 20% reduction in the SBAP would correspond to a
26% decrease in log value per ha, assuming a 45-year
rotation with waste-thinning and typical log prices.
This analysis was made possible using a new model for
predicting the growth of Douglas-fir in New Zealand
which uses the SBAP as an index for basal area growth.
Because it was apparent that there had been a substantial
decrease in growth after the advent of the disease, the
model incorporates only post-infection data. However, it
can be used to estimate the SBAP from plot
measurement data, and hence to quantify the effect of
the disease as shown above.
The results of this work will be published in full after
further analyses are completed, but in the meantime a
number of conclusions may be drawn. Strong evidence
is provided confirming that
Phaeocryptopus gaeumannii
is the direct cause of growth loss in Douglas-fir in New
Zealand. The loss in growth is substantial, and has
significantly reduced the profitability of growing
Douglas-fir. However, there are indications that there
are regions in the country where the disease is less of a
problem. There are also grounds for optimism that
breeding and selection may enable most of the lost
growth potential to be recouped. The current move to
using seed sources originating from the Californian fog
belt should go a long way towards offsetting the effects
of the disease.
(Leith Knowles, Mark Kimberley, Ian Hood, Forest Research)



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