Lisø PhD Dissertation Manuscript - NTNU

Wood decaying fungi will only be active if there is water present in the material.
Growth of most wood rotting fungi requires moisture content in the material
exceeding 20%. The low temperature limit for growth of wood decay fungi varies
from type to type. Some develop at several degrees below the freezing point, others at
more than 60 °C. All types of wood decaying fungi are heat sensitive and will die at
high temperatures, and normally they survive at low temperatures. Most wood rot and
mildew fungi in buildings in Norway is found to be growing at temperatures between
5 and 30°C (Norwegian Building Research Institute and Mycoteam as, 1997;
Sedlbauer, 2001).
The moisture factor of the index is chosen based on the fact that duration or
distribution of precipitation (and thus wetting) is generally more important than total
precipitation in determining the moisture content of wood above ground. The chosen
precipitation measurement is supported by field experiments (Scheffer, 1963). The
moisture factor, the mean number of days in the month with 0.254 mm or more
precipitation, was found to yield indices most proportional to actual rates of decay in
experimental units in three different exposure sites in the US with significantly
different climates. Relative decay potentials were estimated from rates of decay in
untreated units of decay-susceptible wood species. The days of precipitation were
randomly reduced by 3 to keep the index for the driest regions of the US near zero.
Climate indices are calculated according to Equation 1 for 115 weather stations in
Norway, all having at least 10 years of high quality observations in the 30-year period
1961-1990. These calculations were based on mean monthly air temperatures (Tmean)
and number of days with more than 0.3 mm precipitation (D), both being registered
on a regular basis by the Norwegian Meteorological Institute. Tmean are calculated
using a daily average (N) of air temperatures registered at 0600, 1200 and 1800 UTC
and monthly minimum temperatures (Tmin), as given in Equation 2:
T mean
T0600
+ T1200
+ T1800
= N − k(
N − Tmin
) where N =
(2)
3
k is a local value to compensate for lack of hourly observations of air temperatures.
The calculated climate indices were used in a spline interpolation to create a smooth
coverage of the index when producing the national decay risk map presented in Figure
1. This interpolation is fairly coarse with no adjustments for topographic effects,
except what is captured in the observations at each weather station. Three index levels
are outlined on the map (with boundaries delimited according to Scheffer):
Low Decay Risk (less than 35, the least favourable conditions for decay),
Medium Decay Risk (35-65)
High Decay Risk (values greater than 65, favourable conditions for decay).
Index values, both monthly products and the year total, for a selection of weather
stations are provided in Table 1 (all shown on the climate index map for Norway
given in Figure 1).
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