Supplementary discussion
APPROPRIATE RESOURCE RESOLUTION DEPENDS ON THE ECOLOGY OF THE STUDY
SPECIES
Local dynamics of wood-decomposer species are influenced by the
characteristics and abundance of the resource units the species
primarily uses. Of the cell-scale covariates selected in our fitted
models for the colonisation probability, P. viticola responded to
the density of logs, whereas P. ferrugineofuscus responded to the
volume. At this smallest spatial modelling scale (20 m x 20 m cells) a
large volume at one cell, relative to that at other cells, will often be
caused by one or two large logs. Higher log densities, on the other
hand, will often be made up of several smaller deadwood units. In a
previous study of colonisation and extinction events at the individual
log level, Jönsson et al. (2008) found that for P. viticola,colonisation probability increased with connectivity to occupied logs
regardless of log size, whereas for P. ferrugineofuscus,colonisation probability increased with log diameter. During the first
round of model selection, we found a positive effect of mean log
diameter for P. ferrugineofuscus when based on the cell-level
fine scale deadwood data (results not shown), but once volume had been
included in the model this effect was no longer significant. At the
level of individual dead trees, Jönsson et al. (2008) found thatP. viticola had a substantially lower log-level extinction risk
than P. ferrugineofuscus (with annual mortality rates of 13.1%
and 29.6%, respectively) on similar-sized logs, probably reflecting the
longer fruiting period and later peak in fruiting in P. viticolathan P. ferrugineofuscus (Jönsson et al. 2008, Nordén et al.
2013, Ovaskainen et al. 2013). In our data, P. viticola had
higher extinction rates than P. ferrugineofuscus at the cell,
plot and patch scale especially when including also 5-15 cm logs. The
difference is likely because in our data P. viticola occurs on
average on smaller, fast-decomposing logs than P.
ferrugineofuscus that leads to more frequent extinctions in P.
viticola . The rate of deterministic extinction due to log disappearance
increases with decreasing log size.
POTENTIAL BIAS IN MODEL PREDICTIONS
Our predictions of the future population trends may be biased for
several reasons. Firstly, connectivity decreases for 20 years after 2020
(Fig. S4-2). Some populations acting as dispersal sources will disappear
during that time which will slow down the colonization rates more than
what is expected by our models. Secondly, the forest currently
voluntarily set aside may in fact not be set aside until the year 2100.
For example, the mean length of private landownership is 22.5 years
(Ingemarson et al. 2006) and new owners often start by cutting the
oldest forest (Eriksson 2008). Thirdly, we ignored edge effects from
clear cuts into set-asides which will have a negative effect on
sensitive wood-decomposer fungi (Ruete et al. 2017). Fourthly, we may
have misestimated the colonisation and occurrence probabilities in
deadwood-rich forests as our models are based on field data from forests
with low or intermediate amounts of deadwood. Fifthly, we assumed thatP. ferrugineofuscus colonizes all dead trees with diameter ≥5 cm
with the same probability while other studies have shown that the
colonization and occurrence probabilities increase with log diameter for
this species (Jönsson et al. 2008, Nordén et al. 2013). Finally, future
climate is likely to be adverse for our model species (Mair et al. 2018)
even if our data lacked the power to detect climate effects.