Results
COLONISATION AND EXTINCTION EVENTS
We observed several colonization events, especially in the mature
patches/plots/cells, and several extinction events, especially in the
clear-cut patches/plots/cells (Table 1). The clear-cut forests lost
almost all of their occurrences between the two surveys, and very few
colonisations took place.
The species with the highest occupancy in the landscape, P.
viticola , had higher colonization rates and higher ratios of
colonization/extinction than the less frequent species, P.
ferrugineofuscus at all spatial modelling scales and both resource
resolutions (Table 1). We observed the highest extinction rates forP. viticola at the fine resource resolution (≥5 cm) and the
smallest spatial modelling scale (20 x 20 m). For P.
ferrugineofuscus, the colonisation and extinction rates were comparable
between the fine resource resolution at the smallest spatial modelling
scale and the coarse resource resolution at the largest spatial
modelling scale. At the fine resource resolution, the colonization rate
increased and extinction rate decreased with increasing spatial
modelling scale. Similarly, at the coarse resource resolution, the
largest spatial modelling scale had the highest colonisation rates and
the lowest extinction rates. Extinctions resulted both from host logs
disappearing due to decomposition and stochastically where suitable logs
were recorded in both surveys.
SUMMARIES OF FITTED MODELS
For P. ferrugineofuscus , the responses – probabilities of
occurrence and colonisation – were explained by the volume of spruce
logs at the cell scale (here reflecting the presence of large logs),
whereas the plot-scale responses were explained by stand age, and the
patch-scale responses by stand age or connectivity (Table S2-1). The
amount of data available allowed estimating the effects of one or two
covariates (Table S2-1) and hence only one or two rounds of model
selection were required. For P. viticola, the same responses were
explained by the density of spruce logs (here reflecting many small
logs) and connectivity at the cell and plot scales, and the density of
spruce logs at the patch scale. The best fitting measure of connectivity
for P. ferrugineofuscus was the presence/absence of old (≥120
years) spruce forests within a distance that corresponds to a mean
dispersal distance of 1 km. For P. viticola , two measures of
connectivity were important: the volume of spruce or presence/absence of
spruce in old forests within a distance that corresponds to a mean
dispersal distance of 10 km. Deadwood resource resolution had an
influence on whether the density of logs was selected or not in the
models for P. viticola.
FUTURE PROJECTIONS
We predicted higher occurrence probabilities and relative changes from
the projections based on the occupancy models than the
colonisation-extinction models. The general probability of our model
species increasing across all forest land was higher using the occupancy
models than the colonization-extinction models. The main reason for this
was the predicted smaller decrease in production land when using the
occupancy model (Figs. 2 and 3, Table 2, Figs. S3-1-4). For both model
species, the probability of an increase over all forest patches was
unity for the projections based on the occupancy models but ranged from
0.59 to 0.96 for P. ferrugineofuscus and from 0.88 to 1 forP. viticola based on the colonisation-extinction models.
Occupancy models were more sensitive to the chosen resource resolution
and spatial modelling scale than
colonisation-extinction
models. Specifically, there was larger variation among the projection
trajectories when using the occupancy models
(Figs.
2
C-D, 3 C-D) than when using the colonisation-extinction models (Figs. 2
A-B, 3
A-B).
For both model species we projected an increase in the set-asides across
all spatial modelling scales and resource resolutions, owing to
increasing density and volume of deadwood, stand age and connectivity
for set-asides (Figs. S4-1 and S4-2). The relative change in occurrence
across all the forest patches depended on the degree to which the
positive trends in the set-asides could compensate for the declines
(where predicted) in the production land (Figs. 2 and 3, Table 2, Figs.
S3-1-4). We predicted higher occurrence probabilities for P.
viticola than for P. ferrugineofuscus , and an increase in future
occupancy was more likely for P. viticola than P.
ferrugineofuscus across the two types of models, spatial modelling
scales and resource resolutions.
The differing forecasts of the occupancies of the two polypore species
resulted from a combination of the general occupancies or
colonization-extinction rates (Table 1) and the forecasts of the
covariates of the fitted models (Table S2-1). For a description of these
links, see Appendix S1.
The magnitude of future increase in occupancy depended strongly on the
spatial modelling scale (cell, plot, patch) (Figs. 2 and 3). For both
model species in mature forests, colonisation rates were the lowest at
the cell scale and increased going up via plot to patch scales, while
the opposite was true for the extinction rates (Table 1). In clear-cut
forests, colonisation events were rare and extinction events were common
at all spatial modelling scales (Table 1). The consequence of these
overall colonization and extinction rates were that the probability of
future increase across all forest land was higher for both model species
when the projections were conducted using the plot- or patch-scale
models than using the cell-scale model (Table 2, Figs. S3-1-4). Applying
the fine resource resolution (≥5 cm) colonisation-extinction model forP. viticola , the probability of a decline in the production land
was much lower using the cell- than the plot-scale models (0.08 versus
0.66). However, here the 0.08 probability of decline also means a
1-0.08=0.92 probability of increase, which is thus detected by the fine
resource resolution.
Resource resolution (all or only large deadwood included) had a great
impact on the future predictions. For P. viticola , we predicted
clearly a more positive future population development with the fine
resource resolution (≥5 cm) than with the coarse resolution (≥15 cm),
and the precision of the prediction was higher for the coarse resolution
(Table 2, Figs. S3-3-4). For P. viticola we predicted, probably
erroneously, a decline in the production land when we did not account
for the small resolution deadwood units. For P. ferrugineofuscus ,
future decline in production land seemed certain based on the coarse
resolution (both model types) but less probable (0.47 and 0.38) based on
occupancy models that used the fine resolution. Projections based on the
occupancy models for P. ferrugineofuscus showed a decline in the
production land only for the models based on the coarse resource
resolution; for the fine resolution the trends were more stable (Fig. 2;
Table 2, Fig. S3-2). Projections based on the occupancy models forP. viticola similarly showed an almost certain (probability
≥0.99) decline in the production land only when we modelled at the
coarse resource resolution; at the fine resolution the probability of
decline was zero (Fig. 3; Table 2, Fig. S3-4). We predicted the greatest
increase in future occupancy when modelling at the fine resource
resolution. The effect of resource resolution was less pronounced in the
colonisation-extinction models than in the occupancy models.