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.