Abundance and viability of fungal spores along a forestry gradient: responses to habitat loss and isolation?

• Regional variation in spore deposition and viability was studied for two fungi, Fomitopsis rosea (Alb. & Schwein.: Fr.) P. Karst. and Phlebia centrifuga P. Karst., both confined to old-growth spruce forests in the boreal zone. Seven regions in Sweden were studied along a north-south transect in which the historical impact from forestry increases and the amount old forests decreases towards the south. The two southernmost regions were located outside the distribution border of the species. Spore deposition was measured species specifically as heterokaryotisation of homokaryotic mycelia growing on wood discs. There was a significant decline in spore deposition towards the south for both species. F. rosea deposited an average amount of 111 spores m-2 24 h-1 in the northernmost region compared to less than 1 spore in the four southernmost regions. The corresponding values for P. centrifuga were 27 spores m-2 24 h-1 in the north compared to less than 2 spores in the 4 southernmost regions. No deposition was found south of the distribution borders. The viability of spores from local populations within each region was measured as germination success on nutrient media. Individual fruiting bodies from large populations in the north generally produced spores with higher germinability than fruiting bodies from geographically isolated populations in the central and southern regions. However, there was a high variation among the southern populations. Our data suggest that some populations in mid- and south Sweden may suffer from negative genetic effects, possibly associated with fragmentation and loss of habitat. Thus, the combination of low spore deposition and low germinability of spores may be a threat to the long-term persistence of F. rosea and P. centrifuga in southern Sweden. Several other species may experience the same situation, especially when considering the severe decline of dead wood in Swedish forests.

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