Impact of root-rot pathogens on forest succession in unmanaged Pinus mugo stands in the Central Alps

Abstract

In the mountain pine (Pinus mugo subsp. uncinata (DC.) Domin) forests of the Swiss National Park in the Central Alps, disease centers associated with the root-rot fungi Heterobasidion annosum (Fr.) Bref. and Armillaria spp. are characteristic elements. We assessed the impact of these pathogens on forest dynamics by studying transects running across disease centers into the adjacent forest. Overall, mountain pine was the most abundant regenerating tree species and accounted for 84% of all seedlings (<20 cm high) and 93% of all saplings (20–130 cm high), whereas Swiss stone pine (Pinus cembra L.) was less frequent (15% seedlings : 7% saplings). The density of mountain pine seedlings did not differ significantly between the disease centers and the adjacent forest, whereas mountain pine saplings were more frequent within the disease centers, indicating that growth from the seedling to the sapling stage was favoured in disease centers. There was significantly more dead wood and a greater diversity of plant species in the disease centers than in the adjacent forest. The results suggest that root-rot fungi slow down succession towards stands with a higher proportion of P. cembra by causing premature mortality of mountain pines and creating disease centers with dense mountain pine regeneration.