Decline of silver fir (Abies alba Mill.) stands in the Spanish Pyrenees: role of management, historic dynamics and pathogens
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The present study examines the influence of ecological conditions and silvicultural practices on the health of silver fir (Abies alba Mill.) stands, with special attention to the implications of Viscum album, Armillaria and Heterobasidion annosum. Health of individual trees was estimated by measuring
defoliation, chlorosis, needle retention, cone production, stork nest, epicormic sprouting and lichen cover. For stand health we measured regeneration. We used a systematic sampling throughout the Spanish Pyrenees, and analyzed the data by means of mixed models. We observed damage on 15% of the trees based on defoliation levels and on 10% of the trees based on chlorosis. Defoliation, chlorosis and cone production were more frequent in dominant trees while epicormic sprouting was more concentrated in the dominated layer. Stork nest and lichen cover may be linked to low growth in low quality sites. The incidence of V. album on trees was 13.9%, and positively related to defoliation and chlorosis. We did not find a clear relationship between H. annosum or Armillaria and decline symptoms. Both pathogens appeared to be widespread in silver fir forests. No regeneration difficulties were observed anywhere. At stand level silver fir does not seem to be a species in decline. Silver fir was favoured during the last period of intensive cuttings, whilst Pinus sylvestris and Pinus uncinata nearly disappeared. At tree level, fir decline and health problems are more localized and advanced on seed trees left after the last cuttings.