Biotic, abiotic and management factors involved in "Pinus pinaster" decline in the Iberian Peninsula

Abstract

Forest decline is a complex disease caused by the interaction of both abiotic and biotic factors leading to gradual deterioration of the forest. These factors have been classified as predisposing, inciting or contributing factors. Pinus pinaster Ait. is one of the most important forest species in the Iberian Peninsula taking into account both the protection it provides and the timber it produces. It is also the main conifer species in Spain in terms of area covered. In recent years, several Maritime pine stands in the center of the Iberian Peninsula have experienced a general decline, manifested as transparency at the crown, smaller needles than normal, foliage discoloration and premature tree death. The main objective of this work was to develop a better understanding of the influence of both biotic (pathogens) and abiotic factors, including anthropogenic ones (climate, soil properties and forest management) on Pinus pinaster decline in the Iberian Peninsula, shedding light upon the role each factor plays in this process (i.e. if it acts as a predisposing, inciting or contributing factor). Various parameters were measured in 27 circular plots with radii of 15 m in natural stands of Pinus pinaster ssp. mesogeensis. Regarding abiotic and management factors; the results suggested that predisposing factors such as competition and water deficit have triggered the decline of Pinus pinaster. On the other hand, the silvicultural and environmental factors influencing the health status at the tree level were: size of the tree (diameter at breast height), stand structure (mean total height) and climate (seasonal summer precipitation). Correlations between tree growth and aridity index generated significant responses related to health status conditions and defoliation levels; Thus, drought events at the beginning of the 1990s, in 1994 and in 2005 could have acted as inciting factors to Maritime pine decline. Regarding biotic factors; Heterobasidion annosum, one of the most important causal agents of root and butt rot in the Northern Hemisphere, was found to be involved in Maritime pine decline, and was described on record for the first time in Spain as Heterobasidion annosum on Pinus pinaster. On the other hand, fifteen species were identified during the study and the main pathogenic species belonged to the group of Ophiostomatoid fungi: Ophiostoma minus, Ophiostoma ips, Ophiostoma piliferum and Ophiostoma ranaculosum. The root rot pathogen, Heterobasidion annosum, was also identified in the study area. We tested the pathogenicity of two Spanish isolates of Heterobasidion annosum sensu stricto in two-year-old Pinus pinaster seedlings. The mortality rates, lesion lengths and the percentage of fine root growth in the inoculated seedlings differed significantly from the control seedlings. These results demonstrated the susceptibility of Mediterranean Maritime pine to Heterobasidion annosum and indicated that this fungus could be the most important contributing factor to Pinus pinaster decline in the Iberian Peninsula.