A window to the future of Sweden’s pine forestry? : development of declining Scots pine after a severe drought and the presence of Diplodia sapinea

Abstract: Swedish forestry is dominated by uniform conifer stands, a structure generally more susceptible to biotic and abiotic stressors. Among the agents causing forest decline, pathogenic fungi have the most substantial impact. This thesis presents an example of a “disease triangle”; the interaction between Scots pine, the fungus Diplodia sapinea, and the environmental aspects affecting them. Scots pine has low demands on nutrient and water supply and is a strong competitor in poor soils. D. sapinea causes Diplodia shoot blight, a common pine disease that can lead to severe damages. The fungus is particularly infectious when the weather is wet and warm, and when the trees experience stress. The study focuses on Gotland, an island where D. sapinea was found in connection to a severe drought period in 2018. The aims were to identify drivers of and monitor the decline in Scots pine on four symptomatic and four asymptomatic sites on the island, by estimating defoliation levels, assessing D. sapinea spore release, and analysing the local climate at the sites during one year after the drought period, as well as testing correlations of the decline with soil properties. Trees with a high level of defoliation just after the drought period experienced a larger increase in defoliation the following year than trees with initially low defoliation levels. Trees did generally not recover during the experimental period. The spore load was overall higher at symptomatic sites, but the difference between site types by season was only significant in summer. The spore load was positively correlated to precipitation and wind speed. Relative humidity was overall higher at asymptomatic sites, and asymptomatic sites experienced less extreme temperatures. The soil type differed between the site types, although soil analyses showed no significant differences between asymptomatic and symptomatic sites regarding nutrients, pH, loss on ignition and water-holding capacity. Conclusively, this study shows that the decline in Scots pine related to D. sapinea is strongly driven by drought stress and that an inadequate site may be what enables a disease triangle to be complete.