Healthy forests in a changing climate – the path to resilient conifers

Swedish coniferous forests are vital for the climate, biodiversity, and the forestry industry. But as climate change becomes increasingly pronounced, the threats from fungal diseases that attack our most common tree species—pine and spruce—are also growing. In her new doctoral dissertation, Matilda Stein Åslund has examined why some trees are more severely affected than others—and how we can establish forests that are better able to withstand the challenges of the future.

Genetics and environment affect tree susceptibility

At the core of the research are three major pathogens causing problems in the Nordic region: Diplodia sapinea; a pathogenic fungus that causes disease in stressed pines, Melampsora pinitorqua; a rust fungus responsible for pine twisting rust and Heterobasidion parviporum, the most common cause of root rot in spruce. By combining field studies, genetic analysis, and modern DNA techniques, Matilda has explored how trees' vulnerability is influenced by both heredity and environment.

Diplodia sapinea can exist in healthy trees without causing damage. The disease it causes, Diplodia tip blight, may then develop when the trees are subjected to stress. One of the main symptoms of Diplodia tip blight is the dieback of new shoots. In a study following an outbreak of D. sapinea after the drought summer of 2018, Matilda and her colleagues found that drought stress increased the risk of pines contracting Diplodia tip blight. However, the study also showed that even severely affected trees could recover and produce new shoots once the drought’s effects subsided.

– In my research, I have primarily worked with pines affected by both pine twisting rust and D. sapinea. Our initial hypothesis was that the rust predisposed trees to Diplodia tip blight —basically that the infections stressed the trees and paved the way for Diplodia tip blight. But that was not quite the case. Disease severity was more strongly influenced by the trees’ genetics, says Matilda.

An open door toward pine breeding

Matilda and her colleagues followed a pine stand planted in 2015 after the major wildfire in Västmanland, and observed that the same trees consistently avoided both pine twisting rust and Diplodia tip blight year after year. To understand what distinguished the disease patterns, Matilda studied differences in both the trees' metabolite profiles (the composition of metabolites in the tissues) and their genetics.

In one of her studies, Matilda used genome-wide association studies (GWAS) to identify specific genes linked to disease patterns. She and her colleagues found several genes associated with resistance to Diplodia tip blight. They then examined whether offspring from trees in the pine breeding program carrying these gene variants were more resistant than offspring from trees that lacked them.

– The connection between several of these genes and better resistance to D. sapinea was confirmed in material from the breeding program, which opens the door to breeding for resistance against D. sapinea in pine.

Staying one step ahead

A similar strategy—genomic selection—was also tested on spruce in the fight against root rot. The goal was to assess whether genomic selection could support resistance breeding. By analyzing trees' DNA and resistance phenotypes, the researchers built models to predict which individuals had the greatest resistance based on their DNA—a potentially valuable tool for the forestry industry’s future breeding work. The results are promising, though the models were not  accurate enough to identify the best progeny in a cross of two trees, suggesting more data is needed to build robust predictive models.

Fungal pathogens can exploit both external stressors and genetic vulnerability in trees, but there is also considerable variation in how host trees respond. By combining ecology, forest pathology, and population genetics, Matilda’s research enhances our understanding of forest disease dynamics and proposes practical strategies for breeding more resilient conifer populations.

– It is about staying ahead. By understanding how fungi interact with their host trees, and which genes make trees more or less susceptible, we can create more robust forests—something that is becoming increasingly important in a world with a changing climate, Matilda concludes.

Matilda will defend her doctoral dissertation on May 23, 2025, at Ultuna in Uppsala. Read more about the thesis defense here.