Climate is a major driver of susceptibility and response of forest trees to pathogens and insect pests. Pathogen pressures from foliage diseases are greater in warmer, wetter climates. Consequently, natural selection against trees susceptible to foliage diseases in warm, wet climates might be expected to lead to local adaptation and differences among populations in resistance or tolerance. This, in turn, could lead to increased mortality and decreased productivity of forests as populations encounter major changes in climate, either through climate change over time or from moving seed sources to new climates.
Tapping into the Douglas-Fir Seed Source Movement Trial, a long-term, multi-state study, a team of scientists with the Forest Service and Oregon State University assessed disease severity and symptoms for Swiss needle cast and Rhabdocline needle cast in Douglas-fir in relation to climatic difference between planting sites and seed-source locations.
Tree populations showed stark differences in their ability to resist Rhabdocline and tolerate Swiss needle cast. These results suggest that local seed sources are adapted to local climate and pathogen pressures and that seed sources from regions with high foliage disease pressure are most resistant/tolerant to those foliage diseases. Source movements from drier, more continental, to milder, more mesic climates are expected to lead to increased losses due to these foliage diseases.
Reforestation managers might consider moving tolerant or resistant populations from more maritime climates to mitigate some of the losses. Results suggest, however, that reforestation managers should be cautious about moving potentially drought-tolerant populations from areas with dry summers to areas with wetter summers.