Short-term climate trends and the Swiss needle cast epidemic in Oregon's public and private coastal forestlands

Swiss needle cast (SNC) is a fungal disease of Douglas-fir (Pseudotsuga menziesii) that is having important consequences on tree growth in the Pacific Northwest (PNW) region of the USA. Once considered innocuous in PNW forests, SNC symptom expression has increased rapidly in extent and intensity in recent decades. Previous research has linked the disease epidemiology of SNC to climate, and observations indicate a link with forestry practices of the 20th century as well, particularly with the conversion of old growth and mature mixed-conifer forests to young monocultures of Douglas-fir on private forestlands. Given the sensitivity of SNC intensification to forestry practices and changing climatic conditions, it seems plausible that disease behavior response to short-term directional climate changes could differ between contrasting forest management regimes. We compared the relationship between trends in canopy energy and water flux parameters detected during the spring and summer months (May-August) along the Pacific Coast of Oregon from 2003 to 2012, and the distribution of SNC symptoms in 2012 on private and public lands. Canopy energy and water exchange parameters were calculated with MODIS Land Surface Temperature (LST), and evapotranspiration (ET) data, and with Parameter-elevation Relationships on Independent Slopes Model (PRISM) precipitation data. We found that a higher level of deviance in SNC presence/absence could be explained on private land compared with public land. Proximity to coast explained 9.3% of the deviance on private land and 6.7% on public land. Trends in LST during May and August emerged as important and explained 7% of the deviance in SNC symptom distribution on private land compared with 2% on public land. When combined with proximity to coast, May and August LST explained 14% of the deviance in SNC symptom expression on private land and 8.7% on public land. We found a significant difference between public and private ownership for the proportion of SNC (p = 0.0006), and a significant interaction between ownership and distance to coast (p = 0.0019), such that across public and private ownership, distance to coast has a different effect. LST may provide valuable information on leaf wetness, or thermal properties of the canopy, possibly capturing both early season and late season dynamics important to SNC epidemiology. We find evidence that recent short-term directional climate changes may have contributed to differences in symptom development in Douglas-fir forests on private and public land, with symptoms more prevalent on private land.