Douglas-fir foliage retention dynamics across a gradient of Swiss needle cast in coastal Oregon and Washington

Abstract

Swiss needle cast (SNC) is an important foliage disease of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) caused by the native pathogen Nothophaeocryptopus gaeumannii (T. Rohde) Videira, C. Nakash., U. Braun & Crous, which has been present in epidemic proportions in coastal Douglas-fir forests since the 1990s. Under conducive environmental and stand conditions, the fungal fruiting bodies emerge on young needles, inhibiting gas exchange and causing premature needle casting and subsequent growth losses. Using a new regional plot network, which extends and approximately doubles the area of SNC-susceptible coastal forest sampled, we investigated the distribution of SNC disease indices across the region and throughout individual tree crowns. Foliage retention varied from 1.15 to 3.9 years, and disease severity index (incidence × % occluded stomata) ranged from 0.05% to 52.11%. Foliage retention was positively correlated with distance from the coast and elevation, while foliage retention and disease severity were negatively associated across the study area. Within crowns, disease severity was negatively associated with crown depth, and foliage retention was positively associated with crown depth, regardless of distance from coast. Across the entire study area, foliage retention was found to decrease and disease severity increase with latitude, all else being equal. Tree growth metrics are positively associated with increasing foliage retention, and normal growth occurs greater than ∼3.2 years.