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Severity of Swiss needle cast in young and mature Douglas-fir forests in western Oregon, USA
Citation:
Lan, Y., D. Shaw, Peter A Beedlow, EHenry Lee, AND Ronald S Waschmann. Severity of Swiss needle cast in young and mature Douglas-fir forests in western Oregon, USA. FOREST ECOLOGY AND MANAGEMENT. Elsevier Science Ltd, New York, NY, 442:79-95, (2019). https://doi.org/10.1016/j.foreco.2019.03.063
Impact/Purpose:
Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) is a dominant PNW tree species that is infected by the foliar fungus Nothophaeocryptopus gaeumannii (Rhode) Petrak which causes Swiss needle cast (SNC) disease. There is currently a SNC epidemic occurring in the PNW which has raised concerns that SNC is increasing in severity, frequency, and range in association with rising temperatures. However, very little is known about the severity of SNC in mature and old-growth Douglas-fir stands. WED scientists in collaboration with forest health specialists at the Department of Forestry, Oregon State University (OSU) investigated SNC disease patterns in young, mature, and old-growth forests across an elevation and precipitation gradient in western Oregon, USA. WED and OSU scientists show that: 1) disease severity is much greater in young forests than mature forests; 2) incidence of disease is highest for two-year-old needles in young trees and 3-5-year-old needles in mature trees; and 3) no single factor including total foliar nitrogen concentration and leaf wetness can explain the age differences in SNC severity. We speculate that multiple factors including needle anatomy, chemical and thermal properties, and possibly, increased tolerance to SNC with increasing exposure may have caused the stand age differences in disease severity. Our work is important to the timber industry which has introduced Douglas-fir plantations in coastal Oregon and Washington within the SNC impact zone. SNC in combination with climate stress are predicted to decrease productivity, forest health and condition in both young and old Douglas-fir forests in western Oregon and elsewhere.
Description:
Swiss needle cast (SNC), caused by Nothophaeocryptopus gaeumannii, is an important foliage disease of Douglas-fir (Pseudotsuga menziesii) forests of the Pacific Northwest. The fungus lives endophytically within the foliage, until forming reproductive structures (pseudothecia) that plug stomates and cause carbon starvation. When pseudothecia appear on 1- and 2-yr foliage, significant foliage loss can occur, which reduces productivity of the tree. The fungus is everywhere Douglas-fir grows, and foliage age class of pseudothecial emergence appears to relate to disease expression. While there is considerable evidence of SNC disease in coastal Douglas-fir plantations, the severity of SNC in mature and old-growth forests is poorly understood. We compared tree crowns of mature and old-growth conifer forests and nearby young forests by tree climbing at three locations in the Oregon Coast Range and four locations in the western Cascade Range of Oregon. We assessed disease severity for N. gaeumannii on 2-yr foliage, incidence by presence of N. gaeumannii on all foliage, foliage retention for the first 4-yrs, and foliar nitrogen of 1-yr foliage. We also compared leaf wetness in one mature and one young tree at five of the seven sites. Disease severity was greater in young forests than mature forests at all sites. Incidence of disease was highest for two-year-old needles in young trees and 3-5 year-old needles in mature trees, except for one coastal site. Retention of 1-4 year-old needle cohorts did not differ between young and mature trees, but mature trees had much larger complements of >4 year-old needles. Total foliar nitrogen concentration did not differ in needles of young and mature trees, but at some locations total N differed between canopy positions. Leaf wetness differences were not consistent between young and mature tree crowns and did not inform disease severity differences. However, at one study site in the core epidemic area, the younger stand had longer periods of wetness in the upper crowns than a nearby old stand. Leaf wetness and foliar N were hypothesized to play a role in SNC disease severity, but they do not explain differences in adjacent young vs mature trees. In younger stands, the fungus appeared to block stomates earlier than in older stands and stomatal occlusion was always greater on younger than older trees for two-year old needles. We speculate that multiple factors may have caused the observed differences, including stand age differences in thermal properties, needle anatomy, chemical differences, and/or genetics. It is also possible that older trees are less impacted by N. gaeumannii because they have experienced exposure to the fungus over a longer period, which could influence host-fungus interactions. Also, four of the sites that we examined were outside the current epidemic area and were not considered diseased, so this may have reduced disease influence on foliage retention. Although the fungus is present in old and young trees, the likelihood of disease expression and lower foliage retention appears to be greater in younger plantation forests than mature and older forests in western Oregon Douglas-fir forests.
URLs/Downloads:
DOI: Severity of Swiss needle cast in young and mature Douglas-fir forests in western Oregon, USA