Citation:

Shaw, D., Y. Lan, J. Hatten, Randy L. Comeleo, AND G. Ritokova. Needle retention in Oregon Coast Range Douglas-fir plantations across elevational and disease severity gradients. 2018 Society of American Foresters National Convention, Portland, OR, October 03 - 07, 2018.

Impact/Purpose:

Swiss needle cast is a plant disease caused by a fungus which adversely affects the needle-like leaves of Douglas-fir trees in the Pacific Northwest, USA. The disease causes trees to prematurely shed, or cast, its needles. The severity of the disease and its impacts on tree growth may be assessed by evaluating the number of years the trees are able to retain their leaves. The objectives of this research are to confirm that reduced needle retention is related to increased disease severity, determine which portion of the tree crown best indicates disease severity, develop a map of foliage retention for the Oregon Coast Range based on observations of leaf loss and factors such as land elevation. Foliage retention was assessed for the upper, middle, and lower crowns of young Douglas-fir trees located in 106 tree plantations found at different elevations in the Oregon Coast Range. The relationships between needle retention, tree crown level, disease severity, and land elevation were examined. The mid-crown portion of the tree was the most consistent for estimating needle retention. Disease severity was significantly related to mid-crown foliage retention. Foliage retention increased with increasing elevation. A needle retention map for the Oregon Coast Range will aid managers in estimating the potential for disease impacts and help ensure that Douglas-fir trees remain a productive component of the coastal forests of Oregon and Washington in the presence of Swiss needle cast disease.

Description:

Swiss needle cast, a foliar disease affecting Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in the Pacific Northwest, USA, is caused by the ascomycete fungus Nothophaeocryptopus gaeumannii. (T. Rohde) Videira et al. Disease severity and growth impacts are assessed using estimates of foliage retention in years. Growth declines below normal begin under 3.2-yrs, and at 1-year of foliage retention, growth losses exceed 50%. On the landscape, foliage retention varies with site productivity and elevation. The objectives of this research were to confirm the relationship of foliage retention to disease severity, determine which crown level is most consistent in representing disease severity, develop a foliage retention map based on empirical data, and test the relationship of needle retention to elevation. We used the Swiss needle cast Cooperative research and monitoring plot network to evaluate foliage retention across an elevational gradient of the Oregon Coast Range within 55 km of the Pacific Ocean. The plot network consists of 106 plots systematically located from the California-Oregon border into SW Washington State. We estimated needle retention from 5-10 trees per plot for the upper, middle and lower crowns of young plantation trees between 10 and 25 yrs old. We tested the relationship of needle retention to crown level (upper, mid, lower), disease severity (estimated by counting fungal reproductive structures emerging from stomates), and across elevation (50 m – 810 m) using linear mixed models. We found that the mid crown region was the most consistent in estimating needle retention, perhaps due to variability in the upper crown from weather and disease factors, and shading in the lower crown. Disease severity was significantly related to mid-crown foliage retention. Foliage retention increased with increasing elevation as predicted, but there is still unexplained variation, perhaps complicated by disease epidemiology. A needle retention map for the Oregon Coast Range will aid managers in estimating potential for disease impacts in plantations. Needle retention remains a useful tool for plantation managers in the Oregon Coast Range.

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