PARENT SESSION
Oral Session #76: Plant Ecology: Nutrient Uptake, Movement, Use. Presiding: J. Schade.
Friday, August 10, 2001. 8:00 AM to 11:45 AM. Hall of Ideas G.
Nitrogen surplus in Douglas-fir forests: Consequences for susceptibility to an endemic foliar fungus.
Rose, Cathy1, Kavanagh, Kathleen2, Waring, Richard 1, Manter, Dan1, 1 2
ABSTRACT- Persistent and severe defoliation has been documented in approximately 200,000 hectares of coastal Douglas-fir (Pseudotsuga menziesii) forests. Symptoms are associated with heavy infection of foliage by a common endophyte, Phaeocryptopus gaeumannii, the causal agent of Swiss needle cast disease. A mechanism is lacking to explain forest vulnerability to this normally endemic fungus. Our study tests the hypothesis that P. gaeumannii infection is associated with supra-optimal availability of nitrogen (N) in young Douglas-fir forests. A causal mechanism involves the accumulation of free amino acids (FAA) in needles of trees grown at high N levels. Free amino acids are known to be an excellent source of nutrition to other species of fungal endophytes related to P. gaeumannii. We collected needles from four 12-20 year-old Douglas-fir plantations with varying levels of P. gaeumannii infection. All needle age classes were sampled two times during the dormant season, just prior to budbreak, and just after bud break. Seasonal patterns of foliar FAA concentrations were as expected. Expanding needles had the highest concentrations. All foliage had higher concentrations of FAA compared to reported values for coastal Douglas-fir saplings. In some cases, FAA values exceeded levels reported for fertilized stands. Unusually high concentrations of foliar FAA in Coast Range Douglas-fir support our hypothesis that a nutritional imbalance, particularly high N availability may explain the susceptibility of Coast Range forests to Swiss needle cast disease.
KEY WORDS: excess nitrogen, endophyte, Douglas-fir, foliar free amino acids