Use of tree-ring stable isotopes to quantify Swiss Needle Cast disease severity in Douglas-fir

Background/Question/Methods

Swiss needle cast (SNC) is a fungal disease that has damaged Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands in coastal Oregon since the early 1980s. The distribution of the disease appears to be associated with climatic variables, though no single environmental factor thoroughly explains variation in SNC occurrence. Previous research using tree-rings to understand the history of SNC in Oregon has been limited to ring-width analysis. SNC primarily affects photosynthesis through reduced gas exchange via partial blockage of stomata. Because the stable isotope ratio of photosynthate δ¹³C records both assimilation rate and plant water status we hypothesize that the tree-ring stable isotope ratio of δ¹³C will provide an integrated measure of SNC symptom expression and its relationship with climate. We collected 90 increment cores from treated and untreated trees at a site where a fungicide trial designed to treat SNC took place from 1996 to 2000. The tree-cores were measured and cross-dated according to standard dendrochronological procedures. The δ¹³C of cellulose of earlywood and latewood of each ring was compared with ring growth characteristics of treated and untreated trees in relation to past climate variables.

Results/Conclusions

Prior to the application of fungicide in 1996, there was no difference in the average ring width between stands of treated and untreated Douglas-fir. From 1998 through 2002, the average ring width of trees growing in the fungicide treated stand was nearly twice that of untreated trees. In the five years following the first fungicide application, earlywood δ¹³C of treated trees was more negative than trees in the untreated plot. Differences in latewood δ¹³C between treated and untreated trees were not as strong. These results suggest that the fungicide effectively unblocked stomata during treatment years and that the effects of the disease are most apparent during spring before infected needles abscise and current year foliage growth is complete. Analyses are underway regarding the relationship of δ¹³C to climate in treated and untreated trees. These results provide evidence that the δ¹³C signal in earlywood can isolate a physiologically driven signal of SNC infection that may be more reliable than ring width measurements for detecting past disease severity. This signal has potential applications toward investigating regional histories of SNC and the climate variables that exacerbate disease severity.