Spatial reconstruction of western spruce budworm outbreak dynamics in the central western Cascades of Oregon since AD 1600

Background/Question/Methods

Although the western spruce budworm (Choristoneura occidentalis Freeman) is known as an important disturbance agent in dry, mixed-conifer forests of eastern Oregon, little is known about budworm activity in mesic forests west of the crest of the Cascades. Despite abundant host trees and documented endemic budworm populations, outbreaks observed since the mid 1900s have been small in extent and concentrated primarily near the crest of the Cascades. Because the documentary record is short relative to the frequency of historical outbreaks, dendroecological methods were employed to provide proxy records of budworm outbreak dynamics on a time scale that better captures recent climatic fluctuation and associated disturbance cycles. The study was conducted in the 240-km² Blue River watershed in the central western Cascades of Oregon. Outbreak reconstruction was based on quantitative comparison of growth patterns in tree cores collected from more than 200 Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and 50 western hemlock (Tsuga heterophylla (Raf.) Sarg.) trees, host and non-host, respectively. Tree cores were collected at 47 sites. Using the mean host-tree growth reduction at each site as a measure of outbreak severity, inverse distance weighted interpolation was applied to map the severity of each inferred outbreak over the last 400 years.

Results/Conclusions

Since AD 1600, 10 inferred outbreaks were identified based on criteria of 35% of the host trees meeting a growth reduction threshold for at least 8 consecutive years. The reconstructed outbreaks exhibited patchiness in severity at a finer scale than documented in recent outbreaks in drier forests of eastern Oregon. Growth reduction was spatially contagious regardless of elevation, slope gradient, topographic position, or aspect. Three reconstructed outbreaks decreased in severity from east to west, as expected if west-side outbreak dynamics were driven by a “spill-over” of budworm populations during major outbreaks in drier, eastern Cascades forests. However, severity decreased from west to east or north to south during other outbreaks, thereby supporting a role of endemic budworm populations responding to favorable climate or other factors. The most severe reconstructed outbreak, in the mid 18^th century, led to higher growth reduction over a larger area than previously documented in the western Cascades. Long-term reconstruction of outbreak dynamics suggests that budworm outbreaks have contributed to the development of most old-growth forests in the western Cascades. Possible influences include synchronous releases of suppressed shade-tolerant trees and temporarily increased susceptibility to fire following defoliation due to reduced within-stand humidity and reduced fuel moisture.