Hemlock woolly adelgid (Adelges tsugae Annand) is causing defoliation and mortality of eastern hemlock (Tsuga canadensis (L.) Carrière) in the eastern United States. The objectives of this study were to quantify changes in tree-ring width and wood anatomy for trees that survived adelgid infestation, and to contrast dendroclimatic relationships across a latitudinal gradient. Six sites spanning the current range of hemlock woolly adelgid (HWA) infestation were selected. At each site, 23 infested eastern hemlocks were cored and two trees were felled at the Virginia site and thin-sectioned using a sliding microtome for analysis of wood anatomy. Tree cores were cross-dated and ring widths were measured. For each site, t-tests were used to determine if there was a difference in radial growth pre- and post-HWA arrival. To compare differences in pre- and post-HWA cell properties, t-tests were used. For dendroclimatic analysis, Pearson correlation coefficients were calculated between radial growth and monthly climate variables. Three sites showed significant suppression in radial growth after HWA arrival and latewood produced post- HWA arrival had significantly smaller cells with reduced cell-wall thickness than latewood produced before HWA arrival. This indicates that HWA can reduce a tree's photosynthate production. The relationship between hemlock growth and climate also varied with latitude and site, with trees growing further south or on shallower soils being more sensitive to moisture levels. This sensitivity to drought can also partially explain the variation in hemlock response to adelgid feeding, as trees affected by moisture stress tend to be more sensitive to insect attack. / Master of Science

I assessed the relationship between natural disturbances and climate in Douglas-fir forests in western North America. I quantified synchrony within disturbance types, explored the potential for synergism between disturbance types, and analyzed changes in disturbance dynamics that have occurred following Euro-American settlement of western North America. I used new and previously published dendrochronological reconstructions of disturbance histories and climatic variability to explore these complex interactions at multiple spatial scales over the last three centuries. I used dendroentomological methods to reconstruct western spruce budworm outbreaks at thirteen sites along a transect running from central Oregon to western Montana. These forests experienced repeated, often decadal-length western spruce budworm outbreaks over the last three centuries. I compared my records with previously published outbreak reconstructions and found widespread synchrony of outbreaks at stand-level, regional, and sub-continental scales. At ten of my sites, I also reconstructed or obtained previously published reconstructions of fire dates. I compared these disturbance histories with dendroclimatological drought records to quantify the influence of moisture availability on disturbances. I found that fires were more likely to occur during drought years, while western spruce budworm outbreaks were most likely to begin near the end of droughts. After approximately 1890, fires were largely absent from these sites and western spruce budworm outbreaks became longer-lasting, more frequent, and more synchronous, likely due to land-use induced changes in the structure and composition of forests. My results show no discernible impact of defoliation events on subsequent fire risk. Any effect from the addition of fine and coarse fuels during defoliation events was too small to detect given the overriding influence of climatic variability. If there is any relationship between the two disturbances, it is a subtle synergistic relationship wherein each disturbance type dampens the severity but does not alter the probability of occurrence of the other disturbance type over long time scales. This dissertation includes unpublished co-authored material. / 10000-01-01