The Emerald Ash Borer (EAB) is an invasive beetle that has killed millions of ash trees (Fraxinus spp.). Anecdotal evidence suggests that our local ash populations died almost simultaneously following rapid spread of beetles. However, no research has determined a mortality date for these trees, and we observed variation in tree decay. Based on these observations, we hypothesized that variation in ash mortality reflected preferential colonization by EAB, perhaps due to different habitat types. We predicted that a chronological reconstruction would show variation in mortality dates. Between September 2017 and January 2018, we obtained cores from over 200 dead Fraxinus trees and 20 living white oak trees (Quercus alba) throughout our 8-hectare study site. We measured the width of growth rings in all the cores to the nearest 0.001mm using a Velmex system. We developed a chronology of oak growth using COFECHA and ARSTAN software, which we used to determine Fraxinus mortality dates. We then obtained GPS coordinates for all the trees and performed one-way ANOVAs to test for spatial variation in ash mortality dates. We also tested for correlations between ash populations/ring widths and temperature/precipitation using data obtained from Oregon State University’s PRISM climate database.
Results/Conclusions
Our results indicate that ash tree mortality spanned 1985 through 2012, with a significant increase in mortality rate after 2001 (p=0.0124), with peak mortality in 2009. Because peak mortality tends to lag invasion by 7-8 years, we proposed that EAB entered our study site between 2001 and 2002. Neither ash populations nor ring widths correlated with temperature/precipitation, but 3 of the 4 greatest droughts since 1950 occurred during the period of pre-EAB ash loss (e.g. 1985-2000), suggesting that drought stress killed these trees in the years following the drought. Although we found variation in post-EAB mortality dates, these dates were not significantly correlated with habitat characteristics, and we concluded that this variation was due to gradual increases in EAB populations over time. The density of rotted trees, however, was significantly greater in the slope portion of our grid than in the upland or floodplain (p=0.008), which suggests that the deep, dry soil of slope plots may increase the susceptibility of trees to drought stress. From these results, we propose that EAB most likely entered Chance Creek Preserve between 2001 and 2002, and that prior ash tree deaths were due to climate-related stress.