Using dendroecology to disentangle forest changes pre- and post-EAB

Background/Question/Methods

The Emerald Ash Borer (Agrilus planipennis - EAB) invasion has decimated ash (Fraxinus spp.) populations, including in our 8 ha NE Ohio study site, where ash trees were previously the third most abundant taxa. However, permanent plot surveys show ash declines prior to the 2001 EAB invasion, perhaps due to severe droughts in 1988, 1991, & 1994. We used tree-ring data in an effort to understand differences in growth patterns between trees that died pre-EAB and those that survived to die post-EAB. Since ash trees grow in both upland and floodplain habitats at our site, we cross-dated ash tree-ring data with cores from living white oak (Quercus alba) and tulip poplar (Liriodendron tulipifera). We used GPS locations of all the ash, white oak, and tulip poplar trees in the preserve in order to test our hypothesis that site variation played a primary role in the pre-EAB ash decline. We predicted that trees on drier sites would have developed to deal with severe droughts. We measured growth rings to the nearest 0.001mm using a Velmex system, developing master files with COFECHA. We used dplR for crossdating and to produce Basal Area Increment (BAI) values for comparing chronologies with ANOVA.

Results/Conclusions

All three species show BAI decreases (n.s.) during the three drought years. Both ash and tulip poplar also had significant (p< 0.05) growth declines following the 2001 EAB invasion, while oak continued with slight growth increases. We compared growth patterns in both ash and tulip poplar separated by habitat (floodplain vs. upland) and by time interval (pre-drought vs. post drought - pre-EAB). The two species differed in responses, with upland tulip poplars showing no response (p=0.814) but floodplain trees showing significant declines (p=3.41x10-5), while ashes had significant declines (p=5.7x10-8) in the upland and no decline in the floodplain (p=0.407). We offer two potential explanations: 1) floodplain and upland ashes could be different species (F. pennsylvanica and F. americana, respectively), a distinction not made in earlier surveys, and 2) since the tulip poplar cores are all from trees that were living in 2019, the data do not include trees dying pre- or post-drought, as the ash data do. The permanent plot surveys show that tulip poplars and sassafras (Sassafras albidum) experienced declines similar to ash pre-EAB. Subsequent surveys will include living and dead cores from these two species in an effort to better understand community composition changes pre- and post-EAB.