Effects of climate on the radial growth of white ash infested with emerald ash borer

- White ash in the northern regions had a negative response to winter snowfall.
- Growth was negatively related to prior summer precipitation and moisture index.
- The prior year summer influence was observed in all insect infestation groupings.
- Growth had a negative response to current year's (early fall) moisture index.
- The current year moisture effect was found in the high severity infestation level.

Emerald ash borer (EAB) (Agrilus planipennis Fairmaire), is a non-native beetle responsible for widespread mortality of several North American ash species (Fraxinus sp.). The objective of this study was to examine the effects of climate on the radial growth of white ash (Fraxinus americana) infested with emerald ash borer. Sites with infested white ash from different land-use categories (i.e., natural forests vs. high-use recreational areas) were selected along a latitudinal gradient in the eastern Lower Peninsula of Michigan. Infested trees were selectively sampled based on predetermined crown condition (vigor) classes and categorized into two levels of EAB infestation severity (i.e., low vs. high). Sampling of 3 control regions in the northern Lower Peninsula, and western Upper Peninsula were based on measuring trees that were visually healthy (no major crown mortality or visible signs and symptoms of EAB). Dendroclimatic analyzes indicated that radial growth of white ash in the three northern control regions were the only groups to have a negative response to winter snowfall, indicating a latitudinal sensitivity. Dendroclimatic analyzes also indicated that white ash in Michigan across the control regions and EAB impacted latitudinal regions, land-use categories, and EAB infestation levels, was shown to have a strong negative relationship with precipitation and available moisture in the summer of the previous year. A negative response in the current year's radial growth due to the climate moisture index at the end of the growing season for trees with higher severity EAB infestation levels was also observed. The negative relationship with summer precipitation and moisture index suggested that crown canopies may be damaged by strong winds associated with storm events characteristic of Michigan summers. Increased risk of branch failure due to EAB infestation may further increase ash susceptibility to wind damage. Since moisture levels have also been shown to be important for EAB larval development, it is possible that the negative responses of radial growth to precipitation at the end of the growing season may benefit EAB larval development, thereby reducing radial growth of white ash.