Forest responses to emerald ash borer (Agrilus planipennis) induced ash mortality

As emerald ash borer (EAB; Agrilus planipennis) continues to spread throughout North America, land owners and managers are faced with the loss of untold millions of ash (Fraxinus spp.) trees. It is yet unclear how such widespread, simultaneous mortality will alter forest communities. Our objective was to determine if gap formation due to ash mortality has altered the successional trajectory of the plant community. We established 42 transects, each containing three 0.1 ha plots, in seven natural areas in southeast Michigan. Transects were classified according to soil moisture from dry xeric uplands previously containing white ash, to more organic mesic sites previously containing green ash, to wet hydric sites previously dominated by black ash. Plots also spanned a gradient of ash density (prior to EAB-induced mortality) and time since EAB infestation. In this region, ash tree mortality exceeded 99% by 2009, and the ash seed bank was depleted by 2007. Gap fraction values were calculated from hemispherical photographs taken throughout the transects. Ash importance values (IVs) (relative basal area × relative density × relative dominance) were calculated using plot data collected prior to ash mortality; values were compared across moisture levels with the highest IVs in hydric plots and the lowest in xeric plots. Gap fraction was positively correlated with IV in xeric plots, but was not correlated in hydric or mesic plots. Gap fraction was negatively correlated with canopy cover in the subcanopy (2-5 m) layer, suggesting that any increase in light due to canopy gaps formed by ash mortality is likely being intercepted before reaching the ground or understory layers. Ongoing research is focused on the response of non-ash overstory and understory trees to determine what effects, if any, ash mortality and canopy gap formation had on their growth. We are analyzing trunk cores collected from a variety of dominant, co-dominant, intermediate, and suppressed shade-tolerant and shade-intolerant trees to quantify their growth release in response to gap formation.