Nearly 15 years after emerald ash borer (EAB; Agrilus planipennis) was initially detected in southeastern Michigan, it has killed millions of ash (Fraxinus spp.) trees and has the potential to eliminate the entire genus from forest and urban communities across the nation. What is yet unknown, is the long-term ecological impacts that EAB will have on the affected communities. In particular, foresters and land owners are interested in which species will respond favorably to the loss of ash trees, and how plant community composition will be altered. To address these questions, 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 and spanned a gradient of ash density (prior to EAB-induced mortality). Ash importance values (IVs) (relative basal area × relative density × relative dominance) by transect were calculated using data collected prior to ash mortality. Gap fraction values were calculated from hemispherical photographs taken throughout the transects. In 2013, over 850 trees comprising a variety of non-ash species, shade-tolerance levels, and canopy positions were cored using a standard increment borer. The cores were then analyzed for growth releases indicated by increased ring widths.
Results/Conclusions
In this region, ash tree mortality exceeded 99% by 2009, and the ash seed bank was depleted by 2007. Importance 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. Initial analyses of the tree cores indicate species-specific responses to ash mortality. The majority of species exhibiting positive correlations between ash IV and increased ring widths (trunk growth) are either shade-tolerant or intermediate species, whereas the ring widths for most of the shade-intolerant species were not correlated with ash IV. Furthermore, the canopy position of the trees (whether dominant, codominant, intermediate, or suppressed) may have an affect on their response to ash mortality, with ring widths of some codominant species increasing in size; however, additional analyses are being performed to clarify the initial observations.