Mechanisms promoting persistence of ash in the presence of an invasive wood borer (emerald ash borer)
Emerald ash borer (EAB) has killed millions of ash trees near its introduction point in southeastern Michigan, making the region critical in understanding the ecological impacts of EAB as its range continues to expand. Existing ash regeneration established prior to the onset of EAB would be sufficient to replenish overstory ash to pre-EAB levels without additional mortality. However, recent studies in the area have provided evidence for depletion of ash from the seed bank and a reduction of new recruits as canopy trees experience heavy mortality. Long-term ash persistence is therefore unclear because of uncertainties in the future dynamics of EAB that include whether existing ash regeneration will be killed before it reaches reproductive capability. Potential mechanisms for persistence of ash in the presence of EAB were examined by sampling 17 small, nearly monospecific stands of green ash in three consecutive growing seasons, including the years before and after a mast year for ash. This study examined the current status and condition of surviving, mature ash trees, the amount of seed produced by ash trees in stands highly affected by EAB, and the level of ash seedling recruitment that occurs as a result of current seed production.
Some canopy trees survived the initial EAB attack (mean basal area lost = 74%), but 18.6% of the remaining live trees exhibit symptoms of EAB infestation, including 55% of seed-bearing trees during the mast year. Regeneration occurred as basal sprouts originating from killed trees, advanced regeneration that established prior to EAB, and seedlings that established since EAB infestation. Sprouting dominated ash regeneration in every year (p < 0.001), and 27% of large sprouts produced seeds during the mast year in 2011. Advanced regeneration represented a lower proportion of regeneration than sprouts but more than post-EAB seedlings, which were significant only following the mast year (p < 0.001). Seed dispersion was correlated to seed production (p = 0.004) and source dispersion (p = 0.03) when seed production was low, and was related to seed production alone in the mast year (p = 0.04). A lack of spatial correlation between seeds and recruits suggests that seed dispersal does not limit seedling recruitment. These results suggest that the seed-producing ability of small trees and basal sprouts may allow ash to persist in the presence of EAB, although individual ash trees are unlikely to ever resemble the stature of those prior to EAB.