Ongoing regeneration of ash and co-occurring species 20 years following invasion by emerald ash borer

Abstract

Emerald ash borer (Agrilus planipennis Fairmare) is a destructive invasive insect pest of ash trees (Fraxinus spp.) in North America. Monitoring ash regeneration within post-invasion forests is essential to assess ash persistence in North America. We recorded density of overstory ash [> 10 cm diameter at breast height (DBH)], ash recruits (2–10 cm DBH), ash saplings (≥ 45 cm in height; < 2 cm DBH), and ash seedlings (< 45 cm in height), along with canopy dieback of overstory ash and recruits in four post-invasion areas in south-central Michigan, USA. We also recorded density of all other overstory trees, recruits, saplings, and seedlings by species. Ash regeneration was abundant in recruit (470 ± 68.9 stems per ha), sapling (2599 ± 336.1 stems per ha), and seedling strata (4557 ± 557.9 stems per ha). Overall, 47 % of overstory ash, 17 % of ash recruits, and 7 % of ash saplings were dead. More than half of the live overstory ash (54 %), and 43 % of ash recruits had < 30 % dieback, although 33 ± 3.0 % of ash recruits had external signs of EAB infestation. Living ash basal area was inversely associated with Quercus rubra and Tilia americana density. Dead ash basal area was not related to tree species composition, indicating that stands with low ash density, and subsequent effects on community structure, were not due to canopy gaps resulting from EAB mortality. Overall, we documented substantial ash regeneration in post-invasion forests despite high mortality of trees > 10 cm DBH.