Ash regeneration capacity after emerald ash borer (EAB) outbreaks: Some early results

Abstract

The emerald ash borer (EAB), a wood-boring beetle native of Asia, has killed millions of ash trees in North America since its detection in 2002. The rapid spread of the infestation and the widespread distribution and importance of ash in North America, coupled with the genus' high vulnerability raise the crucial question of ash regeneration capacity following EAB infestation and its potentially broad ecological implications. We report on ash regeneration and infestation at the epicentre of the initial EAB invasion in Canada (near Windsor, Ontario). Up to 12 years after detection of the outbreak, we found abundant ash regeneration in the impacted area. However, the likelihood of these stems reaching maturity appears low. Dissection of a subsample of saplings allowed us to confirm the presence of an active residual EAB population, with infestation in 19% of regenerating stems (including stems as small as 2 cm in basal diameter). A vigorous regenerating ash cohort comprising a large portion of stump resprouts might allow for survival of the genus in the landscape for decades, but it is likely that the key functional role played by ash species will be definitively altered as a result of persistent EABcaused mortality in maturing trees. Keywords:Agrilus planipennis, Fraxinus species, invasive forest pest, post-outbreak ash establishment, ash infestation. RÉSUMÉ L'agrile du frêne (EAB), un coléoptère indigène de l' Asie, a tué des millions de frênes en Amérique du Nord depuis sa détection en 2002. La propagation rapide de l'infestation, l'importance du frêne en Amérique du Nord, combinées à sa grande vulnérabilité, soulèvent la question de la capacité de régénération du frêne et des implications écologiques d'une telle infestation. Nous présentons ici des résultats portant sur la régénération du frêne et de son niveau d'infestation à l' épi-centre de l'invasion au Canada (près de Windsor, en Ontario). Jusqu'à 12 ans après la détection de l' épidémie, nous avons trouvé une régénération abondante de frênes dans la zone touchée. Cependant, la probabilité que ces tiges atteignent la maturité semble faible. La dissection d'un sous-échantillon de gaules nous a permis de confirmer la présence d'une population résiduelle active d'agrile du frêne, avec une infestation chez 19 % des tiges (y compris des tiges aussi petites que 2 cm de diamètre basal). Une vigoureuse régénération comprenant une grande partie de rejets de souches pourrait permettre la survie des frênes dans le paysage pendant plusieurs décennies, mais il est probable que le rôle fonctionnel clé joué par les frênes sera définitivement modifié en raison de la persistance de l'agrile causant la mortalité de la régéné-ration au fur et à mesure de sa maturation.

Buying Time: Preliminary Assessment of Biocontrol in the Recovery of Native Forest Vegetation in the Aftermath of the Invasive Emerald Ash Borer

“…Although parasitoid prevalence in some ash trees is as high as 35% for O. agrili and >90% for T. planipennisi at some study sites, the mature ash trees still experienced high mortality as they were already dead or dying when parasitoid releases began [31,37]. In areas no treated with biocontrol, up to 19% of the stems in the regenerating sapling layer can also be infected [76]. However, younger, thin barked ash trees and saplings growing at these release sites seem to be protected by the dominant biocontrol agent T. planipennisi, a small parasitoid with a short ovipositor that successfully parasitizes EAB larvae in ash trees <10 cm DBH [37,40,77,78].…”

Ecological Impacts of Emerald Ash Borer in Forests at the Epicenter of the Invasion in North America

“…In addition, the 58% ash mortality documented by Kashian [33] would have generated larger canopy gaps than observed by Klooster et al [13], where ash was a significantly lower component of more diverse forest stands [14]. In southeastern Ontario, Aubin et al [35] also observed substantial ash regeneration from basal sprouting. However, inter-and intraspecific competition experienced by regenerating ash would have been limited there as well, because the amount of pre-EAB ash basal area in the sampled stands was greater than twice that of all other species combined, and more than 99% ash basal area died following EAB establishment [35].…”

“…Epicormic basal sprouting can contribute to ash regeneration [34] and is a common response of ash trees that have had their canopies killed by EAB [33,35], especially for open-grown trees ( Figure 2). However, no such regeneration was observed by Klooster et al [13] in the closed-canopy mixed deciduous forests of the Upper Huron River watershed, where basal sprouts exhibited low vigor and died with the canopy or soon thereafter, perhaps due to strong interspecific competition for light and other resources in the understory of these diverse forests [14].…”

Loss of ash trees in riparian forests from emerald ash borer infestations has implications for aquatic invertebrate leaf-litter consumers

“…Our field surveys showed that ash can be a common, sometimes dominant, riparian tree species and a significant contributor to leaf litter deposition in the agricultural landscape of southwestern Ontario where EAB infestations are burgeoning (Aubin et al 2015). Nisbet et al (2015) predicted that because ash is likely a high-quality food source for aquatic consumers based on its nutrient quality, the loss of ash litter from EAB infestations in riparian forests would have measurable consequences for litter consumers.…”

“…Elm was relatively common in riparian plots of our study area (fourth most common riparian tree in terms of stem density) but is susceptible to Dutch elm disease which is still a serious threat to native North American elm (Knight et al 2012). However, the risk to litter consumers from the loss of elm trees is not as high as from the loss of ash trees because Dutch elm disease only affects matures trees and kills them relatively slowly whereas EAB rapidly kills even small ash saplings thereby virtually eliminating ash canopy trees (Aubin et al 2015). Nevertheless, the rapid loss of ash from EAB coupled with a slow loss of mature elm from Dutch elm disease where both species are present in riparian forests will undoubtedly exacerbate the reductions in high-quality food resources for consumers.…”