Towards silvicultural mitigation of the European ash (Fraxinus excelsior) dieback: the importance of acclimated trees in retention forestry

Abstract

The European ash (Fraxinus excelsior L.) dieback is an acute forest pathology problem caused by the invasive ascomycete Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz, Hosoya. There are no practical solutions yet, but selection for resistant genotypes and intensive care have been highlighted as options. Our aim was to assess the disease mitigation potential of silvicultural harvests, which influence stress levels in retained trees. We annually monitored 577 retention trees on Estonian cut areas for 13 years, including 9 years impacted by the dieback. Sixty-five percent of the trees survived and 15% retained healthy crown, despite all sampled trees being infected. The damage was smallest in the trees retained near precut edges. Former forest-interior trees that were left in central parts of the cut areas suffered high initial damage but smaller disease progression than trees near postcut edges. Tree size and secondary infection by Armillaria spp. were not related to disease progression, but rapid decline was observed in the region with the highest density of ash trees retained. Our results indicate a significant silvicultural potential for tree resistance. Ash trees tend to be healthiest in open conditions, which probably inhibit the pathogen and provide better resources for the tree. Location near precut edges is an important tree retention criterion, which can mitigate initial harvest-induced stress.Résumé : Le dépérissement du frêne commun (Fraxinus excelsior L.) est un problème aigu de pathologie forestière causé par l'ascomycète invasif Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz, Hosoya. Il n'existe pas encore de solutions pratiques mais la sélection de génotypes résistants et les soins intensifs ont été mis en évidence comme solutions possibles. Notre but était d'évaluer le potentiel des interventions sylvicoles qui influencent le niveau de stress chez les arbres retenus pour atténuer la maladie. À chaque année pendant 13 ans, incluant 9 années durant lesquelles le dépérissement a sévi, nous avons suivi 577 arbres retenus dans une aire de coupe en Estonie. Soixante-cinq pourcent des arbres ont survécu et 15 % ont conservé une cime en santé même si tous les arbres échantillonnés étaient infectés. Les arbres retenus près de la lisière de la forêt avant qu'elle soit coupée étaient les moins endommagés. Les arbres situés à l'intérieur de l'ancienne forêt et qui ont été laissés dans le centre des aires de coupe ont initialement subi des dommages sévères mais la maladie a moins progressé que sur les arbres situés près de la lisière de la coupe. La taille des arbres et l'infection secondaire par Armillaria spp. n'avaient pas de lien avec la progression de la maladie, mais un dépérissement rapide a été observé dans la région où la rétention du frêne atteignait la plus forte densité. Nos résultats indiquent que la résistance des arbres offre un fort potentiel sylvicole. Les frênes ont tendance à être plus en santé dans les endroits ouverts où le pathogène est probablement inhibé et les ressources plus a...

Biological Flora of the British Isles:Fraxinus excelsior

“…But Rosenvald et al . () did not find Armillaria spp. contributing to the death of isolated retention trees.…”

“…(Bakys et al 2011) or environmental stresses (Bakys et al 2009a,b;Schumacher, Kehr & Leonhard 2010) exacerbated by ash dieback-infected ash having narrower vessels in the xylem (reducing from 0.15 mm in healthy to 0.12 mm in infected trees) with consequent reduced hydraulic conductivity (Tulik, Marciszewska & Adamczyk 2010). But Rosenvald et al (2015) did not find Armillaria spp. contributing to the death of isolated retention trees.…”

“…As noted above, more open stands are less damaged by ash dieback and the reduced competition for water, light and nutrients should make the trees more resistant to the disease. Rosenvald et al (2015) followed the fate of 577 ash trees left after felling (retention trees); after 13 years, 65% remained alive and 15% were healthy despite all trees being infected. The most isolated individuals fared best, particularly those that had been exposed to semi-open conditions for longer such as those along forest edges, which holds out hope for mature retention trees surviving as seed parents.…”

Landscape epidemiology of ash dieback

“…Several authors showed that high moisture promote ash dieback (Husson et al 2012;Marçais et al 2016) and it has been hypothesized that dense stocking could favour disease severity by enhancing air humidity (Havrdová et al 2017). Rosenvald et al (2015) showed trees in stand edge were less affected by H. fraxineus and hypothesised that high temperatures could limit the disease development in these environments. Last, high summer temperature were demonstrated to limit ash dieback was in the Rhône valley in France at the southern limit of the disease presence .…”

“…Reports on the impact of host density on the disease severity have been conflicting (Skovsgaard et al 2017). Nevertheless, it is clear that dense stands with a closed canopy are very susceptible to the fungus while ashes set up in open habitats or near forest edges appears to be less affected (Havrdová and Cerný 2013;Rosenvald et al 2015;Vacek et al 2015). H. fraxineus has also been shown to have a poor survival at temperatures above 35°C, which affects the disease epidemiology (Hauptman et al 2013.…”