Potential alternative tree species to Fraxinus excelsior in European forests

Abstract

Common ash (Fraxinus excelsior L.) is a keystone tree species in Europe. However, since the 1990s, this species has been experiencing widespread decline and mortality due to ash dieback [Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz and Hosoya]. Besides H. fraxineus, emerald ash borer (Agrilus planipennis Fairmaire), an invasive alien pest already devastating ash trees in western Russia, is spreading westward and becoming an emerging threat to the remaining European ash populations. While efforts to control ash dieback continue to be a priority, it is becoming crucial to compensate for the loss of ash and its ecosystem services by elaborating restoration strategies, including the search for alternative native and non-native tree species. This review summarizes available knowledge on potential alternative tree species to common ash to help forest managers to cope with ash dieback. Although using natural regeneration and promoting tree species diversity can reduce the impacts of ash dieback in European forests, our review indicates that no native species alone or in combination can fully replace the ecological niche of common ash and its associated ecosystem services (e.g., biodiversity and timber). To fill this gap, forest managers have considered using non-native ash species that are tolerant to both H. fraxineus and A. planipennis and have similar ecological and forestry values as common ash. Of the 43 ash species reviewed, few non-native ash species (e.g., Fraxinus mandshurica Rupr. and Fraxinus platypoda Oliv.) have similar ecological characteristics to common ash and are tolerant to H. fraxineus and A. planipennis. However, the performance of non-native ash species in European forests, their invasiveness potential, and the risk of hybridization with native ash species are still unknown. With the current state of knowledge, it is thus too early to recommend the use of non-native ash species as a suitable option to deal with ash dieback. The priority should be the conservation, regeneration, and breeding of tolerant common ash populations to H. fraxineus, as well as the use of the natural regeneration of other native tree species. Our review highlights the need for controlled experimental plantations to better understand the regeneration ecology and invasiveness potential of non-native ash species prior to their utilization in natural forests.

The ecological role of Fraxinus for species diversity in floodplain forests

“…This would include analysing other forest types, e.g. Hordelymo-or Carici-Fagetum on calcareous ground, where Fraxinus occurs in greater abundance due to its high physiological plasticity (Lévesque et al 2023). In contrast to Q. robur and U. laevis, only six specimens of T. cordata were sampled, which proved insufficient for a reliable diversity analysis, as the high variability within the beetle communities prevents a clear assignment to either the Q. robur-U.…”

Genetic barcodes for ash (Fraxinus) species and generation of new wide hybrids

“…Worldwide species of ash differ in their levels of resistance to ADB and EAB (Koch et al ., 2007; Kowalski et al ., 2015; Rigsby et al ., 2016; Nielsen et al ., 2016; Plumb et al ., 2019; Kelly et al ., 2020). Thus some non-native Fraxinus species could be suitable to replace susceptible populations (Jepson & Arakelyan, 2017; Marzano et al ., 2019; Lévesque et al ., 2023) or used as parents in hybrid breeding programmes to transfer resistance among species (Koch et al ., 2007, 2012; Plumb et al ., 2019).…”

Priming of ash saplings with a low virulent Hymenoscyphus fraxineus strain as a possible disease control approach for reducing symptoms of ash dieback

“…However, ash saplings produced in nurseries are also sometimes used for replanting in certain areas (Pautasso et al 2013). This is, in part, because it has been determined that there are no viable native species that can fully replace, alone or in combination, the ecological niche and ecosystem services of the European ash (Lévesque et al 2023). Therefore, the preservation of the species is critical for maintaining associated biodiversity and ecosystem services (Pautasso et al 2013;Mitchell et al 2014;Broome et al 2014;Hultberg et al 2020;Turczański et al 2022;Lévesque et al 2023).…”

“…This is, in part, because it has been determined that there are no viable native species that can fully replace, alone or in combination, the ecological niche and ecosystem services of the European ash (Lévesque et al 2023). Therefore, the preservation of the species is critical for maintaining associated biodiversity and ecosystem services (Pautasso et al 2013;Mitchell et al 2014;Broome et al 2014;Hultberg et al 2020;Turczański et al 2022;Lévesque et al 2023). Furthermore, replanting may be critical in some cases for maintaining genetic diversity of ash populations (Semizer-Cuming et al 2021).…”