Fungal Presence and Changes of Wood Structure in Bark Stripping Wounds Made by Red Deer (Cervus elaphus L.) on Stems of Fraxinus angustifolia (Vahl)

Fungal Presence and Changes of Wood Structure in Bark Stripping Wounds Made by Red Deer (Cervus elaphus L.) on Stems of Fraxinus angustifolia (Vahl)

Fortunately, narrow-leaved ash seems to heal inflicted wounds relatively successfully and quickly, which has been reported for common ash as well [18]. Out of ten randomly selected wounded stems used for fungal isolation in this study, six displayed fully closed wounds where discoloration and decay were restricted to the annual rings that grew before the stripping, as described for European beech by other authors [58]. Three samples displayed freshly inflicted wounds, and only one represented a partially closed wound. This is probably related to tree vigor and associated intensified cambium growth, which are at their peak in younger trees, enabling faster wound healing [18,19]. Nevertheless, the further spread of discoloration and decay are possible with tree aging and loss of vitality, as certain fungal species continue to dwell in wooden tissue and advance even after wound closure [32]. Moreover, because bark-stripping causes physiological stress [18] and activates the allocation of resources to healing [15], tree growth is reduced and it is possible that less carbon is invested into the root system [59], making trees more prone to the effects of other abiotic and biotic stresses. A relationship between tree mortality caused by pathogenic fungi and previous damage to the trunk caused by deer has been observed in aspen (Populus tremuloides Michx.) [60].
Most of the fungi isolated from the wounded wooden tissue in this research were ubiquitous generalists and known saprotrophs or endophytes, similar to in the young bark stripping wounds on Quercus robur in Lithuania studied in [32]. Several of these species and genera (Clonostachys rosea, Fusarium spp., Trichoderma spp.) were reported to be endophytic in narrow-leaved ash in a previous study [9], indicating that they might have been present in the ash wood prior to bark stripping. However, further research is required to determine the exact colonization process and patterns of the fungi found in this study. Relatively young trees and wound ages might be responsible for the low incidence of several potentially pathogenic and decay fungi found in this research, as reported by Burneviča et al. [61] and Marčiulynas et al. [32]. Only two white-rot fungi were isolated, Bjerkandera adusta and Hyphodermella rosae, with rather low incidence; both have already been reported on common ash [12,62]. Restricted spread of decay following bark stripping has been reported for common ash as well [18]. This indirectly corroborates the finding that summer bark stripping leads to a greater risk of decay development in comparison to winter wounding [37]. Similarly, only two designated woody plant pathogens were isolated: Peroneutypa scoparia (syn. Eutypella scoparia) and Eutypa lata. The first is associated with canker, stem blight, and dieback symptoms on blueberry, grapevine, and fig [63], while the latter is considered to be a pathogen with a wide host range, and is particularly fatal for grapevine and apricot [64]. Fungi of the genus Eutypa are quite abundant in silver fir trees (Abies alba Mill.) damaged by bark stripping, although mostly in older wounds [65]. While these pathogens were relatively abundant regarding the number of isolates obtained in this study, each occurred in only one sampled stem. The most frequently isolated taxa which colonized more than two stems were Boeremia spp., Clonostachys rosea, Fusarium spp., and Trichoderma spp., all of which are reported to be endophytes and potential opportunistic pathogens of ash. Boeremia isolates were found to be the most abundant fungi in necrotic common ash lenticels in [66], with the species B. lilacis and B. exigua reported to be endophytes with an ability to become pathogenic and cause minor damage to the host [8,66,67]. Clonostachys rosea has been isolated from both healthy and necrotic wooden tissues of common and narrow-leaved ash in previous studies [9,68], confirming its ability to spread in symptomatic tissue. Members of the genus Fusarium are widespread and colonize a great number of plant hosts, often without causing severe disease; these include F. acuminatum [69], which was the only Fusarium species identified to the species level in this study. This species has been found in old bark stripping wounds on silver fir as well [65]. Fusarium species have a tendency to become pathogenic, especially in stressed hosts, which has been confirmed in inoculation experiments on common ash, with three species able to cause necrotic lesions on the stems of some seedlings [8]. Fusarium species have been isolated from healthy and necrotic tissues of narrow-leaved ash as well [9]. The most frequently isolated fungi in this study were members of genus Trichoderma, which are known to be ubiquitous saprotrophs and endophytes in plants [70] and in certain cases antagonists of fungal pathogens [71]. Their abundance in narrow-leaved ash stems of different health status has already been reported [9], as has their presence in freshly inflicted and healed bark stripping wounds on silver fir [65]. Without further identification to a species level, it is hard to conclude whether they act antagonistically to other fungi and prevent the further spread of discoloration and decay or opportunistically contribute to the advantage of the mentioned symptoms.

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