The invasive forest pathogen Hymenoscyphus fraxineus boosts mortality and triggers niche replacement of European ash (Fraxinus excelsior)

Abstract

Determining the impacts of invasive pathogens on tree mortality and growth is a difficult task, in particular in the case of species occurring naturally at low frequencies in mixed stands. in this study, we quantify such effects by comparing national forest inventory data collected before and after pathogen invasion. in norway, Fraxinus excelsior is a minor species representing less than 1% of the trees in the forests and being attacked by the invasive pathogen Hymenoscyphus fraxineus since 2006. By studying deviations between inventories, we estimated a 74% higher-than-expected average ash mortality and a 13% slower-than-expected growth of the surviving ash trees, indicating a lack of compensation by the remaining ash. We could confidently assign mortality and growth losses to ash dieback as no mortality or growth shifts were observed for co-occurring tree species in the same plots. the mortality comparisons also show regional patterns with higher mortality in areas with the longest disease history in norway. considering that ash is currently mostly growing in mixed forests and that no signs of compensation were observed by the surviving ash trees, a significant habitat loss and niche replacement could be anticipated in the mid-term.www.nature.com/scientificreports www.nature.com/scientificreports/ forest have co-evolved with native pathogens, which target individuals weakened by inter-and intra-specific competition for resources. Changes in the population of the host would occur if the new pathogen caused a higher mortality than that occurring under natural conditions in the forest. Manion and Griffin 14 obtained the baseline mortality from a Liocourt curve calculated from tree inventories carried out over large areas. This model shows how the density of trees decreases as they increase in diameter. The underlying process behind this curve is that weaker trees eventually die, and the surviving trees increase their size as they occupy the newly freed space. The model is adjusted so that the same decay or mortality operates regardless of tree size, and thus can be used as a 'baseline mortality' value to be compared with when analysing areas affected by a new pathogen. Manion and Griffin 14 applied this approach to widespread tree species in eastern US; however, this model can be difficult to apply in the case of minor species, such as Alnus glutinosa, Fraxinus excelsior or Ulmus minor, affected by exotic pathogens in Europe. These species are poorly represented in national forest inventories (NFIs), and often appear in mixed natural stands where baseline mortality may be difficult to estimate.In this study, we focus on European ash (Fraxinus excelsior) in Norway. Ash, a keystone species with wide distribution and habitat range in Europe, is being affected by the non-native fungus Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz & Hosoya. European ash grows over a wide range of altitudes and soil moisture contents, depending on climatic and site conditions. It is frequently mixed with other broadleaved trees fr...

European-wide forest monitoring substantiate the neccessity for a joint conservation strategy to rescue European ash species (Fraxinus spp.)

“…The extent to which the European population of F. excelsior and F. angustifolia has been declining since the arrival of the pathogen is largely unknown, which makes targeted conservation efforts at pan-European level challenging. National and regional reports on mortality rates were so far based upon literature reviews 14 , experimental forest sites 15 , 16 , and national forest inventory data 17 – 19 . However, the different approaches, datasets, and their specific sampling biases make it difficult to draw a conclusive pattern for European ash and narrow-leafed ash across their entire distributions.…”

“…In this study we make use of survey data from the ICP Forests Level I network, providing detailed information on crown condition and diseases in annual resolution since 1987 20 . In total, we analyzed 36,170 observations on ash trees from 1987 to 2020 in 27 countries and incorporated stand information as well as climate data in order to test for covariation between ash dieback-induced mortality and ecological variables that may accelerate or decelerate the decline as was recently suggested in earlier studies at regional scale 17 , 19 , 21 . We hypothesize that the mortality of both ash species has significantly increased throughout the last three decades resulting in a devastating ash dieback driven by ADB rather than by other abiotic and biotic agents.…”

Drought in the forest breaks plant–fungi interactions

“…The infection of shoots with the pathogen is less likely during prolonged dry periods, but the pathogen usually persists on pseudosclerotic petioles in some years (Kirisits 2015). The problem is still open, particularly since Díaz-Yáñez et al (2020), in turn, question the influence of drought on ash development and survival.…”

European-wide forest monitoring substantiate the neccessity for a joint conservation strategy to rescue European ash species (Fraxinus spp.)

“…Coker et al 2019), experimental forest sites (Stocks et al 2017, Cleary et al 2017), and national forest inventory data (e.g. Díaz-Yáñez et al 2020; Klesse et al 2021; Enderle et al 2018). However, the different approaches, datasets, and their specific sampling biases make it difficult to draw a conclusive pattern for European ash and narrow-leafed ash across their entire distributions.…”

“…In total, we analyzed 36,170 observations on ash trees from 1987-2020 in 27 countries and incorporated stand information as well as climate data in order to test for covariation between ash dieback-induced mortality and ecological variables that may accelerate or decelerate the decline as was recently suggested in earlier studies at regional scale (e.g. Klesse et al 2021; Díaz-Yáñez et al 2020; Chumanova et al 2019). We hypothesize that the mortality of both ash species has significantly increased throughout the last three decades resulting in a devastating ash dieback driven by ADB rather than by other abiotic and biotic agents.…”