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

Abstract

European ash (Fraxinus excelsior) and narrow-leafed ash (F. angustifolia) are keystone forest tree species with a broad ecological amplitude and significant economic importance. Besides global warming both species are currently under significant threat by an invasive fungal pathogen that has been spreading progressively throughout the continent for almost three decades. Ash dieback caused by the ascomycete Hymenoscyphus fraxineus is capable of damaging ash trees of all age classes and often ultimately leads to the death of a tree after years of progressively developing crown defoliation. While studies at national and regional level already suggested rapid decline of ash populations as a result of ash dieback, a comprehensive survey at European level with harmonized crown assessment data across countries could shed more light into the population decline from a pan-European perspective and could also pave the way for a new conservation strategy beyond national boarders. Here we present data from the ICP Forests Level I crown condition monitoring from 27 countries resulting in > 36,000 observations. We found a substantial increase in defoliation and mortality over time indicating that crown defoliation has almost doubled during the last three decades. Hotspots of mortality are currently situated in southern Scandinavia and north-eastern Europe. Overall survival probability after nearly 30 years of infection has already reached a critical value of 0.51, but with large differences among regions (0.20–0.86). Both a Cox proportional hazard model as well as an Aalen additive regression model strongly suggest that survival of ash is significantly lower in locations with excessive water regime and which experienced more extreme precipitation events during the last two decades. Our results underpin the necessity for fast governmental action and joint rescue efforts beyond national borders since overall mean defoliation will likely reach 50% as early as 2030 as suggested by time series forecasting.

Rapid polygenic adaptation in a wild population of ash trees under a novel fungal epidemic

“…Damage from the fungus had further increased by 2021 especially in juvenile trees (Figure 1). This fits with widespread documentation of the progress of the ash dieback epidemic throughout Europe (34, 35).…”

“…If we assume that the narrow sense heritability of ADB susceptibility is 0.4, as previously estimated by several field-trial studies 25 , it would require selection eliminating the most susceptible 13% of the juvenile population. This level of selection appears plausible, given the 85% mortality seen in some mature ash plantations due to ash dieback over a 20 year period 34,33 , of which only a small portion would need to be conditional on the loci underlying our GEBV score to have produced the response that we have detected. If heritability in the woodland population is lower than the field-trial estimates, due to greater environmental variability and lower genetic diversity, the estimate of selection would be correspondingly larger.…”

“…The invasive fungal pathogen ( Hymenoscyphus fraxineus ) causes ash dieback disease in European ash ( Fraxinus excelsior ) populations. The disease is frequently lethal 11,12 , but larger trees persist longer 11,13 , often with reduced reproductive output 13 . Here, using whole genome resequencing of an infected natural ash tree population in England, we measure allele frequencies at 7985 single nucleotide polymorphisms (SNPs) previously associated with resistance to the pathogen 14 .…”

Regionally Varying Habitat Relationships in Lichens: The Concept and Evidence with an Emphasis on North-Temperate Ecosystems

“…We exemplify this point by considering the dieback of the European ash ( Fraxinus excelsior ). The dieback, caused by the invasive exotic ascomycete Hymenoscyphus fraxineus , has roughly halved the ash populations in the continent in two decades, and continues to expand [ 270 , 271 ]. Multiple assessments indicate that this process can threaten the specific epiphyte assemblages of ash (e.g., [ 272 , 273 , 274 , 275 ]).…”

Long‐term forest monitoring reveals constant mortality rise in European forests

“…For the analyses shown in this study, this dataset comprises a total of 20,455 survey plots across Europe. We removed European ash and narrow-leaved ash from the dataset as mortality in these two species is currently driven by an invasive pathogen rather than by climate (George et al 2022). We also removed all mortality instances that were attributable to planned utilization or felling and selected only survey years from 1992 onwards in order to obtain a constant sample size across the continent for each year.…”