Co-extinction effects of mass tree mortality – a case study of lichens on ash

When a species of tree suffers mortality as a result of a pest or disease, and such mortality occurs on a large scale, there is significant risk of co-extinction. Other species reliant upon the tree for habitat, be it exclusively (they spend their whole life using that tree species) or partially (a segment of a species’ lifecycle is spent on that tree species), stand a higher chance of also suffering large scale mortality, either immediately or soon after (a lag period). With around 10% of all tree species being threatened with extinction by a pest or pathogen, understanding the co-extinction effects of mass tree mortality is critical.

In this study, located on Gotland (an island in the Baltic Sea; off of the coast of Sweden), the authors look at the relationsip between Fraxinus excelsior mortality associated with ash dieback (Hymenoscyphus fraxineus), and the species’ epiphytic lichen community. In total, 20 wooded sites (including meadows, pasture, and of course woodlands of varying canopy closure levels) were selected that contained ash populations (at least 50 individuals of the species), and all 20 sites had been inventoried for lichens in 1989-1991. This allowed for the authors to compare current populations of lichen with populations prior to the onset of ash dieback.

The study was undertaken during July of 2009. By completing the study during this time, symptoms of ash dieback could be seen more readily, and the ash populations could be segmented into categories: dead (16%), dying (18%), infected (36%), and visually healthy (28%). In total, 1,066 ash were surveyed, and all 20 sites were found to be home to ash infected with ash dieback (though managed sites showed higher infection rates at 84%, compared to 63% in unmanaged sites and 66% in grazed sites – likey as managed trees, such as pollards, have an abundant amount of sprouts that may be more susceptible to the pathogen). The authors expect that the infected ash will all succumb to the pathogen in the coming 10 years. With the current figures from this study, the authors suggest that at least 34% of the ash (classed as dead and dying) will die, though as trees do not appear to routinely recover from infection, a further 36% may also die.

Ash pollards such as this are considered to be at higher risk of infection (and subsequent mortality). How will this impact upon local lichen populations, and the populations of other tree affiliates? Source: Explore South Lakeland.

As a result of this anticipated mortality, the authors suggest that the best case scenario will see 12-95% of the lichen species to suffer the same fate as the ash, though it is likley that at least 38% of the lichen community will disappear. Once 60-65% of the host ash are infected on a site, lichen communities will begin to suffer significantly as a result. Of course, lichen species that are not exclusively associated with ash will not suffer so greatly compared to those species reliant perhaps exclusively on ash, though their populations will still suffer losses. Concerningly however, most of the lichen species likely to suffer huge losses are red-listed species found only on ash. Additionally, extinction of lichen species is most likely on traditionally-managed sites, given their higher infection rates. The below tables outline such predicted extinction rates.

Graph ‘a’ shows how lichen species, either fully or partially reliant upn ash for habitat, will fare as a result of the infection and subsequent extinction of ash (both optimistic and expected co-extinctions are displayed). Graph ‘b’ shows how lichen species, again both exclusively found on ash and found across all tree species, will fare in different land use types.

Because extinctions only begin to significantly rise in occurrence at around 60-65% of ash being infected, the fact that few co-extinctions of lichens has been observed is perhaps not surprising – dead and dying individuals, at least in the area surveyed, were not high enough to cause such large-scale mortality of lichens. However, the authors note that lichen species may in fact go extinct before the ash do, at least on a local level, which highlights the need for further research into the relationship. Furthermore, sexually-reproducing lichen species are more likely to suffer extinction than vegetatively-reproducing species, as the latter is a much more rapid means of colonisation on a host tree. However, all species will ultimately be caught in a ‘bottle-neck’, where many lichen species are seeking to colonise few remaining ash hosts (particularly mature individuals), and this will see host-specific lichen species suffer most markedly.

Such associated mortality in fact crosses over into other affiliate species associated with ash, however (such as insects, fungi, and other epiphytes), meaning that lichen will not be the only affiliate to suffer as ash dieback takes a hold. Perhaps, as highlighted in this research, areas with infection rates of above 60% should be targets for conservation management, in an attempt to reduce the loss of affiliate species. Such management may in fact encourage traditional management methods, such as pollarding, to cease upon ash, given managed ash are at a higher risk of infection.

Source: J├Ânsson, M. & Thor, G. (2012) Estimating coextinction risks from epidemic tree death: affiliate lichen communities among diseased host tree populations of Fraxinus excelsior. PLoS One. 7 (9). e45701.

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Co-extinction effects of mass tree mortality – a case study of lichens on ash

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