Will natural resistance result in populations of ash trees remaining in British woodlands after a century of ash dieback disease?

Abstract

Novel pests and diseases are becoming increasingly common, and often cause additional mortality to host species in the newly contacted communities. This can alter the structure of the community up to, and including, the extinction of host species. In the last 20 years, ash dieback (ADB) disease has spread into Europe from East Asia. It has caused substantial mortality in ash tree ( Fraxinus excelsior L . ) populations. However, a proportion of the individuals in most populations appear to be less susceptible to ADB and resistance seems to have high heritability. These observations have led to suggestions that ash populations may be sustainable after the disease. In order to test this hypothesis, I modified an existing model of UK woodland (parametrized for Wytham Woods, Oxfordshire) to take into account the impact of ADB and allowed offspring to inherit resistance traits from their parent. The results suggest that ash populations would still exist in 100 years, but at lower levels than they are currently. For example, when the initial proportion of resistant individuals is about 10% and heritability of resistance is 0.5, then the population of ash falls to about one-third of present levels. The proportion of individuals initially resistant to ADB had a larger effect on population size after 100 years than the heritability of resistance. The fact that the initial size of the resistant population is important to achieve a high population size in the presence of ADB suggests that a selective breeding programme with the intention of augmenting the natural ash populations would be beneficial.

Genetic Diversity and Differentiation of Pedunculate Oak (Quercus robur L.) Populations at the Southern Margin of Its Distribution Range—Implications for Conservation

“…In the past decades, pedunculate oak forests in Europe have been subjected to numerous stress factors, such as low underground water level [7], drought stress [8], pest and diseases [9,10], and climate change [11]. As a consequence of their negative impact, trees' health status and growth rate have frequently deteriorated, sometimes resulting even in premature death [7,12].…”

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

“…Although a minor proportion of ash trees is likely inheriting resistance against ADB (Plumb et al 2019; Kjær et al, 2012), this alone will not guarantee that the ash population will recover after the disease has reached an equilibrium state. Evans (2019) simulated that long-term recovery of ash is highly dependent on the proportion of ash trees carrying natural resistance and secondly on the degree of heritability of resistance. However, even under extreme high heritability assumptions, long-term recovery under natural conditions will remain low when the founder population consists of only few trees.…”

Gene flow and reproductive success in ash (Fraxinus excelsior L.) in the face of ash dieback: restoration and conservation

“…This verifies that disease resistance is partially under genetic control. Assuming that the heritability of resistance is 0.5 and the frequency of resistant trees in pre-diseased ash forests is about 10%, model simulations estimated that ash populations in the UK woodlands would still exist in 100 years but decrease to about one-third of current levels (Evans 2019). It has thus been suggested that genetic variation in resistance may prove to be a long-term solution to ash dieback if natural selection is coupled with artificial selection (McKinney et al 2014;Evans 2019).…”

“…Assuming that the heritability of resistance is 0.5 and the frequency of resistant trees in pre-diseased ash forests is about 10%, model simulations estimated that ash populations in the UK woodlands would still exist in 100 years but decrease to about one-third of current levels (Evans 2019). It has thus been suggested that genetic variation in resistance may prove to be a long-term solution to ash dieback if natural selection is coupled with artificial selection (McKinney et al 2014;Evans 2019). However, the frequency of ash trees with reasonable to high levels of genetic resistance (defined as crown damage below 10%) within natural populations is low, 1-5% (Kjӕr et al 2012;McKinney et al 2014), and due to their long lifetime, no studies have yet measured the realised lifetime fitness of healthy trees with respect to their more susceptible neighbours in the surroundings (Kjaer et al 2012).…”