Negative correlation between ash dieback susceptibility and reproductive success: good news for European ash forests

Abstract

European ash ( Fraxinus excelsior L.) trees with low susceptibility to ash dieback have higher reproductive fitness compared to highly susceptible trees, although most pronounced for female success. Selection at generation turnover therefore supports the future recovery of ash forests. The introduced invasive pathogen Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz, and Hosoya cause extensive damage on European ash (Fraxinus excelsior L.). Heritable variation in susceptibility to ash dieback has been observed among ash trees in natural and planted populations, but it is not clear how variation in susceptibility influences reproductive fitness. We hypothesize that healthier male and female trees contribute more gametes to the following generation compared to unhealthy ones. We tested the hypothesis by studying gender, seed production, and paternal success in a clonal field trial with 39 replicated clones. In the trial, the susceptibility level of each clone has been recorded in terms of percent crown damage since 2007. We used a linear regression model to explore the relationship between susceptibility and reproductive success (female and male). The clones revealed a clear gender dimorphism with an approximate 2:2:1 male/female/hermaphrodite ratio. Females with low levels of crown damage produced substantially more seeds compared to highly damaged females. The male clone with the lowest level of susceptibility was the most effective pollen donor, but highly susceptible males also sired some offspring. The results overall represent good news for the potential recovery of ash forests: selection against most susceptible genotypes at generation turnover is expected to facilitate building up disease resistance in ash populations.