Does ash dieback affect the reproductive ecology of Fraxinus excelsior L.?

Abstract

Forest tree species reproduction is a key factor in maintaining the genetic diversity of future generations and the stability of forest ecosystems. The ongoing ash dieback disease could affect the reproductive ecology of Fraxinus excelsior L. and have a major impact on the quantity and quality of pollen and seeds. In this study, we investigated pollen production and viability of pollen and seeds of ash trees with different health status from 2018 to 2022. Inflorescences were collected from 105 trees (pollen production), pollen from 125 trees (pollen viability), and seeds from 53 trees (seed quality) in two seed orchards and in one floodplain forest in southern Germany. Not all parameters were examined at every site every year. The average pollen production per tree was estimated at 471.2 ± 647.9 billion pollen grains. In addition, we found that a high number of inflorescences did not equate to high pollen production per inflorescence. Pollen production of healthy and diseased trees did not differ significantly, although only 47% of severely diseased male trees (vs. 72% for healthy trees) produced flowers. With regards to pollen viability, the TTC test showed an average viability of 73% ± 17%. Overall, there was a slight tendency for diseased trees to have less viable pollen. However, a significant difference could only be calculated for trees in the floodplain forest. The percentage of germinable seeds in 2018 was 38% in the floodplain forest and 57% in one of the seed orchards. The percentage of viable seeds (TTC test) ranged from 17 to 22% in the orchards in 2020. Non-viable seeds were usually heavily infested by insects. In general, seed quality was not significantly different between healthy and diseased trees. Our results indicate that ash dieback affects flower formation and pollen viability but not pollen production or seed quality. Nevertheless, the fact that hardly any flowering was observed, especially for trees that were seriously affected, suggests a negative effect of ash dieback on reproductive performance. Thus, severely diseased trees will transfer their genes to a smaller extent to the next generation.