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.

Does the health condition of the common ash tree affect its pollen viability?

“…However, as ash dieback induces the development of additional epicormic shoots during disease progression, further investigations explicitly studying the impact of epicormic shoots on pollen viability would give more insights. However, severely affected trees exhibited a substantial reduction in ower production, as demonstrated by Eisen et al (2024). This observation was con rmed in our study, as pollen samples could only be obtained from nine out of the total 119 trees from vitality score class 4 across all study sites and years.…”

“…Regarding the long generation times in natural forest landscapes, exploring pollen production is crucial as it provides valuable insights into phenomena such as the alternating patterns in mast and non-mast years (Dahl et al 2013). Additionally, it provides insights into the effects of air pollution or climate change (Ziska et al 2003; Darbah et al 2008;Talwar et al 2022) or the in uence of diseases (Kozlowski 1971;Eisen et al 2024). For example, CO, SO 2 , NO 2 and O 3 were often associated with negative effects on pollen production (Darbah et al 2008), while CO 2 repeatedly showed a positive effect (LaDeau and Clark 2006; Darbah et al 2008).…”

“…For example, CO, SO 2 , NO 2 and O 3 were often associated with negative effects on pollen production (Darbah et al 2008), while CO 2 repeatedly showed a positive effect (LaDeau and Clark 2006; Darbah et al 2008). Both Kozlowski (1971) and Eisen et al (2024) found a link between the vigour of forest trees and their ability to produce owers. However, pollen production alone is not su cient to conclude on the effective fertilisation of owers in the plant kingdom.…”

“…The authors con rmed an accelerated decrease in pollen viability with increased or prolonged UV radiation and under warmer conditions. This methodological background was utilised in the eld study by Eisen et al (2024). The authors investigated whether ash dieback affects pollen production and the viability of pollen and seeds in two seed orchards and a riparian forest.…”

“…Castiñeiras et al 2019, Buchner et al 2022, Eisen et al 2024). The 1% TTC solution is made from 1 g 2,3,5-triphenyltetrazolium chloride and 60 g sucrose in 100 ml of distilled water(Buchner et al 2022).…”