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

Abstract

Pollen viability plays a crucial role in reproduction. Given the enormous threat posed to the common ash (Fraxinus excelsior L.) by ash dieback, it is important to investigate the disease's effects on pollen viability and germination. Thus, we conducted an analysis of these pollen characteristics across three distinct forest stands in southern Bavaria, with a maximum of 23 ash trees per study site. These ash trees exhibited varying degrees of ash dieback-related damage symptoms, enabling us to assess differences between mildly and severely affected trees (via Mann-Whitney-U/Wilcoxon test). Pollen viability was assessed using the TTC test, while pollen germination capacity was evaluated using a sucrose agar solution. Our findings revealed no significant differences in pollen viability between healthy and diseased trees, as indicated by both the TTC test and pollen germination assay. However, a tendency towards higher pollen viability was observed in healthier, more robust ash trees across both methods. Non-significant differences, however, suggest that ash trees can produce viable pollen necessary for successful fertilisation irrespective of their health status. Nonetheless, it was observed that severely diseased trees were linked to less inflorescences, as the severely diseased or dead shoots produced few to no flowers. Consequently, the likelihood of pollen from severely diseased trees fertilising other ash trees is substantially diminished. In conclusion, it is evident that flower and pollen production are most important in the reproductive ecology of ash trees.