Annual growth of male and female individuals of the Common Ash (Fraxinus excelsior L.)

Abstract

Background: In general, females of woody dioecious plants are expected to grow more slowly than males because of the high cost associated with fruit bearing. Yet wind-pollinated species may invest relatively heavily in male reproduction, and various compensatory mechanisms can offset incurred costs, therefore the apparent growth of genders might be similar over a longer period of time. Any cost-related disparities are expected to be most pronounced in mast flowering years, when female reproductive resource demand is highest.

Aims: We aimed to investigate gender-related response in relation to the cost of reproduction, and as a function of climate.

Methods: Using dendrochronology techniques and analysing growth parameters at the branch level, we compared long-term and seasonal growth of male and female Fraxinus excelsior trees.

Results: There was no observed decline in trunk growth of either of the genders over a 55-year period, but male and female trees had distinguishable annual growth patterns. The tree-ring widths of both genders were smaller in masting years than in other years. The difference between genders could be explained by their response to different climatic variables: females are more sensitive to spring frost and drought in early summer, while male growth depends more on late summer conditions.

Conclusions: Vegetative development and flowering create a feedback system that minimises the cost of reproduction and assures that expenses are covered. Hence, growth differences between genders may be related to their phenological and physiological disparities rather than reproductive cost differences per se.

Keywords:

• Fraxinus excelsior
• masting
• secondary sexual characters
• trade-offs
• tree rings
• wind pollination

Acknowledgments

We thank Åslög Dahl and Eva Wallander for helpful comments, Mats Töpel for help in data collection, employees of Skogsvårdsstyrelsen Mälardalen, district Västerås, for help with locating suitable ash stands. Special regards to R. Ochoterena, R. Thomas and other friends for statistical advice. This work was supported by The Royal Swedish Academy of Agriculture and Forestry Fund (Stiftelsen Mary Franken-Gustafson fund, Gff-170-FG).