Patterns of sex expression, health and mortality in a sex-changing tree

The ability of individuals to change sex during their lifetime is known as environmental sex determination (ESD). This ability represents a unique life history trait, allowing plants to allocate resources differentially to male and female functions across lifetimes, potentially maximizing fitness in response to changing environmental or internal cues. Blake-Mahmud and Struwe investigated Acer pensylvanicum, a species with an unconfirmed sex determination system, to see what patterns in sex expression existed across multiple years, if there were sex-based differences in growth and mortality, and whether this species conformed to theoretical predictions that females are larger and in better condition.

Many coloured bands
Sankey diagram of sexual transition in Acer pensylvanicum showing male trees in blue, female in pink, monoecious in yellow, non-reproductive in green and dead in grey. Full details in Blake-Mahmud and Struwe 2019.

In this study, 54 % of the trees switched sex expression during a 4-year period, with 26 % of those trees switching sex at least twice. Consistently monoecious trees could change relative sex expression by as much as 95 %. Both size and condition were influential in predicting sex, with condition exerting three times more relative influence than size on expressed sex. Contrary to theoretical predictions, the model showed that full female sex expression did not increase with size. Healthy trees were more likely to be male; predicted female sex expression increased with deteriorating health. Growth rate negatively correlated with multiple years of female sex expression. Populations maintained similar male-skewed sex ratios across years and locations and may result from differential mortality: 75 % of dead trees flowered female immediately before death.

This study shows conclusively that A. pensylvanicum exhibits ESD, and that femaleness correlates with decreased health, in contrast to prevailing theory. The mortality findings advance our understanding of puzzling non-equilibrium sex ratios and life history trade-offs resulting from male and female sex expression.