Ecophysiological differences and the cost of reproduction in a native sex-switching species

Over the lifetime of an individual, resources are allocated to a variety of functions, such as maintenance and reproduction. When resources used for reproducing in the present negatively affect health or longevity, and therefore the chance of reproducing in the future, this is considered a cost of reproduction. In most dioecious species with stable sex expression, these costs equate to differences in early versus late paternity or to early versus late maternity. But in rare species that switch sex during their lifetimes, individuals may trade off between all four of these options. The costs of reproducing may be visible in differences in sex-based physiology.

We investigated differences in sex-based physiology in five forest populations of the striped maple, Acer pensylvanicum, in northern Michigan. This species exhibits environmental sex determination, a mechanism by which individuals may change sex from year to year based on environmental cues or physiological conditions. For >100 study trees, we measured photosynthetic rate, growth and fluorescence in spring and summer over 3 years.

In comparison to non-flowering trees, we found that flowering trees grew less and had higher rates of photosynthesis. Reproductive trees flowering female had significantly lower growth rates and photosynthetic rates than male trees. Females were especially impacted by the season, demonstrating a larger decrease in carbon assimilation in the summer compared with the spring.

These data are consistent with the idea that there is a substantial cost to reproducing as a female, noticeable especially during the time when females are developing seeds. These results suggest that striped maple females might be disproportionately affected by climatic stressors compared with males, an observation that is in line with predictions for stable dioecious species.