In dioecious plant species, an individual is either male or female, producing either staminate or pistillate flowers, respectively. Rarely, however, the sex of the plant can be affected by its environment, a phenomenon known as environmental sex determination. Striped maple trees, Acer pensylvanicum, are known to change sex, and to increasingly flower as females as their health deteriorates. Female trees of this species in turn have a higher mortality rate. So far, no direct causative link has been established between trauma and sex change in striped maples, nor the degree to which a tree must be injured to trigger this change.
In a new study published in the American Journal of Botany, authors Jennifer Blake-Mahmud and Lena Struwe explored the effect of varying levels of stress on sex expression in striped maples. The researchers inflicted four different degrees of damage on groups of trees to mimic environmental stressors, including (from most to least severe,) crown pruning, full defoliation, partial defoliation, and vascular restriction through bark removal. They then monitored the trees for two years, noting the sex before and after.
Two years after treatment, the trees that had sustained the most severe damage – pruning and complete defoliation – and survived, had a much higher rate of female flowering. In the case of the pruned trees, none of the originally male trees flowered male two years later. Fully defoliated trees were four and a half times more likely to become female. The less severe treatments did not cause a significant shift to female flowers compared to untreated trees. This seems to suggest a threshold of physical damage above which a change in sex occurs in male trees. Blake-Mahmud explains that the shift is likely, but not necessarily, a permanent one. “We have seen trees that switch from female back to male in the field. Nevertheless, variables are rarely equal and trees generally maintain or decrease in their level of health. Furthermore, the finding that most trees that died were female before dying would suggest there is a directionality to the patterns we are observing.”
But why become female when females have a much greater risk of dying? “It might have to do with the sex ratios of the populations or the imminence of death. Striped maple populations don’t have equal sex ratios. Males outnumber females by over three to one, meaning that there is likely a lot more competition to contribute pollen than to contribute ovules. If a tree is in poor health and its prognosis is already bleak, those trees in the population that can contribute ovules instead of pollen might have a better chance of getting their genes to the next generation of trees. Essentially, if a tree is going to die anyway, contributing ovules to the next generation might be the safest bet, evolutionarily.” Blake-Mahmud says.
The tendency to become female in response to stress when females have a higher mortality rate may spell trouble for striped maples as the progression of climate change increases the frequency and severity of those stressors. “Most forests don’t have marauding botanists coming through and randomly chopping up trees, but they do have increasing numbers of storms that topple large canopy trees and drop large branches. In the understory, these falling giants can crush smaller trees like striped maples. Rapid warming can also harm trees by splitting open their trunks. Overpopulated deer use trees as antler rubs; insects eat trees’ leaves. We’ve shown that severe damage of different kinds can increase the odds of a tree being ‘female’ over ‘male,’” Blake-Mahmud says. “In the short term, this might seem beneficial for a population. More females mean more seed. But females also have a much higher mortality rate in this species. So if damage-inducing climatic events continue to increase in frequency, we’d expect to first see more female trees, then more dead trees in striped maple populations.”