Environmental dimensions of variable sex expression in striped maple, Acer pensylvanicum (Sapindaceae)

Background/Question/Methods

An organism’s gender has important repercussions for its health, longevity, and ultimate fitness. Gender ratios within a population has implications for population demographics, stability, and long-term evolution.  In plants, gender is expressed via flowers and is usually a fixed characteristic present throughout a plant’s reproductive lifespan. In rare cases, gender expression is flexible and may be influenced by size or environmental factors. A better understanding of the mechanisms that influence shifts in gender is essential to our understanding of theory regarding sex allocation and the evolution of sex ratios and separate sexes.

My objective is to elucidate how complex environmental signals and stress affect labile sex determination in plants, using striped maple (Acer pensylvanicum; Sapindaceae) as a study system. Striped maple is one of these rare species that can switch sex expression throughout its lifetime. The timing of the change in sex expression and correlation with environmental cues remains largely unexplored. Due to the energetic demands of being female (setting seed), striped maple populations are projected to be highly male skewed. 

Results/Conclusions

Physical damage is an important factor in gender determination for this species; other forms of environmental stress may also affect the gender switch. I observed a canopy gap experiment in which dominant canopy trees were removed from the forest in the summer of 2014.  The resulting increase in light levels and catastrophic damage caused by falling trees significantly affected sex expression of the remaining striped maples trees in the following flowering year.  Trees experiencing severe damage and increased light switched sex expression at a higher frequency than did other striped maple control trees.

Given the projected increase in frequency of severe storms due to changing climatic patterns, the resulting tree damage may increase the abundance of females within populations. This shift may improve seed set, recruitment, and persistence of A. pensylvanicum, but slow the recruitment of other desirable hardwoods due to increased shading.  Therefore, increased storm damage may impact not only striped maple populations, but also the ecology, composition, demographics, and sustainability of northeastern hardwood forests.