During the fall, a change occurs with maple trees that is prominent and apparent.  As the daylight hours decrease green leaves turn to colors of vibrant yellow, burnt orange and an array of shades of red. 

There is a list of species of Maples that add to this colorful splendor, from Sugar, Norway, Amur and more but one in particular changes more than its leaf color — the Striped Maple. 

Many people have a hard time identifying the different species of maple by the bark in Summer but the Striped Maple possess a smooth, variegated green, reptilian-looking bark that can be noticed with ease. 

Small tree, big leaves

The leaves are three-pronged and seem disproportionately large for the size of the tree, dwarfing an adult human’s hand.  Striped Maple have more than one name, it is also known as moosewood, moose maple and goosefoot maple.  It is one of the smaller known deciduous trees, growing from 16-30 ft. tall and is among the most shade-tolerant of the many deciduous trees here in the Adirondacks. 

This tree is capable of germinating and persisting for years as a small shrub, then growing rapidly to its full height when an opening in the canopy occurs but does not grow high enough to become a canopy tree and once the gap above it closes it responds by flowering and fruiting profusely. 

Changes below the surface

All of these changes can be seen by the eye but there is a change occurring within this tree that is rare and remains hidden.  The striped maple is a sequential hermaphrodite, meaning that it can change its sex throughout its lifetime.  There are less than 0.1% of recorded cases in which plant species entirely change their sex. The Patchy Environment Model and Size Dependent Sex Allocation are the two environmental factors which drive sequential hermaphroditism in plants.

The Patchy Environment Model states that plants will want to maximize the use of their resources through the change of their sex. For example, if a plant will benefit more from the resources of a given environment in a certain sex, it will want to change to that sex. Size Dependent Sex Allocation outlines that in sequential hermaphroditic plants, it is preferable to change sexes in a way that maximizes their overall fitness compared to their size over time.

Scientists have known for some time that trees not only have a sex, but can sometimes switch between sexes. But they haven’t always known why. During a biology field course, ​Jennifer Blake-Mahmud, a PhD student at Rutgers University, learned that the striped maple, Acer pensylvanicum, could switch from male to female.

In order to determine the sex of trees, researchers turn to the flowers, which can take up to 11 months to start blooming. While many trees have both male and female reproductive parts, others have just one or the other. So the duo cut branches of striped maple trees, which are common in the northeast United States, southeastern Canada, and various parts of New Jersey.  The cut branches were placed in sugar water in a greenhouse and watched until they bloomed, publishing their results in the journal Trees: Structure and Function.

Research findings

The first surprising find was in the development of the flowers. These trees can finalize the development of flower bud parts within three weeks prior to flowering, compared to other species who do this many months in advance. Even more unexpected was the sex of the branches. Although all of the trees sampled from were male, all of the blooms were either female or female and male.  In a follow-up test, the researchers examined branches, growing them both indoors and outdoors. The cut branches were also placed in different solutions, either in sugary water or plain water. The results were the same: branches turned female or female and male. Something about the branches, not the trees, were causing them to change sex. 

This research showed that one of the reasons a striped maple may change sex is that it would need to bloom as quickly as possible, producing offspring before it ultimately dies from damage or sickness.  There is always something amazing happening in nature, some of which we see and some we don’t.