Trembling Aspen

One of PEI’s most common trees is Poplar, aka Trembling Aspen (Populus tremuloides). The ‘trembling’ name comes from the habit of this tree’s leaves to flutter in the barest of breezes; the gust in the middle of this video wasn’t even enough to ruffle the fur of my cameo-appearing dog. That leaf movement is not just some random trait – it’s a useful adaptation.

Within sight of my front deck is a stand of young Poplar surrounded by a mixture of Pin Cherry, Red Maple, and White Birch. If there’s a breath of wind at all, those Poplar trees stand out from the rest, shimmering as if they were behind hot air rising from pavement. Poplar’s leaf stems (petioles) are unique in that they are flat rather than round. The narrow edge pivots into the wind like a flag, creating the trembling movement we see.

While the physical cause of Poplar’s fluttering leaves has long been known, the biological reason for it was a mystery until about 30 years ago. It turns out there are at least four reasons why it’s an advantage.

First, the outer and upper leaves of trees shade inner and lower leaves, reducing the sunlight reaching them and thus their ability to photosynthesize. Less photosynthesis means less food for the tree. Poplar’s moving leaves allow more sunlight to reach more parts of the tree, increasing its overall photosynthetic efficiency.

Second, leaves that move stay cooler than those that stand still under the hot summer sun. Tree leaves lose water to cool down, just like we do. Moving in the breeze is a bit like sitting under a fan to keep cool and reduce water loss – especially useful during hot, dry years like this one.

Third, the more leaves move, the less prone they are to being eaten by insects or having fungal spores land and establish. Poplar’s fluttering leaves help keep pests away, not unlike us shooing away flies and mosquitoes.

And lastly, those leaves help with wind resistance. The large surface area of broad leaves maximizes photosynthesis but is a liability in high winds. Poplar’s leaves turn in the wind, reducing drag, dispersing wind energy, and helping reduce broken branches or toppling during storms.

With all those advantages, the real mystery is why more trees don’t have flat petioles. In nature, there’s no such thing as a free lunch and it’s likely that this adaptation comes with some costs as well. Or Poplar may just be an early adopter, and in a few hundred thousand years more tree species may get on the flat petiole bandwagon. In the meantime, I’ll enjoy watching and listening to the soft rustling of this part of PEI Untamed!