Winter Comes and Autumn Leaves

November 1, 2022

I think we’re all guilty of indulging our inner-treasure hunter this time of year, seeking out that single unique, beautiful, and perfectly colored autumn leaf, hiding amongst the uncountable thousands strewn about our feet. Who could resist? I certainly can’t. All those vibrant colors and shapes constantly shifting above and below in the crinkling sea of fall foliage are enough to drive anyone into a wonderous frenzy.

I recently fell into one such moment of bliss while trash-picking at the Schiffendecker Farm Preserve and was struck by the variety of the leaves in an area as small as that parking lot. In your leaf collection safaris (which we all know you’ve done),have you ever encountered this? I hold, here in my hand, two perfect leaves, which you may recognize as having come from Tulip trees, Liriodendron tulipifera. You may be surprised to learn however, that despite the striking size difference, these fell from the same single tree.

What could cause such a dramatic size difference in these tulip leaves? It’s not the age of the tree, or some genetic anomaly, or disease, or drought; it’s not even unique to this species of tree. This is a common sight amongst many of our eastern deciduous, and even some evergreen, trees.

It all comes down the one of the primary functions of the leaf: catching sunlight. What we’re seeing here is the result of countless years of evolution at work, but which really boils down to this simple concept: where on the tree the leaf grows often determines it size. It might help to think of the leafy canopy of a tree as a ball formed from several layers; picture a jawbreaker, or those cross-cut images of the earth’s layers you were shown in science class.

Surface area is the name of the game. Plants need to find the balance between surface area for capturing as much sunlight as possible but also limiting exposure to harsh conditions.

In this photo, we see what are commonly referred to as “shade leaves” (left), and “sun leaves” (right). The larger the leaf, the more sunlight it can capture- makes sense, right? By applying that logic, you might think that the biggest leaves would grow on the outside layer of the tree, capturing as much sunlight as quickly as possible. The opposite is true, however, and typically the leaves growing at the outside layer are smallest, while the internal layers made up of the larger, sometimes gargantuan leaves.

Striking size difference of leaves found on the same Tulip tree.

There are many other factors that come into play with this tiered arrangement in the canopy. What you probably can’t tell from this one photo is that the sun leaf on the right, while smaller, is also waxier and much thicker. Growing at the outside of the canopy, sun leaves are exposed to greater levels of heat from sunlight and wind, but a reduced surface area and smaller size helps prevent overheating, and the thick waxy coating helps to avoid water loss from the drying winds. The thickness of sun leaves is also due to the higher density of chloroplasts, the cellular machinery that converts sunlight into chemical energy for the tree, meaning these leaves have the right equipment for dealing with the excess sunshine.  

The shade leaves in the lower layers of the tree, can grow much larger without the risk of overheating and losing water. The larger size helps them capture more of the sunlight that bypassed the smaller leaves above. Compared to the sun leaves, shade leave shave less dense concentrations of chloroplasts and thus tend to be relatively thinner.

Interestingly, sun and shade leaves will often grow at specific angles on the tree! Shade leaves in the lower levels tend to grow horizontally, and parallel to the ground, again to capture as much of the sunlight penetrating in from above as possible. Sun leave, on the other hand, will actively track the sun as it moves across the sky and remain upright/vertical in order the present the broadest side of the leaf to the sun throughout the day.

As the tallest-growing species of deciduous tree in New York forests, the Tulip Poplar is a great candidate for displaying the variety of leaf size and canopy structure, but as noted above, many of our trees have adapted this strategy for efficiently capturing sunlight. While you’re out visiting our preserves and giving into the autumnal whimsy, see if you can spot the sun and shade leaves on the forest floor. Pair the leaves together as I have and share them with us on social media!

Marshall Lefebvre
MHLC Stewardship Coordinator