SCI-NY: Fallen leaves, clever trees, and stockpiling for the winter.

Trees really are clever. Everywhere you look in New York City these days, you’re reminded of that fact. Last time around, we looked at how tree leaves change colors and why. It pretty much came down to chemistry. During the fall, the chlorophyll in leaves begin to break down and as the rich green color their chloroplasts provide fades, the colors that were there the whole time but were outshone by the green get their moment to shine, That’s how leaves become yellow or orange. The purple and reds that appear are the products of a whole separate chemical reaction not associated with the process called senescence in leaves.

The reason the chlorophyll in leaves breaks down is because they are no longer needed. With the days (and hours of sunlight) shortening and the temperature falling to winter lows, the tree decides keeping its leaves isn’t worth the trouble. Sure, if they stuck around, they could still photosynthesize light into nutrients for the tree. Winter sun and summer sun are pretty much the same. However, in order for the tree to benefit from photosynthesis, it needs to transport the nutrients from up in the tree leaves down to the rest of the tree. That process involves water. By now, it’s pretty clear what happens to water during those brutal February days when the whipping wind numbs all sensation in any body part exposed to the elements. 


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As you might imagine, having water freezing in a tree’s insides is far from ideal. In fact, it’s pretty much fatal. That’s because the frozen leaves would die and just be dead, unable to return during spring. Then come springtime, when plant life is bursting back to life with brand-spankin’ new leaves, the tree with frozen leaves will just sit around leafless. Aside from looking pathetic, it would also have lost its way of feeding itself by harvesting sunlight. Eventually, the tree just dies. Sad.

Fortunately, trees have figured it out over time. Rather than clinging to their leaves, they undergo a process called abscission in which the tree essentially clips off its leaves at the point called a petiole where the leaf stem meets the branch. Disaster averted. Clever, isn’t it?

This brings us to those fallen brown leaves that are ubiquitous during mid- to late-autumn months. While they may not be the prettiest looking or smelling things in the world, They are goldmines of nutrients for plant life, including the trees that dropped them. 

A few years back, a team at Rutgers University studied the main components in leaves. This is the composition they discovered.

NutrientAverage Concentration (%)Range (%)
Carbon4736-52
Nitrogen10.66-1.62
Potassium (K2O)0.380.09-0.88
Phosphorous (P2O5)0.380.02-0.29
Calcium1.640.13-3.04
Magnesium0.240.02-0.46
Sulfur0.110.01-0.21

There’s a lot of potential in those fallen leaves and composting them into mulch has been a popular and environmentally friendly way of disposing of them for gardeners for a long time. There’s no shortage of gardening sites with tips and tricks on how to make the most of them the way trees do naturally.

The concentrations of those nutrients in leaves are small, but they can still be a valuable component to promoting soil health and fertility. They help feed worms, who convert the leaves into leaf mulch. With the help of microbes in the soil, the broken down leaves eventually decompose, depositing the nutrients overtime.

The additional nutrients provided by fallen leaves helps the trees make it through those long, cold winter days when the ground is frozen and the promise of spring is still months away.

 Like we said, clever.

IMAGE CREDIT: Scientific Inquirer.


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