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Without its 'understory' layer, the forest will collapse

Forrest Gump claimed life is like a box of chocolates; we believe a healthy forest is like a layer cake.

At the bottom of the cake is the “understory” layer, with seedlings, saplings, woody shrubs and other plants. Next up is the “mid-story,” with taller young trees and larger shrubs. The “canopy,” with the oldest, largest trees is the top layer of the cake!

What happens to a cake if the lower layer is pulled out? It collapses. And, ecologically, that’s what could happen to forests in central New Jersey.

According to Dr. Jay Kelly, a biology professor at Raritan Valley Community College who is studying forest health, many forests are in trouble because their lower understory layers are disappearing. “They’re being decimated by deer and invasive plants,” he explains.

Kelly assesses forest health by comparing current conditions to those of 50 to 70 years ago.

This study uses a “treasure trove of data” collected by former Rutgers professor Murray Buell from 1948 to 1972. Buell studied forests at 13 sites in four central New Jersey counties, including the Watchung Reservation, Jockey Hollow, Hacklebarney State Park, Voorhees State Park, Duke Island Park, Johnson Park, Mettlars Woods, Cushetunk Mountain, Musconetcong Mountain and Herrontown Woods.

“Those studies were conducted prior to the deer population explosion,” Kelly said, noting that white-tailed deer essentially vanished from New Jersey prior to 1948 and didn’t rebound until decades later.

Kelly and his students surveyed these same forests … and the differences today are astounding!

While Buell counted an average of 10 deer per square mile in central New Jersey forests, the number today is closer to 70 deer per square mile!  “We found deer numbers over 300 per square mile in some places,” Kelly said. “It’s been catastrophic for the understory plants.”

The number of medium and large trees has decreased only slightly since Buell’s time, but saplings have plummeted by 85 percent and small trees by 90 percent.  “If this trend continues, we’re actually going to be losing forest as the older trees die, because there are no new trees to replace them,” Kelly said.

When native saplings and plants are eaten by deer, they often don’t grow back. Instead, invasive plants spring up in their place. These invasives aren’t appealing to deer, or to native insects and birds. “Our forests are actually more invasive than native at this point,” laments Kelly.

Why is it important to keep native plants in our forests? Without them, the forest loses its rich diversity and resiliency to droughts and floods. A healthy, biodiverse forest also helps break down dead plants and recycles them into soil that further promotes a healthy understory for more native plants.

What can be done?

Kelly evaluated ways to control deer and keep New Jersey’s forest understory healthy. Deer fences are expensive, he said, but effective. When deer are kept out, native plants regenerate and out-compete invasive plants. This is good news not just for the understory, but for the entire forest.

Controlling deer populations through contraceptives is still experimental and is expensive and not very effective. Allowing recreational hunting in public forests is not highly effective. Culling deer with trained sharpshooters is effective, but costly and controversial.

It’s clear to Kelly that some combination of these deer control measures is needed. If the number of deer per square mile isn’t reduced, he said, thousands of forest plants and animals will not survive.

Kelly said that humans also benefit from fewer deer. Towns with reduced deer populations have fewer deer-automobile collisions and lower rates of tick-borne illnesses like Lyme disease.

On March 2, Dr. Kelly will speak about forest ecology at the NJ Land Conservation Rally in New Brunswick, along with Dr. Emile DeVito of NJ Conservation Foundation and Eric Karlin, a professor of plant ecology at Ramapo College. He will also address municipal leaders on March 29, along with Michael Van Clef of the NJ Invasive Species Strike Team, at a program sponsored by Raritan Headwaters.

To see a copy of Kelly’s presentation, go to

And to learn more about preserving New Jersey’s land and natural resources, visit the New Jersey Conservation Foundation website at or contact me at



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