Dappled Light and Shades of Green

Posted: August 29, 2016

Ferns, low growing shrubs and trees, and canopy trees all change the quantity of light, but they also change the quality of light.

A summer walk under a woodland canopy where the shade is dense and only flecks of dappled sunlight reach the forest floor is a gift of the season. There is something comforting about investing time in such a landscape where the light changes as you move through the landscape. Then, too, there are relaxing shades of green all around. Most walkers probably give little thought to the role of light and the color green. Ultimately, though, the quantity and quality of light are important to the success of woodland plants.

A few years back, while visiting Philadelphia, I picked up a paper and read an editorial by an enraged person writing about a timber harvest on the Allegheny National Forest. The anger was not so much about cutting trees; rather, it was about how the equipment was destroying acres of ferns. Lovely, pretty, waving ferns drew her to the forest.

I have to admit, those fields of ferns covering the forest floor in many of Pennsylvania’s forests are kind of pretty, inviting, troubling, depressing, scary! What? How could ferns convey negative thoughts?

A fully closed hardwood forest canopy reduces the intensity of light reaching the ground. Those leaves from the very top of the tree crown through each layer below reduce light intensity. You might have seen this as you walk into the woods from an edge. The immediate edge is verdant and full of plants; as you walk further, the species composition might change and there is less vegetation. Eventually, the light intensity reaches a level where little grows on the forest floor.

As light intensity drops, the forest becomes “shady.” Shade in a forest is not sharply defined in the same way as it is under a picnic pavilion roof, for example. There, shade is well defined and sharp. Scattered and reflected light in a forest does not create the shadow—the intensity is seemingly even. However, as the plant canopy is closer to the forest floor, the shade becomes more defined and “deeper” or less scattered. So, in a forest, a layer of ferns or shorter trees under the high canopy can “wring out” the last little bit of useable light reducing intensity to the point where other plants cannot survive.

That is part of the story: light intensity changes under layers of plants and is especially trying as those plant layers occur closer to the forest floor. The other part of the story is the quality of the light. Kermit the Frog lamented “it is not easy bein’ green.” Hearing Kermit lament about being green is one thing; imagine how all the plants “feel” living in a green-dominated world.

Ferns, low growing shrubs and trees, and canopy trees all change the quantity of light, but they also change the quality of light. Summer woodlands are full of green. From the top of the canopy to the forest floor, green dominates.

Ever wonder why almost all leaves are green? Everyone admires rainbows, and, as a kid, prisms made sunlight special. Simply, rainbows and prisms demonstrate the components of visible light. At one end of the visible spectrum is purple, which represents short wavelengths and at the other end is red, which indicates the long wavelengths.

Understand that when we see colors, we are seeing the wave length reflected from the object; all of the other wavelengths are absorbed. Most plants are green. This means they absorb all the colors of the rainbow, except green. Leaves and sometimes twigs and bark on green plants are comprised of cell structures containing chlorophyll, which is green. Green plants don’t use green; essentially, they turn that color away and absorb the rest of the rainbow—violet, blue, orange, and some red. These absorbed wave lengths or light colors are essential for photosynthesis. The energy they bring into the plant excite chemical activity that allows plants to create sugars necessary for growth.

So, why are ferns troublesome? In Pennsylvania, acres and acres of forests, based on research, have fern cover that is so dense that the quantity and quality of light under them essentially eliminates the ability of tree seedlings and other plants to germinate and grow. Under ferns, light intensity is greatly reduced and the quality of light shared beneath their fronds lacks the wave lengths of light needed for growth.

Hasn’t all of this been in place for thousands of years? Haven’t plants always competed for light, water, nutrients, and space? The answer to both questions is yes; however, selective browsing by white-tailed deer has shifted plant competition, invasive non-native plant species have moved from yards to woodlands, acid rain has changed soils, and changing climate conditions are bringing even more unknowns to forest growth and development.

One study found that about 20 percent of the state’s forest land have too many ferns. Other research has found that both native and non-native plants are creating additional problems as they shift light conditions under existing tree canopies. A recent study conducted by the USDA Forest Service found that about 6 of 10 forest inventory plots across the state have at least one non-native plant, which could change forest growth and development processes.

In the long run, competition for light, both its quantity and quality, in our forests is a significant forest health challenge. We have become accustomed to the conditions we see in our woodlands – we enjoy the green vegetation and the shade. But not all is well.

The next time you walk in the woods. Think about Kermit and shades of green. Enjoy the shady places and explore how the forest is growing and pay attention to where the green may be preventing native trees and shrubs from growing. Aldo Leopold, the author of A Sand County Almanac and early environmentalist wrote “One of the penalties of an ecological education is that one lives alone in a world of wounds. Much of the damage inflicted on land is quite invisible to laymen. An ecologist must either harden his shell and make believe that the consequences of science are none of his business, or he must be the doctor who sees the marks of death in a community that believes itself well and does not want to be told otherwise” (1949). Once you know, you can’t not see.

For more information, visit two articles in the Forest Science Fact Sheet Series. Controlling Understory Fern Competition for Regeneration Success describes three species of fern in detail and outlines the reasons why we have so much fern present in our forest understories today. The fact sheet also provides research based information on the impacts of fern as well as how to successfully control it. Regenerating Hardwood Forests: Managing Competing Plants, Deer, and Light looks at how an understanding of competing plants, deer, and light can lead to successful forest regeneration and the sustainability of hardwood forests. Contact the Center for Private Forests at Penn State and Natural Resources Extension Office for more information.

Contact Information

James Finley, Ph.D.
  • Professor Emeritus of Forest Resources
Phone: 814-863-0402

Article Details


Dappled Light and Shades of Green

This publication is available in alternative media on request.