Living in the dim
Stand in a dense forest in summer and you will notice how little direct sun reaches the ground. The canopy absorbs and reflects most of the light, and what filters through is dim, shifting and patchy. In many closed forests, only a small fraction of the light above the canopy reaches the floor; the exact share depends heavily on the forest type, species and season. Yet the ground and the space below the canopy are rarely bare. Shrubs, ferns, grasses, sedges and wildflowers form a layer called the understory or herb layer, and these plants have found ways to make a living in the shade.
Light is not just dim, it is uneven
Light on the forest floor comes in several forms:
- Diffuse light that has passed through or bounced off leaves. It is weak and has less red and blue light, the colors most useful for photosynthesis, than direct sun.
- Sunflecks, brief patches of direct sunlight that pass through gaps in the foliage as the sun moves and the leaves shift in the wind. They may last seconds to minutes, yet in some forests they supply a large portion of the energy available to plants beneath.
- Gaps, larger openings made when one or more trees fall, which let in much more light for months or years. Such canopy gaps let shaded plants respond.
Plants in the understory must be able to use all three, and the way they do so shapes where each species can grow.
Strategies for the shade
Beat the canopy: spring ephemerals
In deciduous forests of temperate regions, trees leaf out in spring, but before they do, the floor gets plenty of light. Many wildflowers exploit this window. They sprout early, flower, take in sunlight and make food, then fade back before the canopy closes. Some store energy in underground bulbs, tubers or rhizomes for the following year. These plants are often called spring ephemerals. The timing matters, and shifts in the date of tree leaf-out, which scientists track as spring phenology, may influence them. Many ephemerals also depend on early-season insects for pollination, and some have seeds carried by ants, so changes in any one link can matter.
Stay evergreen
Other understory plants keep their leaves through winter, using the sunlight available before the canopy leafs out and after it falls. Some ferns, wintergreens and shrubs follow this approach. They tend to have tough, long-lived leaves. The cost is that leaves must survive cold, but the gain is that the plant can photosynthesize in fall and early spring, when the floor is bright.
Be efficient in low light
Many true shade plants have larger, thinner leaves with a high amount of chlorophyll to collect scarce light, and a low respiration rate so they do not waste what they make. Some leaves are dark green, held flat and arranged so they overlap little. Others make good use of sunflecks, being able to switch on photosynthesis rapidly when a patch of light arrives. These plants usually grow slowly, since their energy budget is small.
Borrow from fungi
A few forest plants have taken an unusual route. Some lack chlorophyll entirely and obtain carbon through fungi connected to nearby trees, a relationship related to mycorrhizal fungi. Examples include certain pale, ghostlike plants of shaded forest floors. They are uncommon and relatively specialized, and they are not typical understory plants.
Wait for a gap
Tree seedlings and saplings are also part of the understory. Shade-tolerant species can persist for years or decades, growing very slowly until a canopy gap opens and gives them a surge of light. Some may grow only a few centimeters in a year while waiting. This waiting behavior is a core part of how forests renew themselves.
More than light
Light is only one limit. The forest floor also has:
- Competition from tree roots. Large trees take much of the water and nutrients, so understory plants must share a thin supply. In dry weather, root competition can matter as much as shade.
- Leaf litter. A thick layer of fallen leaves can block seedlings or protect them, and it affects moisture and nutrients, as with any leaf litter.
- Soil conditions. Acidity, moisture and nutrients vary across short distances, which is why different plants appear on slopes, hollows and ridges.
- Browsing. Deer and other herbivores often feed on understory plants. In places where deer are abundant, browsing can sharply reduce wildflowers, shrubs and tree seedlings, which changes the structure of the forest layer.
Who depends on the understory
This layer is more than decoration. Shrubs and herbs offer food and cover to insects, birds, small mammals and reptiles. Berries and seeds feed wildlife in late summer and fall, and dense shrubs give nesting sites that many birds use. Roots help hold soil and reduce erosion on slopes. The layer also takes up nutrients that would otherwise be lost, and contributes to the cycling of nutrients through the forest.
Understory richness often reflects overall forest condition. Plant communities that change after heavy browsing, trampling, past land use or the arrival of invasive species can be used as clues to the history and health of a woodland, and they are among the things that people look at when judging forest health indicators.
Changing the light changes the understory
Because so many understory plants are tuned to a particular light level, changes in canopy density tend to cause changes beneath it.
- Thinning or harvest raises light on the floor. Many shrubs and herbs respond, often with a flush of growth, while some shade-loving species decline if the change is large or long-lasting. Results vary with how much canopy is removed.
- Dense, closed planting or heavy evergreen shade can keep the floor so dark that few plants survive.
- Storm damage and tree death open the canopy and bring in light, favoring different plants.
- Non-native shrubs that leaf out early or hold leaves late can shade out native plants, which is a common concern in managed woodlands and an example of how invasive species affect forests.
Seeing it for yourself
A good way to understand the understory is to visit the same woodland at different times of year. In early spring, notice the flowers on the open floor. In midsummer, see which plants remain when the canopy is full. In fall, watch the evergreen species stay green after the trees go bare. A few return visits show clearly how closely plant life on the floor follows the schedule of the light.