Plant successions occur regularly in the forest. More vigorously growing plants shade out the slower-growing species. Shade-tolerant species occupy the forest floor. The lower limbs on trees in a mature forest do not exist because they have been shaded out, often early in the forest’s development. The result is tall trees with clean trunks. In the absence of branches, the trunks increase in diameter because new sapwood is formed where branches once protruded.
More than shade-tolerant, the species that occupy the forest floor survive and thrive because they can function under different light qualities. As sunlight penetrates through the canopy of forest trees, the quality of light changes.
Sunlight consists of many colors, as can be seen in a rainbow or glass prism that separates the bands of light based on the wavelength of each color.
The chlorophyll in the foliage of the tall-growing species absorbs most of the far-red and red light of the sun as it penetrates the canopy. The remaining light striking the forest floor is both lower in intensity and nearly devoid of far-red and red light. It is rich in blue light. Plants that grow on the forest floor can survive the shade but thrive on mostly blue light. The chlorophyll of higher green plants does not utilize yellow or green light.
Many years ago, a Maryland nurseryman tried to grow spicebush (Lindera benzoin) for shade gardens. Grown under a 60 percent shade canopy, the plants appeared chlorotic with marginal necrosis. Having seen spicebush growing only as an under-story tree, I suggested that he clear a spot in a nearby wooded area and grow them under the shade of the trees. Within a month, the new growth on these spicebushes appeared normal, while the spicebush growing along the edge of the wooded area continued to exhibit problems. I attributed these symptoms to the fact that the plants received direct morning sunlight.
Successions also occur in landscapes. They don’t get much attention until we see growth problems: the crape myrtle does not appear to be flowering as heavily, the roses are getting leggy and have fewer flowers, the juniper appears to be dying out, there are dead and drying branches in the hollies and boxwood, the crabapple is constantly dropping leaves and produces few flowers …
When we establish a landscape, the plants are small and all get about the same amount of sunlight. But all don’t grow at the same rate, and soon some crowd out slower-growing species. As these changes occur, we either ignore the symptoms or try to correct them by applying more fertilizer.
Many of these problems can be avoided by properly positioning plants. Slower-growing species should be planted on the south side of tall-growing species. Shade-intolerant species such as fir, hemlock, juniper, pine and spruce should be planted where faster- and taller-growing hardwoods will not shade them. Shade-tolerant species such as azaleas, andromeda, ground covers, hosta and mountain laurel should be planted where they will be shaded by taller growing species.
A broad palette of shade-tolerant plants can be grown in southern Maryland. Creating a landscape design with compatible species can prevent landscape successions.
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