Tree Polyculture – Mixing Various Kinds of Trees
here is a growing concern in communities in our part of the country about the quantity and quality of the trees on both public and private property that make up what is now called the urban, or community, forest. Through educational programs offered by federal and state Forest Services, the National Arbor Day Foundation, local public gardens, and arboretums residents are more aware of the critical importance of preserving and planting trees in their towns. Collectively, these trees are a major community resource that must be nurtured and perpetuated for the health and welfare of the community.
Losing our community forest
The stunning deciduous forest that blanketed the East Coast until the arrival of European settlers has been continuously fragmented by destruction from environmental, agricultural and commercial causes, threatening the habitat on which both humans and wildlife depend. Guided by local tree commissions and informed municipal leadership, citizens are realizing that it is up to them to maintain what is left of the regional tree canopy by caring for and planting new and replacement trees on a regular basis. The urgency is increasing as we see the threat to our communities when trees are absent– uncontrolled storm water run-off, soil erosion, soil compaction, threatened water supplies, air pollution, excessive heat, reduced wildlife activity and many other problems.
It is not enough just to preserve some trees. We must conscientiously replace those that die or are removed by construction. We must plant more trees to increase the tree canopy. Further, we must choose them and site them more thoughtfully. In the past it was not uncommon in some towns to see miles of streets lined with only stately elms or with only chestnuts. With their tragic demise from Dutch elm disease and chestnut blight respectively, we have learned to avoid such uniform plantings, or monocultures. While arborists and horticulturists have technical definitions of a monoculture, laymen need only to understand “monoculture” as the practice of planting the same type of tree exclusively in a designated, delineated area. Currently, “polyculture,” or mixed plantings of various kinds of trees, is now considered to be the correct practice in these situations.
The advantages of planting trees as a polyculture
Polycultures are natural: The most compelling argument for planting a mix of various kinds of trees in a given area is that this is how forests grow naturally. The model that Mother Nature uses is mingling diverse species that have a variety of genetic strengths. If one species gets attacked by pest or disease, there are other species that are resistant to the problem and they maintain the forest until the threatened one either recovers or is succeeded by another type of tree. This diverse tree population, especially if the trees are native to the region, also hosts diverse populations of beneficial creatures that protect the tree community, which in turn, sustains them.
Polycultures are less expensive: Trees that grow along streets, in parks and in residential and commercial developed areas endure enormous stress. They do not have the expected lifespan that their counterparts in the forest do. They die and must be replaced more often. The problem with monoculture plantings, the same kind of tree all planted at the same time, is also that they all die about the same time. They must all be replaced at the same time, a costly project. In polycultures, trees have different responses to stresses, they have different life spans, so they die at different times. Planting the occasional replacements is less expensive. Best of all, the tree canopy in the area is sustained throughout this natural cycle.
Polycultures are safer: Monoculture plantings of trees of the same species planted at the same time tend to develop problems more often. Their stress and concentrated population draws pest insects and fosters the spread of diseases more easily. This requires more care from township grounds crews by spraying, or other treatments. This, in turn, introduces into the community environment chemical pesticides and herbicides that are expensive and potentially harmful to people and wildlife. Diverse types of trees in polyculture tree plantings each have their own unique susceptibilities and resistances which offset others’ vulnerabilities. Because they harbor a healthy community of organisms and animals that are in balance, pest and disease problems typically are not able to take hold as easily and do not spread as extensively.
Polycultures are beautiful: Obviously, monoculture tree plantings can be lovely and eye-catching. They are appropriate in primarily ornamental situations, where a grove of matching conifers or a small group of similar flowering trees serves as a focal point in a landscape. However, mixtures of different kinds of trees, whether artfully planned for a landscape or informally sited on larger public parks, school or corporate campuses, golf courses, cemeteries or designated open space areas of a community are also stunning. Polycultures offer a continuous display of various tree heights and widths, canopy structures, bark textures and colors. Their flowers, fruits and foliage colors celebrate the beauty and diversity in nature year round.
As we move farther into the 21st century the tree canopy is disappearing so quickly that we have no time to repeat the mistakes of the past. It has taken us a long time to figure out that monocultures present problems. We lost the elms and chestnuts, but then replaced them on denuded campuses and sun-baked streets with uniform plantings of other trees which then also became problematical. Now we realize it is not necessarily the kind of tree that is the problem. It is the fact that so many of the same kind of tree are concentrated in limited areas. Now tree experts recommend that no more than 10% of a community forest should be composed of any specific tree species, no more than 20% of the trees in the same genus. Let us take every opportunity to plant a wide variety of kinds of trees in our communities to preserve wildlife habitat, to save money, to moderate local climate, and to provide great beauty.
By Liz Ball, Marple Tree Commission