WETLAND TREE GROVE
Too Much of a Good Thing
Originally planted as a collection of trees suitable for poorly drained sites, the trees growing here were selected for their adaptability in wet conditions. Whether tolerant of periodic flooding in detention basins, saturated soils in low lying areas, high water tables, or heavy clay soils that never dry out, these trees are some of the best candidates for places with above average moisture.
The presence of high moisture content by itself isn’t necessarily harmful. Trees require water to grow and be healthy, so you would expect extra water to make trees grow extra fast or healthy. However, too much water in soils or even standing water for extended periods of time causes several life threatening problems for some trees. First, water fills up the space between soil particles, leaving no room for air. This makes it so that tree roots have a hard time accessing oxygen which they need to stay alive. Second, wet soils can be a breeding ground for harmful fungi and bacteria which take advantage of roots already stressed by a lack of oxygen. Finally, high moisture content can leach nutrients from surrounding soils and pull minerals from saturated roots, leading to nutrient deficiencies and stunted growth.
While it may be difficult for many plants to grow in poorly drained soils, there are quite a few trees that can tolerate wet conditions. Many of these tree species are native to flood plains, stream banks, or pond edges and have evolved to grow in moist soils. Because of the low oxygen content of wet soils, these trees tend to have very shallow root systems to better access oxygen at the soil surface. Some of these trees grow pneumatophores or “knees” to help access oxygen and provide stability in soft soils. Many of these tree species also readily regrow roots quickly after flooding events or will grow adventitious roots above the surface of the water in prolonged floods.
Some advantages of these trees are that they tend to grow fast, can handle stormwater pollution, and survive extreme runoff like what can be expected in urban areas, near buildings, or by impermeable surfaces. Wetland trees tend to help stabilize soils and prevent erosion, sometimes with penetrating roots that can break up compacted soils. Disadvantages include weak wood from fast growth, fallen twigs can sometimes root in place, shallow roots can lift sidewalks or become mowing hazards, and roots of these trees are notorious for seeking out sewer lines. Some common examples include red maple (Acer rubrum), silver maple (Acer saccharinum), cottonwood (Populus spp.), bald cypress (Taxodium distichum), willows (Salix spp.), birches (Betula spp.), and sycamores (Platanus occidentalis).
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