Trees are a valuable tool in mitigating flood damage, but what’s the right tree for urban areas prone to flooding?

Wetland trees and forests are vital whether they’re in an urban landscape or a floodplain. They remove excess sediment from floodwaters, mitigate the force of floodwaters near levees, stabilize stream banks and improve water quality by buffering pesticide and fertilizer inputs into lakes and streams.

Trees and forests provide habitat, food, cover, nesting sites and travel corridors for wildlife. Forest ecosystems support the communities of many animal species, including migratory songbirds, waterfowl, hawks, eagles, deer and other mammals, as well as reptiles and amphibians. Woody plants provide recreational and educational opportunities for humans and a habitat for threatened and endangered plants and animals.

Wetland forests also sequester carbon from the atmosphere, yield timber products where stems are not damaged by ice and debris in floodwaters and cool and regulate stream temperatures through shading, thereby elevating dissolved oxygen in water for aquatic organisms that require cool, oxygenated water. Wetland forests also provide woody debris that creates important structural habitat for fish and other aquatic organisms, and shed leaves that are an important source of carbon and other nutrients in river and stream food webs. Mangrove forests protect tropical coastal areas from storm surges and are an amazing cradle of biotic diversity, offering food, shelter and spawning habitat for a large number of marine and terrestrial organisms.

Biology and ecology of flood tolerance

Forests and their biotic communities often arrange themselves on the landscape according to their adaptations to soil hydrological regimes, such as those of floodplains, swamps, flatwoods or uplands. Hydrological regimes include xeric (dry) or mesic (moist) uplands and hydric (wet) bottomlands, swamps or floodplains. Tree species of uplands are not genetically adapted to cope with regular, prolonged flooding during the growing season, and cannot thrive in floodplains, swamps or other wetlands. Flood-tolerant trees, however, can grow in upland areas.

Forest communities of wetlands and riparian corridors have distinct sets of tree species able to tolerate various flooding regimes. This includes individual tree species able to tolerate soil water saturation or inundation for years, months, only a few weeks, or only during winter and early spring when water temperatures are cool, root respiration is low, and flowing water is high in dissolved oxygen. Tree species have been classified according to their natural ability to tolerate flooding.

Table 1. Flood tolerance rankings of some eastern North American floodplain tree species for which data is available (adapted from Hook 1983).

Physiological and structural adaptations

Flood-tolerant tree species can keep their roots alive after initial flooding through a greater capacity than upland tree species to support root respiration with limited oxygen (anaerobic respiration).

Flood-tolerant trees exposed to flooding or waterlogging (which interferes with oxygen diffusion) eventually produce special roots and root structures. These structures include thick, white, adventitious “water roots,” such as those seen on plant cuttings rooted in a glass of water. Such root structures are filled with spaces between cells that aid in the diffusion of oxygen from lenticels (porous, wart-like growths induced by flooding) on lower stems and other bark tissue above the saturated zone down to the roots. The roots also aid in the diffusion of carbon dioxide from respiring root cells, preventing buildup of toxic levels of this gas.

Increased oxygen diffusion allows tree roots to resume more efficient aerobic (oxygen-requiring) respiration in support of normal functions. Without sufficient oxygen, root cells are inhibited and cannot support necessary metabolic processes, such as the energy-demanding uptake of nutrients from the soil solution. Over time, flooded tree roots survive in accordance with their species’ flood-tolerance capacities.

Another strategy to avoid flood stress is exemplified by tag alder, a small wetland tree species of the northern U.S. and Canada. Although it is a wetland, bog and streamside species, the tag alder does not tolerate extremely long periods of root flooding. It produces roots near the soil surface to access oxygen in wet soils that do not experience prolonged periods of flooding.

Baldcypress trees and other tree species, such as Shumard oak, produce buttresses in wetlands. These buttresses help stabilize trees and support their mass in unstable, wet soils and resist tipping due to pressure from surging floodwaters. Baldcypress trees also produce anchoring roots that grow downward from lateral roots. These roots help keep trees upright in unstable muck and moving water. Buttresses reflect the level of flooding, and develop most strongly where the base of the tree is regularly exposed to subsequent submersion and drying. Cypress “knees” produced in wetlands extend above soil level to a height approximating the level of periodic flooding and are filled with air spaces.

Ecological principles

How can increased knowledge of tree interactions with flooding and waterlogging aid arborists and other tree-care professionals in managing urban trees? Ecological knowledge pointedly demonstrates the absolute importance of matching tree species to their planting sites. Only flood-tolerant tree species should be planted on lowland sites that flood severely or have poor internal soil drainage with ponding and wet soil.

It’s noteworthy that many urban conditions parallel flooding conditions with respect to the reduction in soil oxygen concentration. Examples include soil compaction, soils high in clays that bind and hold water molecules, slowing oxygen diffusion to roots through small soil pores, and gas-impervious pavement that reduces oxygen diffusion to tree roots. It is no coincidence that many trees selected for planting in urban sites are flood-tolerant tree species and can tolerate low soil oxygen conditions. Examples of flood-tolerant tree species commonly surviving in urban environments include honeylocust, red maple, pin oak, willow oak, plane trees, green ash and baldcypress.

Arizona alder along a stream in a high-elevation canyon.

An illustration of the importance of understanding how trees cope with low soil oxygen diffusion rates comes from home construction activity on sites with beautiful, large, spreading bur oak trees in the Midwestern U.S. Bur oaks live for hundreds of years and have broad ecological amplitude with respect to soil moisture, occurring on dry sites, where their deep sinker roots can access subsoil moisture, and on riparian soils subject to periodic flooding. In many cases, where the trunks of bur oaks were protected from damage during construction activities, but the area under the drip line of the tree was not fenced off to prevent soil compaction, there is clear evidence of injuries resulting from such activities on valuable landscape trees. In this example, the summer shade provided by the trees is attractive for parking cars, trucks and construction vehicles, as well as for stockpiling construction materials. This activity results in soil compaction. Several years after construction, the stately trees that graced a new estate begin to decline and rapidly proceed to die.

Understanding tree interactions with the hydrosphere can be useful in managing and maintaining trees within the urban sphere. This is true at levels ranging from the individual tree and site, to the community, and to the landscape and global levels.

Good to Know

  • Shoreline trees of floodplain species can colonize and help stabilize reservoir banks after impoundment of water by earthen dams, sometimes replacing species unable to tolerate the newly waterlogged soils.
  • Tree growth of any kind on earthen dams must be prevented.
  • Tree roots, especially those of flood tolerant tree species, can penetrate small earth dams creating root channels.

Literature Cited: Hook, D. D. 1983. Forest Ecology. In K.E. Wenger (Ed.). Forestry Handbook. Second Edition. Wiley, New York City, New York. 1335 pp.