News From The U.S. Forest Service
Numerous studies have been conducted to help foresters and natural resource managers understand the impact of flooding on trees.
- The state-of-the-art, however; has not developed sufficiently to warrant a precise statement on the adaptability of a species to a specific flooding situation.
- Conclusions from different studies are often contradictory, caused in part by the physiological responses of the tree as it interacts with environmental conditions.
- Since these environmental conditions are not well understood, as well as the difficulty in categorizing tree species over their entire range, flood tolerance predictions must be carefully evaluated in general terms.
A brief review of flood, soil and tree characteristics indicates the complexity of these interactions after heavy rains.
With news today that 42% of California is now, or is very close to, Life Without The Drought, this wonderful news came at a price: some of the heaviest snowfall and rainstorms in decades, caused multiple flood-damaged locales around the SF Bay Area and Northern California.
Type of Flood
Determining flood tolerance is complicated by the diverse characteristics of floods.
Flooding during the growing season usually is more harmful to woody plants than flooding during the dormant season. Specifically, trees are most susceptible to flooding in late spring just after the first flush of growth. The timing of a spring flood influences species differentiation. For example, since silver maple flushes earlier than green ash, an early flood might be more damaging to silver maple while a later flood more injurious to green ash.
The longer trees are exposed to flooding, the greater the potential for injury. Most trees can withstand only 1-4 months with water being continuously over the soil surface. Short periods of flooding during the growing season can be tolerated by most trees. However, if flooding is recurrent and keeps the soil saturated or prevents recovery from previous flooding, injuries will accumulate and serious damage may occur.
The depth of water influences flood tolerance. The mortality rate is less for trees in saturated soils than for trees with water covering the soil. After water covers the soil, the depth may have little significance until the lower foliage is covered; research results, however; differ on this point. Tolerance to complete submersion is much lower than tolerance to shallower depths of water.
Temperature and Oxygen
Cold water is less injurious than warm water due to cold water’s capacity to hold more dissolved oxygen. Rapidly flowing water (with higher oxygen content) is less harmful than stagnant water.
An often overlooked aspect of flood damage is mechanical injury caused by cur- rent, wave action, and floating debris. Young tree plantings may be especially damaged by current and wave action. Floating debris can injure both small and large trees.
Floods may carry various chemicals that have been picked up as runoff from agricultural fields and other areas or from sewage released when treatment facilities become unable to handle large volumes of water. The impact depends upon the type and dosage of chemicals.
The following soil-related points are important in understanding flooding effects on trees.
Flooding results in poor soil aeration because the supply of oxygen to flooded soil is severely limited. Oxygen deficiency is likely the most important environmental factor that triggers growth inhibition and injury in flooded plants.
Flooding of soil increases the pH of acid soils and decreases the pH of alkaline soils.
The rate of decomposition of organic matter in flooded soil tends to be only half that in an unflooded soil. The major end products of decomposition of organic matter in flooded soils are carbon dioxide, methane, and humic materials. In addition, high concentrations of ethanol and hydrogen sulfide are produced in waterlogged soils which can be damaging to root systems.
Deposits of silt or sand as shallow as three inches may seal over and smother tree roots by limiting the supply of oxygen. Species vary in tolerance to sedimentation, but all seedlings are susceptible to root injury. Eastern cottonwood, baldcypress, tupelo, and black willow seedlings can withstand moderate siltation.
Strong currents, waves, or suspended particulates may cause soil around the base of the tree to be washed away, exposing tree roots. Exposed roots can lead to not only tree stress but can make the tree more vulnerable to windthrow.
The photo above is via CA geologist Gary Hughes’ GeoTripper blogsite. Various characteristics of a tree affect its flood tolerance with the most prominent presented below.
Tree injury increases in proportion to the percent of crown covered by water. Species that can survive standing in several feet of water for months may die in less than one month when their foliage is completely covered. Few species can tolerate more than one month of complete submersion during the growing season.
Trees in the dominant crown class survive flooding much better than trees in lower crown classes.
Adult trees tolerate flooding better than overmature trees or seedlings of the same species. Therefore, some species rated as flood tolerant may be quite sensitive in the seedling stage. Seedlings often die because they are pushed over, buried in mud, or uprooted.
Tree vigor at time of flooding influences tolerance. Vigorously growing, healthy trees withstand flooding better than less vigorous trees. Tree vigor may be irrelevant, however; if the tree is totally submersed in water.
Long-term flooding leads to death and decay of large portions of a tree’s root system (see section on Management Implications for windthrow problems). During flooding, some species can maintain normal roots in an active or dormant condition; others rely upon new secondary and adventitious roots that may form from the root collar or on the trunk near the water surface. Species unable to either maintain normal roots or grow new ones can quickly die.
Flood tolerance variations within a species are not well understood. Flood tolerance may be an inherited trait and this may explain some of the discrepancies in reports on survival. (Research methodologies also may vary from one study to another; contributing to contradictory conclusions). However, it is generally accepted that some species have greater tolerance for flooding than others (see Tables 1, 2, and 3).
…And Yes, Trees Do Help Prevent Flooding
As trees reduce rain’s impact, and hold soil in place, rain runs faster off land forms. The effects of nonstop raindrops onto bare ground gives way to intense soil erosion and quick runoff. Trees soak up water.
Aside from not being able to absorb rainwater…the clear-cutting of trees has another side effect. Since the soil isn’t anchored in place by healthy tree roots, the dirt and soil and small stones gets carried downstream by heavy rainfall. Dirt and mud washes into waterways, rivers and streams….and this means riverbeds get clogged — and carry less water out to oceans. This makes flooding worse.
Trees, Flooding, And Cities
Trees help shoulder the water burden in reducing water damage from flooding in cities too. Bricks, mortar, sidewalks and concrete don’t do much to soak up or stop rain. Water runs across the top of pavement, but water is not absorbed into it! However, soil and trees, act as a sponges.
In a 2013 research study, scientists found that trees soaked up twice as much rain as parking lots full of asphalt — reducing runoff by about 60%.
Are certain trees better than others in reducing flood water accumulations? Bald cypress trees and water tupelo trees are OK in large amounts of water, planting them at the bottom of a basin is a good idea, as they have in Houston, TX. Other tree species and types are not readily adapted to being in standing water for a long time…as these harsh conditions may rot the roots and eventually kill a tree.
In general, you want to plant native trees here in a SF Bay Area flood zone — for a property likely to incur flooding in severe weather conditions, or just to be safe for our next big storm:
- Silver maple
- California laurel
- Sandbar willow
Planting broad-leaf evergreen trees is smart in the Pacific Northwest. Deciduous trees — such as the ones above, are trees that lose their leaves in the winter — which does less to catch rainfall, because their leaves are gone! Talk with a Serpico Landscaping certified arborist about getting trees planted or removed with white glove service!
The benefits of trees don’t stop at slowing water flow. As we recover from the impact of this week’s three back-to-back *atmospheric rivers* — a term for rainstorms we won’t soon ever forget! — Serpico Landscaping wants you to better understand floods effects on trees.