This article provides information on how to grow and manage forests in a wet bottomland or floodplain environment in Iowa. It begins with a review of what floodplains are and how they function, and then addresses forest management topics including establishment, care, and harvest/regeneration systems for bottomland forests.
The floodplain environment
A river or stream and its floodplain exist in a state of constant change. Here in the Midwest these riparian zones occupy some of the most sensitive areas in terms of soils, hydrology, and ecosystem processes, and represent much of the wildlife habitat left in Iowa. Riparian zones have their greatest values as buffers and filters at the interface between agricultural and urban areas and the state’s water resources.
Riparian zones occur as boundaries between aquatic and upland communities but have distinct vegetation and soil characteristics. These zones are most commonly recognized as bottomland hardwoods or floodplain forests in the eastern and central United States. Riparian forests are uniquely characterized by the combination of high timber volume potential and highly productive yet highly variable soil types over a shallow water table.
Flooding can both enhance and stress a riparian ecosystem, depending on frequency, timing, and intensity. Unfortunately, efforts to control floods that involve constructing upstream dams, channelization of the natural meander, or diking the streamside margins often results in a reduction of aquatic ecosystem health. Riparian productivity is reduced when normal seasonal flooding is abolished or reduced by dams. Future management should focus on means of reducing man-made flood stress and restoring the natural flow of water. Channelization of the steam banks force the energy of the water to be transferred downstream at an increased velocity instead of being dissipated at each turn as the stream meanders side to side. Lastly, diking the stream narrows the flood zone which forces more water to move in a smaller area and normally is combined with channelization to move that water in straight lines. The “energy” of the system is kept within the walls of the dike instead of being allowed to dissipate across the floodplain where the water slows down and where it can deposit the suspended sediments.
Three main floodplain site types can be recognized along rivers and streams (figure 1):
- point bars and oxbows
- first bottoms
- second bottoms (terraces or benches)
Understanding and being able to recognize these site types is important for forest establishment and management situations.
Point bars & oxbows are typically inundated by water for several months at a time, each year. Point bars are actually within the stream channel itself and are created by the deposition of sand & silt as the river slows down on the inside bends of the channel and drops sediment. Oxbows are crescent- shaped “bayous” or sloughs that used to be part of the river channel, but became detached when the river bank eroded enough to cut them off. They are usually found up on the first bottom floodplain, away from the present channel, and look like linear-shaped pools that are the same depth as the river channel. Oxbows periodically dry out and become covered by tree seedlings or grass. They usually become inundated again which kills the encroaching trees or they are too shaded for the seedlings to continue growth. Point bars eventually build up enough elevation as the river continues to deposit material and meander in the opposite direction that they will succeed to willow, silver maple, or cottonwood thickets.
First bottoms typically flood every 1-3 years as they are essentially in direct contact with the river. Flooding usually subsides by mid- to late-summer. Soils on first bottoms can vary dramatically in both the vertical profile and laterally across an area. Most commonly they are made up of finer silts and clay particles which drain slowly and do not hold much oxygen in the root zone.
Second bottoms, otherwise known as terraces or benches, are actually former first bottom floodplains which have become separated from the river’s floods over time as the river cut down deeper into its valley. They are located further out away from the channel, near the valley wall before it becomes steep and grades into upland terrain. Second bottoms rarely flood; only under extreme conditions. They often contain soils that drain more quickly than the first bottom, and can grow a wider variety of species that are suited to both upland and bottomland environments. They are often some of the most productive areas for growing black walnut in Iowa.
Flooding impacts on trees
The effects of flooding will vary with the type and age of trees and the magnitude and duration of the flood. Small trees that are completely inundated by water cannot survive more than a few days with their leaves underwater. For this reason, new plantings are especially at risk.
For mature trees that experience flooding, the effects observed in the stem and leaves are merely symptoms of the influence of flooding on the root systems. For most species, the root system either becomes dormant or begins to die immediately after flooding. The adjustment of the root system is dictated by the growth of adventitious roots and new secondary roots under low oxygen conditions. The maintenance of proper root functioning is the factor which determines tolerance to flooding. Very tolerant or tolerant species can maintain their normal roots while also producing new secondary and adventitious roots. Moderately tolerant species are those in which the normal root system deteriorates, but the tree can produce adventitious roots to replace it. The final category consists of species in which the normal root system dies, and the plant is unable to produce adventitious roots. These trees usually die under flooding stress in a relatively short time. Table 1 shows these tolerances for many tree species commonly planted or native to Iowa.
Similar to the effects of flooding, soil texture greatly influences what tree species will grow and thrive in an area because it affects internal drainage through the soil profile, moisture retention, and the amount of oxygen that’s available to tree roots. Soil texture refers to the relative proportions of sand, silt, and clay that make up the mineral soil. Generally, trees that are tolerant to flooding are the same ones that perform best in heavier clay-based soils, while trees that are intolerant to flooding demand a lighter or medium-textured soil (Table 1).
Table 1. Relative tolerance of species to flooding and preferences for soil texture
Forest establishment on floodplains can be accomplished by natural regeneration, artificial means, or a combination of the two. Careful planning is necessary to ensure the species mixture will be appropriate for the site conditions & soil type while also accomplishing the landowner’s goals. The basic steps include site preparation, planting, and maintenance.
Regardless of which approach is used, the main goal of forest establishment should be to achieve crown closure as quickly as possible. This can be accomplished by having high stem density (700-1,000 seedlings or more per acre) and by favoring tree species that grow rapidly and cast heavy shade, such as silver maple, sycamore, cottonwood, and river birch. This will reduce or eliminate the threat from invasive reed canary grass which commonly overtakes bottomland reforestation projects, and it will also buffer impacts from deer and rodents. Slower growing hardwoods such as oak, walnut, and pecan can be interspersed throughout the planting at lower densities (100-200 per acre), and can then be favored to become dominant trees using intermediate thinning treatments as the forest develops.
Bottomland forests are commonly made up of “pioneer” tree species that produce large amounts of seed and attempt to quickly colonize unvegetated land. The seed is dispersed early in the growing season at a time when there is a lot of bare, exposed ground created by recent flooding. The seed is usually either windblown or carried by water over great distances. When the seed lands on a suitable site with ample sunlight, it germinates readily and grows rapidly to gain a competitive advantage over other, slow-growing species. Cottonwood, willow, silver maple, green ash, river birch, and boxelder are all good examples of pioneer trees.
Small reforestation sites that are adjacent to or surrounded by mature bottomland forest make good candidates for natural regeneration. The three basic steps include site preparation, monitoring for success, and maintenance:
- Site preparation. This is the most important step. The goal of site preparation is to create a bare, unvegetated seedbed at the time of year when the surrounding trees disperse their seed (see table 2 below on seed dispersal traits of common bottomland species). On floodplains, this is often best done in late summer or fall when conditions are dry and perennial plants are easiest to kill, so that it’s ready for next spring. If there are undesirable trees on the site, fell or girdle them and apply herbicide to prevent resprouting. The site should have nearly full sunlight. If there are perennial grasses like reed canary or brome, try to mow the site in August and then apply herbicide to the subsequent regrowth 4-5 weeks later when it’s actively growing and 6-12” tall. If possible, take an extra year or two to continually spray out all reed canary grass on site; the ideal situation would be to plant soybeans for a year or two prior to establishing tree cover, since that would gain better control over the invasive grass. Then use light equipment such as a blade, bucket, harrow, or other drag to scarify the surface and expose bare soil, without compacting it. Prescribed burning may also work to remove surface litter and expose bare soil, after it’s been killed by herbicide. (Prescribed burning could also be used to clear dead residue and promote uniform regrowth just prior to spraying in Spring; however, prescribed burning alone is not a sufficient control method for reed canary grass or brome). If there are tree tops or logs on the site, push them into small piles so that they are out of the way and easily visible. If the area floods next spring or is overtaken by reed canary grass before trees can successfully capture the site, this will allow you to easily mow & spray the site again.
- Monitoring for germination. Visit the site in mid- to late May and again in the subsequent weeks to monitor for success. Seedling crops of silver maple, ash, elm, cottonwood, and willow will often have densities of 10,000 to 20,000 seedlings per acre, but anything over 900/acre should be sufficient to capture the site quickly if the perennial vegetation was properly killed during the site preparation step.
- Maintenance. Selective post-emergent herbicides are not ideal during the first growing season due to seedling sensitivity, but if necessary can be used to weaken problematic weeds or grasses (see Environmental Considerations section). Mowing with a high deck setting or sickle mower might also be used to clip off tall weeds (e.g. ragweed) before they go to seed and to keep sunlight on the young tree seedlings. Consider doing weed control until the trees are 2-3 feet tall and are beginning to completely shade the ground, which should be after the second growing season or midway through the third.
Table 2. Seed dispersal traits of common bottomland species
(adapted from Bonner et al, 2008 and Burns and Honkala, 1990)
|Species||Flowering||Fruit Ripening||Dispersal||Typical minimum bearing age (years)||Years between bumper crops|
|Silver Maple||Feb - May||April - June||April - June (Peak Mid-May)||10||1|
|Cottonwood||April - May||May - June||May - June||10||1|
|Willow||Feb - April||April - May||April - May||10||1|
|Green Ash||March - May||Sept - Oct||Oct - May||10||1|
|Sycamore||March - May||Sept - Nov||Jan - April||15||1-2|
|Bur Oak||May - June||Aug - Sept||Aug - Oct||35||2-3|
|Swamp White Oak||May - June||Aug - Sept||Sept - Nov||20||3-5|
|Pin Oak||May - June||Aug - Sept||Sept - Dec||20||1-2|
|Shellbark Hickory||Apr - June||Sept - Nov||Sept - Oct||40||1-2|
|Northern Pecan||March - May||Sept - Oct||Sept - Oct||15||1-2|
|American Elm||Feb - May||March - June||March - June||15||1|
|River Birch||Apr - May||May - June||May - June||15||1|
|Boxelder||March - May||Aug - Oct||Sept - March||10||1|
Artificial regeneration refers to forest establishment either by transplanting nursery seedlings or by direct seeding.
- Site preparation. The steps are essentially the same as those described under the Natural Regeneration section. The site needs to have nearly full sunlight and almost no existing perennial plant coverage. Soybean stubble is ideal. For seedling plantings on former row crop ground, it may be desirable to seed a cover crop of 1-2 bushels/ac of oats or 5-10 lbs/ac of perennial rye to help suppress weed growth in the aisles between tree rows. On pasture ground, spray out 48” wide strips for tree rows. Scarification is not needed if planting seedlings, but will help recruit natural regeneration which will boost tree density.
- Planting. Plant conservation-grade bare root seedlings at a high density (700-1,000 per acre) and use approximately 75% fast-growing, dense shade species such as silver maple, sycamore, river birch, and cottonwood. The other 25% or less can be desired mast species that are slower-growing such as oak (bur, swamp white, pin), black walnut, northern pecan, or shellbark hickory. On sites where flooding is very frequent, fall plantings may be advantageous. The most critical element is to keep the roots cool and moist when handling and get them in the soil and firmly sealed as quickly as possible, before they begin growing. For direct seedings, use hard mast species at a high seeding rate and then rely on natural regeneration of pioneer species to supplement the planting. The surrounding soft maple, ash, cottonwood, and willow will most likely work their way into the planting naturally. Recommended rates for a general wildlife mix would be 1-2 bushels/ac each of bur oak, swamp white oak, and pin oak, 1-2 bushels/ac shellbark hickory, and 2-4 bushels/ac of walnut. It’s critical to recognize the local soil and site conditions when choosing a species mix. Black walnut can only grow & perform well on first bottom floodplain sites where good subsoil drainage exists, like in loam or sandy loam textured soils, or on slightly elevated benches. the landscape position, soils maps, and local vegetation can all be useful to help steer species mixtures for tree plantings.
- Maintenance. Soon after planting, apply pre-emergent herbicides just before a rain event. The most common mix is Princep and Pendulum (aka Prowl) at 2 qts/acre each. Then use mowing and/or post-emergent products during the growing season as needed.
In summary, use a combination of thorough site preparation, very high seedling densities, appropriate tree species for the soils, and good maintenance for the first 3-5 years. The best short-term method to kill invasive reed canary grass is herbicide; the best long-term method is to shade it out.
The two most common intermediate cultural practices needed during the development of a bottomland forest are pruning and thinning.
Pruning can be done on young trees from sapling size to pole size (up to 10-11” diameter) to improve the future log/lumber grade for wood products. Generally, only black walnut warrants pruning and training for economic return, but if time allows it may also be done for oak and other hardwoods with product potential. Remember that not all trees will survive to be harvested some day, so save time and only prune the best crop trees in the stand.
Thinning is done to reduce the density and crowding among dominant trees in a stand to improve their health, seed production, and growth rate. Research suggests that non-commercial thinning treatments in overstocked bottomland stands of pure silver maple and cottonwood are effective: thinned stands of silver maple have been shown to triple their wood volume in the 10 years that follow (Larson, 1968). In cottonwood stands, heavy thinning done early and often through the rotation is recommended (Burns, 1990).
In Iowa, a common form of thinning known as crop tree release is often used in stands that contain desirable hard mast species such as oak, walnut, and hickory. This practice can be used to improve overall forest health, wildlife habitat, and timber production. Crop tree release is most applicable to polesized (4-11”) and small sawtimber (12- 14”) stands that are overstocked and while trees still have the capacity to increase their crown size. Refer to US Forest Service publication Crop Tree Management in Eastern Hardwoods and ISU Extension publication Planning for Wooded Acreages and Woodlands for more information on this practice.
Early release cuttings could also be useful in very young bottomland stands where slow-growing oaks are interspersed with faster growing pioneer species such as soft maple & ash, or in direct seedings that are heavy in walnut and light on oak. This form of thinning can be used to maintain the growth and vigor of the desirable slower-growing crop trees.
Timber harvesting on floodplain sites should be done with the assistance of a trained professional forester. This is not only to ensure the landowner receives a fair price for the timber, but also to make sure the right trees are harvested, at the right time in the stand’s rotation, and that a new generation of young forest will grow up to replace the one that is harvested. However, most timber harvests on bottomlands in Iowa occur without the involvement of a professionally-trained forester and are done by selective cutting of the largest and highest grade trees. This results in maximum short-term profit for both the landowner and/or the logger, but does not guarantee sustainability of the resource. Pure stands of silver maple, cottonwood, or ash on bottomland sites are usually even-aged populations with a normal (bell shaped) diameter distribution. This means that as they grow into a mature size class, half of the trees will exceed 18” in diameter and half will be less than 18” diameter. In a hypothetical “logger’s choice” selective harvest scenario, most or all of the trees taken in the first cutting will be dominant or co-dominant trees 18” and larger. The residual stand will be made up of co- dominant, intermediate, and suppressed trees less than 18”, plus scattered understory shrubs and saplings such as mulberry, elm, and other non-commercial species.
A second cutting will often be available 10-15 years later, made up of the commercially viable co-dominant trees that were too small at the time of the first harvest. After that cutting, the stand is essentially non-profitable because non- commercial shade-tolerant trees & shrubs have regenerated in the partial openings or reed canary grass has taken over the sunny openings. The resulting condition is essentially the same as the climax condition in an unmanaged forest.
This, commercial timber harvesting on floodplains should almost always be done using even-aged practices due to the fact that there are few shade-tolerant commercial species that occur on bottomlands. Small clearcuts (3-5 ac) or group selection cuts are generally the best way to practice sustainable forestry as well as diversify the age structure and create habitat diversity on a bottomland timber landholding. Attempts to use shelterwood treatments in silver maple & cottonwood stands to develop advanced regeneration have not been commonly used or proven successful in Iowa. Seed tree systems can potentially work, but have a high risk for windthrow and may not be needed since seed dispersal by bottomland pioneer species is so efficient anyways. The two most important factors for successful regeneration of bottomlands appear to be timing and luck. Timing refers to creating a seedbed of moist, unvegetated soil at the time of seed dispersal by surrounding desirable trees. Luck refers to the chances of a flood occurring immediately after germination, or a weed problem that precedes it.
Improving wildlife habitat is one of the most popular goals of private landowners seeking forest management assistance in Iowa. Nearly all management decisions (including no management) will have an effect on the habitat and will involve tradeoffs, with beneficial changes for some species and negative impacts on others.
It’s important to recognize that different successional stages of a forest (i.e., young forest, middle-aged forest, mature forest) are utilized by different species of wildlife. Figure 2 below depicts these successional stages and the Wildlife Preferences of Forest Successional Stages section describes examples of wildlife that prefer each stage. Forestry practices such as planting, thinning, and timber harvesting can all be used to optimize wildlife habitat for a wider variety of species through manipulation of this process. Landowners should seek assistance from a wildlife biologist for more information on related wildlife management practices such as food plots, edge feathering, wetlands, and native prairie establishment & maintenance.
Wildlife Preferences of Forest Successional Stages
The seedlings & saplings stage, also known as early successional cover, contains a mixture of grasses, weeds, small shrubs, thorny brambles, and young trees. It is best described as brushy habitat. Many types of small game such as rabbits, mice, voles, & snakes use this cover. Not surprisingly, it’s also preferred hunting ground for avian predators including hawks, owls, and kestrels. Pheasants, quail, woodcock, and ruffed grouse will use this cover at certain times of the year when heavier cover is desired. Deer will use it for bedding, fawning, browsing, rubbing, and staging. Female wild turkeys use it for nesting. Songbirds that prefer this cover include gold-winged warbler, blue-winged warbler, black- billed cuckoo, yellow-billed cuckoo, eastern towhee, and prairie warbler.
During the poletimber stage, the forest canopy closes in and very little sunlight reaches the ground. The grasses, weeds, and other ground plants are shaded out by the dense layer of trees up above, which are all about the same age and fairly uniform in height and form. Consequently, this stage of the forest offers the least amount of diversity for wildlife. Thinning can be used to enhance tree growth and speed up the transition to the next stage, which is the small sawtimber stage. Thinning will also increase acorn & fruit production of favored trees, and make the trees stronger, healthier, and more immune to disease & insect attacks, and get sunlight to the ground to stimulate vegetation and new cover for wildlife. As the forest goes deeper into the small sawtimber stage, the habitat becomes more complex with different layers and new shrubs & saplings emerging.
When the forest reaches the mature sawtimber stage, some trees have begun to die from natural causes like lightning strikes, wind, snow & ice, competition, or old age. Selective tree harvesting can also have this effect. Trees that have died but are still standing are called snags and are very important to cavity-nesting wildlife and woodpeckers. Fallen logs & tree tops house or hide animals on the ground such as whitetail deer, and then rot back into the soil. Wild turkeys roost in the tops of mature trees and eat the sweet acorns of oak trees. Mosses and wildflowers become more abundant and insects find refuge in small microhabitats. Many birds prefer this more diverse habitat structure with its complex layers, such as the acadian flycatcher, cerulean warbler, veery, and black & white warbler among others. Near large rivers, bald eagles and various species of herons may make their nests in mature trees. Various reptiles and amphibians also like mature bottomland forests and the mixture of seasonal ponds, emergent logs for sunning, and hiding places.
Well-managed bottomland forests will naturally serve the environment by enhancing air and water quality, providing wildlife habitat, and performing basic ecological functions. However, forest management activities have the potential to offset these benefits, either temporarily or permanently, and so should be carefully applied.
Herbicides must be used very carefully on floodplains & bottomlands, given the close proximity of groundwater and surface water resources. Any pesticides that have high potential to leach through the soil and cause harm to aquatic life or human health in the water supply should be avoided. Users MUST thoroughly read the product label and understand how to use the product appropriately, or avoid using it altogether. Table 3 below gives a basic summary of some commonly used herbicides in bottomland forestry situations.
Table 3. Commonly used forestry herbicides in floodplain situations.
Always read, understand, and follow product label instructions.
|Burn-down site preparation||Glyphosate (e.g. Roundup, Crop Smart)||Safe to apply to dry soil. If standing water exists or shallow groundwater, use glyphosate approved for aquatic use such as Rodeo.|
|Pre-emergent weed control in young tree plantings||Simazine, Princep 4L, Pendulum, Prowl||Do not apply to wet or sandy/permeable soils. Must apply to dry ground prior to weed emergence.|
|Selective post-emergent weed control in young tree plantings: kills broadleaves/thistles||Stinger, Transline||Do not apply to wet or sandy/permeable soils. Apply early in the season, when vegetation is less than 10” tall.|
|Selective post-emergent weed control in young tree plantings: kills grass||Fusilade, Goal, Poast, Select||Do not apply to wet or sandy/permeable soils. Apply early in the season, when vegetation is less than 10” tall.|
|Cut-stump woody plant control||Pathfinder II, Glyphosate||Do not apply to wet or sandy/permeable soils.|
Beyond herbicide use, there are some other basic considerations to ensure protection of environmental resources during forest management activities on bottomlands.
- The timing of timber harvests and thinning is important for a variety of reasons. The winter dormant season is almost always the best time to do these operations, when the ground is frozen. This will prevent compaction and rutting by equipment, lessen the impacts of physical wounds to residual trees, and greatly reduce the incidence of infectious diseases such as oak wilt.
- Minimize the length of skid roads by planning out the harvest carefully.
- If culverts are to be used to create stream crossings, make sure they are sized properly to accommodate the highest discharge of the stream.
- Remove any and all brush from streams and crossings when operations are complete.
- Leave a 50- to 150-foot wide strip of forest along all rivers and streams as a protective buffer and to avoid dropping tops and debris into the channel.
- Be on the lookout for any unique features or resources such as rookeries, habitat capable of supporting threatened or endangered species such as bats, fen wetlands, or otherwise. Consult a professional biologist or forester before proceeding.
- Do not allow livestock grazing in forests unless it's part of a prescribed grazing system designed by a professional. Livestock eat and trample good tree seedlings and avoid the thorny ones, leading to a buildup of locust, multiflora rose, prickly ash, gooseberry, and others. They compact soil which can kill even mature trees. They can also increase streambank erosion and cause declines in timber value.
Floodplains are dynamic and difficult environments in which to practice forestry. Changing water levels, complex soils, aggressive weeds, and other common challenges are all factors that can affect the outcomes of management efforts. Circumstances that occur upstream can affect projects downstream. Landowners and forest managers must be aware of these challenges and must be persistent, patient, a little bit lucky, and willing to try again if need be. As always, seek professional advice from your state or private forestry consultant.
The presence of emerald ash borer (EAB) in Iowa necessitates some forest management considerations. Check out the publication from University of Minnesota Extension on managing ash woodlands for more information. In general the greatest defense against EAB is to promote diversity and vigor in woodlands. For more information on EAB go to the Iowa State University Horticulture Extension EAB FAQ page or the Iowa DNR EAB page.
Bonner, F.T., Karrfalt, R.P., and Nisley, R.G. 2008. The Woody Plant Seed Manual. USDA Forest Service, Washington, DC. Agricultural Handbook 727.
Burns, R.M., and Honkala, B.H. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654. USDA Forest Service, Washington, DC. Vol. 2.
Larson, H. C. 1968. Some methods of selecting and propagating asexually high quality phenotypes of silver maple (Acer saccharinum L.). In Proceedings, Fifteenth Northeastern Forest Tree Improvement Conference. p.78-84. Northeastern Forest Experiment Station, Broomall, PA.
Spetich, M.A., ed. 2004. Upland oak ecology symposium: history, current conditions, and sustainability. Gen. Tech. Rep. SRS–73. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 311 p.
This article is a web-based modification of the original publication "F-326 Management of Floodplain Forests" by former ISU Extension Forester Jesse A. Randall and Iowa DNR District Forester Joe Herring.