Shelterbelts and Windbreaks

Ernest J. George, Ralph A. Read, E. W. Johnson, and A. E. Ferber.

Shelterbelts and windbreaks are plantings of trees and shrubs that are high and dense enough to reduce wind velocity and prevent the drifting of snow.

They have many values and purposes: To protect the farm home and buildings; to shelter livestock, stack areas, and feed lots; to protect orchards, gardens, and cultivated fields and crops against winds that might erode the soil, injure seedlings, or damage the maturing crops; to hold snow on the land to increase soil moisture; to prevent highways from becoming blocked by drifted snow; to provide habitat for birds; to provide, sometimes, posts, poles, and fuelwood; and to please the eye.

Shelterbelts and windbreaks are planted most extensively in the Prairies and Plains, between the forested regions of the Mississippi River States and the Rocky Mountains. It is primarily a grassland area, sparse in natural tree growth. It has diverse soils, high rates of evaporation and transpiration, extremes in temperature, and droughts.

Only hardy species of trees grow in the Plains, and their growth depends largely on the relationship between water and soil. Soil moisture and soil air greatly influence the vigor, growth, and longevity of trees.

The amount of yearly precipitation and the amount that enters the soil largely determine the degree of success obtained with trees and shrubs in the Great Plains.

The average rainfall in the northern Great Plains is 24 inches a year in the eastern part and about 12 inches in the west. In the southern Great Plains it is 30 inches along the eastern boundary and about 12 inches farther west.

About 76 percent of the average annual precipitation falls during the growing season April to September in the northern section. Another 18 percent falls as snow, which can be of great benefit when it is retained by trees or other vegetation.

Sixty-five percent of the annual precipitation falls during the growing season in the southern Great Plains.

The best soils for shelterbelts and windbreaks in the dryland sections usually are sandy or silty loams. Clay soils usually are considered to be submarginal for tree growth in the drier regions. Extremely sandy soils easily take in water, but they store little moisture. The moisture in sandy loam soils is more evenly distributed to greater depths than in fine-textured soils, and the trees there develop deeper and more extensive root systems. Less rain is required on the porous sandy loam soil than on the fine-textured soils in which water penetrates slowly. A higher percentage of the water held by the soil is available in the coarser textured ones.

We estimate that not more than 25 percent of the annual precipitation becomes available for plants as effective soil moisture, although that varies with the nature of the soil. Part of the precipitation that falls may be lost by runoff, part is lost by interception in the wooded overstory, and part of what does enter the soil is lost by evaporation.

Fine-textured soils, such as clays, have a slow water-infiltration rate, and the runoff may be higher during high-intensity storms. They can hold more water, but a higher percentage of the water is not available for plant use.

Their slower infiltration rate and greater water-holding ability prevent a given amount of moisture from penetrating as deeply as it would on the coarser soils. The soil-held water that is not available for plant use may vary from 10 to 15 percent in clays to 2 to 4 percent in sandy soils.

J. S. Cole and O. R. Mathews, of the Department of Agriculture, reported that the available water for wheat averages nearly 1 inch for every foot in depth of soil (to 6 feet) for sandy soils, almost 1.25 inches for very fine sandy loam, and 1.44 for silty clay loam.

The amounts of available moisture in the area occupied by roots are too small to maintain a vigorous tree growth. Trees in a relatively young planting sometimes have the advantage of drawing on moisture stored in the soil before planting.

Studies at the Northern Great Plains Field Station, Mandan, N. Dak., disclosed that tree roots do not penetrate a dry layer of soil, even though there might be a supply of moisture just below it.

Only in localities where snowdrifts are deep is the exhausted soil moisture replenished beyond the first 2 feet during the dormant period. Trees and shrubs have to live on current moisture after they have exhausted the stored moisture in the root zone. Only in small, isolated places is the water table near enough to the surface to benefit trees and other crops, even in years of highly favorable moisture.

Moisture is not a factor if irrigation water is available, drainage is good, and annual precipitation exceeds 25 inches in the Northern States and 35 inches in the Southern States.

WHEN MOISTURE is as limited as it is in the Great Plains, soil texture largely determines the depth of rooting of trees and shrubs. Rooting is ordinarily shallower in the fine-textured soils (which have a slower water intake rate) than in the coarse-textured ones. precipitation of a given amount does not penetrate so deeply. Species that normally develop a taproot in their natural environment seldom develop such a root system in the Plains.

A. F. Yeager, of the North Dakota Agricultural Experiment Station, studied the depth of rooting of 31 species of trees and shrubs growing in Fargo clay, most of which are commonly planted for shelterbelt purposes. They were 10 to 43 years old. They had been growing under a mean annual rainfall of 22.34 inches. He discovered that 97.3 percent of the roots of all species were growing in the upper 4 feet of soil. The deepest penetration of roots (by a 16-year-old apple tree) was 10 feet, 3 inches and the shallowest penetration (by a 10-year-old butternut tree) was 2 feet, 8 inches. Only three species had roots in the sixth foot of soil. Roots of cottonwood extended out a distance of 75 feet; those of American elm extended 63 feet; and those of other species spread out 8 to 54 feet.

Dr. Yeager made further studies to compare the rooting habits of trees and shrubs when each was grown on sandy and clay soils that had normal rainfall and in low areas that received runoff from heavy rains and melting snow. Rooting was deeper on both types of soil when they received supplemental water. Roots of golden willow grew 6 feet 8 inches deep on clay soils on the site where rainfall was normal and 14 feet on the wet site.

Observations of the rooting habits of cottonwood trees on river sandbars that were partly eroded away by water action revealed that the roots went straight down for at least 15 feet. Cottonwood is shallow rooted when it grows on dryland sites.

Studies made at the Mandan station in the 1930's of trees that had died as a result of drought showed that the trees and shrubs that grew on a fine sandy loam had a high percentage of the roots in the first 3 feet of soil. A plentiful supply of available moisture was present below the roots in the 7th through the 10th foot.

Later investigations in blocks of trees and shrubs that were planted in soil wet to field capacity in the first 10 feet at the time of planting showed that the roots had completely exhausted all available moisture to a depth of 10 feet, and perhaps beyond, by the 12th growing season. Sampling to determine the moisture content by foot intervals in the first 10 feet was done at the beginning and the end of each growing season. Plantings of trees and shrubs at spacings of 5 by 10, 10 by 10, 10 by 15, and 15 by 15 feet all had exhausted the available moisture at about the same time to depths of 10 feet.

SOME SOILS in the Great Plains contain a high concentration of salts, which usually prevent tree growth. Some species are more tolerant of salts than others, but those that do survive usually are stunted and deformed and have a much shorter life span than those growing on less salty soils. Adequate drainage often helps in this situation.

Studies conducted by the Mandan station in farm shelterbelts disclosed that the roots of trees planted in the salt-affected soils did not penetrate to a greater depth than the depth of planting. Roots of trees planted in these soils spread out horizontally or formed a corkscrew type of growth in the hole.

Observations made by the Southern Great Plains Field Station in November 1940, at Tucumcari, N. Mex., indicated that 60 percent of all the roots of Chinese elm were in the top 3 feet when the planting was made in fine sandy soil. About one-third of the feeder roots were in the 0- to 1 -foot zone.

Plantings of windbreaks in the southern part of the Great Plains in soils that had a definite hardpan at a depth of 36 inches survived from 1931 to 1936, when all available moisture became exhausted. Species included Chinese elm, thornless honeylocust, Scotch pine, and Austrian pine. Similar results were noted during the drought of the early and mid-1950's.

Many planting sites on the High Plains are underlain by caliche at various elevations. Tree roots will not penetrate a caliche of a hard, compact form; but they can penetrate a caliche that has considerable gravel content and utilize stored moisture.

ANYTHING ONE CAN DO to improve soil moisture within the windbreaks and on nearby cropland is helpful. In the northern and central parts of the Great Plains, the most feasible methods for increasing and retaining soil moisture when irrigation water is not available are by trapping drifting snow and practicing good tillage to prevent loss of soil moisture by competitive growth and evaporation.

When shelterbelts and windbreaks are planted for purely protective purposes that do not involve the growing of crops, the plantings should be so designed that they will hold the drifting snow within the trees, preferably through the entire width of the belt. Packed snowdrifts that are in trees in early March yield about 1 inch of water for each 3 to 4 inches of snow. Snowdrifts 4 to 6 feet deep are not uncommon in farmstead shelterbelts at that time of year. Their water potential often is greater than the normal annual precipitation of the area.

The additional soil moisture furnished trees and shrubs by melting snowdrifts among the trees explains at least in part why trees in the northern and central Plains will survive in good condition for longer periods than those of the southern Plains, where effective precipitation is about the same.

Trees and shrubs planted for the protection of fields and crops require fewer rows and less density than those planted for the protection of farmsteads. It is well to let some snow filter through and spread over the fields.

Field shelterbelts, as a rule, do not build up the moisture supply among the trees or on the adjacent field as do the drifts that are held within the farmstead shelterbelt, but they hold this potential moisture on the land, rather than permit it to be blown off fields and into ravines and coulees.