Soil Management for Forest Trees

Frank W. Woods, Otis L. Copeland, Jr., and Carl E. Ostrom.

A rising demand for wood is forcing the Nation to increase the productivity of its existing forest land, develop areas previously considered too poor for commercial timber, and rehabilitate eroded lands.

Forest managers, in their efforts to increase the supply of wood, are striving to regulate soil moisture, check soilborne diseases, improve soil fertility and structure, and prevent erosion.

It is not yet practical to manage forest soils as intensively as land planted with field crops. Less is known about the subject, and the value of the timber crop does not always justify large per-acre investments. The forest manager still has relatively few direct controls over his land; he must rely on manipulation of the plant cover to accomplish many things that the farmer does with harrow, fertilizer, and irrigation.

Nevertheless, advances in technology and the pressures on land are bringing a change.

ENOUGH WATER at the right time that is the tree growers' main problem: Some lowlands and hardpan sites have drainage troubles, but such areas are local and occupy a relatively small total acreage. The total annual precipitation is ample for tree growth in Forested regions, but much of the water comes when trees cannot use it.

A deficiency of soil moisture usually is the most important limitation to the survival of tree seedlings in the field. An effective and practical means for increasing soil moisture for seedlings is to minimize the amount transpired by weed plants. The method of doing so varies with the species of tree that is being grown and with the nature and size of the competing weeds.

Scalping removing, surface vegetation fora radius of 10 to 20 inches from the planting spot is used commonly on poor sites and on good sites where competition is severe. It sets back the weeds long enough for the young seedling to become established. A planting lane, prepared by plowing or harrowing strips, gives greater freedom from competition for a longer time.

On the deep sands of the southern Coastal Plain, all competing vegetation (mainly scrub oak and wiregrass) must be removed before pines can be planted successfully. Two trips over the site with heavy root rakes, harrows, or double drum choppers usually will do the job. (Oaks should be allowed to resprout before the second trip.) Pine survival is good on these bare sites, and a part of the cost of site preparation is offset by the greater ease of planting.

When most of the vegetation is removed or turned under, the hazard of fire is small, at least for a few years. So far there is no indication that erosion, whether by wind or water, is a problem on these deep, coarse sands.

On fine-textured and shallow soils, however, the removal of all vegetation can cause large evaporation losses and damage the site through erosion or in other ways.

A good stand of natural seedlings often is obtained by harrowing or otherwise scarifying the soil immediately before seedfall. Properly timed logging operations can serve the same purpose at practically no extra cost.

Scarifying is not recommended in forest types where root damage results in a high incidence of root rot.

After tree stands are established, soil moisture can still be conserved most readily by removing competing vegetation. The individual weed trees are usually chopped down, girdled, or treated with chemicals, such as 2,4-D and 2,4,5-T. If brush or shrubs are abundant, chemical foliage sprays may be necessary. In the Lake States and elsewhere, airplanes have been effective in applying chemicals over large areas to release conifers suppressed by undesirable broad-leaved trees.

Controlled fire is used often to reduce the competition from understory shrubs temporarily. Woody shrubs nearly always resprout after burning, however, and the number of stems per acre may be increased instead of reduced. The fact that burning often kills desirable trees also limits its use.

Irrigation cannot be recommended as a general forestry practice, but open ditches sometimes are used to supply water to special forest lands, such as those set aside for seed orchards. Such high-value land may justify large investments to insure good seed crops in drought years.

Too MUCH SOIL MOISTURE is a problem in thousands of acres of forests in the Eastern States. These wetland forests are of spruce, fir, and tamarack in the Lake States. They are cypress, swamp hardwoods, and pond pine (Pinus serotina) in the South.

Excessive wetness of the soil or a high water table limits the kinds of trees that will live on an area and usually retards their growth. Sometimes the remedy is simply drainage of the excess water. Near Brunswick, Ga., for example, the removal of excess surface water by ditching has permitted the invasion of fast-growing slash pine into slower growing open stands of cypress.

But more often the best remedy is water control, which implies keeping the water table at the desired level through drainage carefully controlled by gates in the ditches. In the subtropical climate of southern Florida, where the summers are extremely wet and the winters extremely dry, a commercial tree nursery is alternately drained in summer and irrigated in winter.

Typical water table profiles from a series of temporary wells extending from a clearcut area into a selectively cut stand of loblolly pine in eastern North Carolina.

The problem of too much water in some seasons and too little in others is accentuated on hardpan soils in times of abundant rainfall. The pan impedes percolation of water down through the soil and results in a perched water table very close to the surface. Then in dry seasons the water in the shallow upper layers soon is exhausted, and the pan prevents the roots from growing into moister depths. A grassy savannah develops on such sites. Tests have been started in Florida to see if planted pines will succeed on the mounds thrown up by subsoil plows that break through the hardpan. The trees have responded favorably, but results have not been conclusive.

Tree species vary in their tolerance of a high water table. On reservoir areas of the Tennessee Valley Authority, all woody species were killed when the root crowns were periodically flooded for more than half the time during a number of successive growing seasons.

Extensive upland swamp areas, called pocosins, occur in eastern North Carolina. They support stands of pond pine, which grow rather slowly and are hard to protect, manage, and harvest because of the high water level. Studies have shown that these lands, if given proper water control, can be converted to a crop of more valuable loblolly pine (Pinus taeda) with a considerably higher rate of volume growth per acre.

Trees themselves can exert a decided effect on the water table by "pumping" water off into the air through transpiration, particularly in low-lying flatwoods areas.

The drawing illustrates the difference in water table level on cutover land compared with that under a stand of loblolly pine on the Bigwoods Experimental Forest on the Coastal Plain of eastern North Carolina.

The water table level is an important consideration in the culture and harvesting of cypress in the Deep South. A lumber company at Harvey, La., creates levees around cypress areas and pumps in 30 inches of water to permit rafting out the logs with the aid of motorboats. The trees are girdled several months before harvest so that they will dry out and float well.

Cypress seedlings will stand only about 3 days of complete submergence when in leaf. It is possible with careful nursery culture to produce 3-foot cypress seedlings in a season for planting on flooded areas.

FOREST DISEASES are often related to soil conditions.

In the northern Rockies, for example, pole blight of western white pine is most severe on shallow, rocky soils or on soils whose dense hardpan restricts root development. It is seldom serious on deep, well-developed soils that store water readily.

The dieback of sweetgum may be connected with soil conditions.

Several dry years may be the cause of a canker of yellow-poplar in Mississippi. It has been suggested that the fungus associated with the canker is always present, but is inactive until drought lowers the vigor of the trees.

Impeded drainage also causes diseases. Loblolly pine plantations in the Southeast are affected by spot die-out on heavy soils that have poor drainage and aeration. Cultural methods to correct these diseases need much more investigation. The best recommendation we can give is to recognize and avoid hazardous sites for these species.