Soil and the Growth of Forests

Earl L. Stone, Jr., and Paul E. Lemmon.

In the soil, trees get nutrients essential for growth, the great quantities of water their leaves evaporate, and anchorage for their roots. The kind of soil strongly influences the character and growth of forests.

Trees may be said to blend with the soil, rather than grow on it. Their great permanent root systems extend widely in and over the mineral soil, and usually as deeply as aeration or obstructions will allow. Fallen leaves and twigs are incorporated into the mineral soil or accumulate above it as a distinctive organic layer. The chemical makeup of these additions brings about effects that are characteristic of the tree species and affect productivity in time.

The environment, or site, in which a forest grows includes all of the permanent or recurring factors of soil, physiography, climate, and biota. Without careful study it is often impossible to know the particular factors responsible for present forest conditions or how far management may be able to change matters. A simple example is found in the jack pine forests, some of which occur on soils too sandy and dry for better trees. But jack pine also dominates more fertile soils temporarily after other species have been displaced by fire. These two kinds of jack pine forests require quite different management. The better we understand such relationships the more effectively we can utilize our soil resources for wood production, water yield, watershed protection, grazing, and wildlife. Ordinarily, extensive forest management can only adjust land use to soil limitations and capabilities. Intensive management includes the possibility of modifying soil characteristics, for example, by drainage or fertilization, in the interest of economic production.

Adequate seed production is the first step in regenerating, or producing, a new forest. Tree seeds also are a major food source for many kinds of wildlife. Many important tree species tend to produce heavy seed crops only at irregular intervals, however, with lesser amounts or none in intervening years. This periodicity creates difficulties in restocking cut over areas before weeds or unwanted species predominate, and it obviously affects wildlife numbers.

The factors controlling seed formation are imperfectly known, but trees low in vigor are poor producers. Flowering and fruiting often make heavy demands on nutrient stores within the tree, and it seems plain that soil factors, particularly fertility, affect fruitfulness. Confirmation is provided by R. F. Chandler's observations of sugar maple and beech on a soil low in available nitrogen in southern New York. On plots heavily fertilized with nitrogen, a calculated seed production index of the larger trees was 0.85 to 1.00, whereas that for adjacent untreated check plots varied between 0.03 and 0.37. Similarly, G. H. Schubert, of the California Forest and Range Experiment Station, found that large sugar pines fertilized with ammonium phosphate bore three times as many cones as the unfertilized controls.

Other soil characteristics affect seed production. Studies by H. H. Chapman with longleaf pine in Louisiana demonstrated that trees on a dry site more favorable for growth produced more than twice the number of seed than comparable trees on the nearby poorly drained soil.

THE GERMINATION of seeds and the establishment of seedlings in a competitive environment are critical stages in the perpetuation of most forests. Germination requires suitable conditions of oxygen, temperature, moisture, and sometimes light. The specific needs differ somewhat according to species. These requirements continue as the new seedlings establish themselves and make increasing demands for soil nutrients and light.

A single soil provides unequal opportunities to the different species of the locality because of their differences in season of seedfall, requirements for germination, and susceptibility to hazards of the environment. Longleaf pine, for example, germinates in late fall and the mild winter months when soil moisture usually is ample. Loblolly pine, however, germinates in spring and is exposed to greater hazards when dry weather occurs in the germination period or soon thereafter.

Destructive fires reduce the selective influence of soils on forest composition and result in establishment of hardy species having seedling or sprouting habits adapted to such catastrophes.

In most American forests the variety of trees is so great that soil features alone seldom prevent perpetuation of some sort of forest after cutting or fire. The number and kind of desirable species in the new forest, however, are often fixed by the combination of soil, seedbed, and weather following seed fall. Successful regeneration of economically valuable trees is the outcome of seed production, germination, and establishment, and in the managed forest is usually considered a tribute to the forester's skill or good fortune.

SOIL TEMPERATURE affects the germination of seeds, establishment of seedlings, and growth of trees. Most tree seeds seem to germinate best between 68 and 75 F. W. R. Adams, at the University of Vermont, observed a tenfold increase in the germination of seeds of white pine when he increased the soil temperature from 57 to 76 . G. A. Pearson's early studies in Arizona disclosed that ponderosa pine germinated slowly until soil temperatures measured in late afternoon exceeded 65 . Douglas-fir, bristle-cone pine, and Engelmann spruce, species that normally grow at higher altitudes, germinated completely at soil temperatures of 5 to 7 lower.

Mr. Pearson also noted that roots of blue spruce grew vigorously when soil temperatures reached 49 or 50 in the afternoon. Douglas-fir roots began growth at 50 to 52 . Ponderosa pine roots, however, began growth only after 4 days with soil temperatures of 52 to 54 . From these and other studies, he concluded that low soil temperatures set the upper limits of elevations at which regeneration of Ponderosa pine occurs.

Other investigators learned that the rate or extent of root growth increases with temperature up to some maxi-mum. W. W. Barney, at Duke University, for example, observed that the daily root elongation of 2-week-old loblolly pine seedlings was greater by 1 millimeter for each increase of 7 to 9 in temperature up to 68 to 77 . A rise in temperature beyond that point caused a reduction in the rate of elongation. Heavily shaded soils, cool northerly slopes, and "cold" soils that warm slowly because of excessive moisture therefore may sometimes retard both germination and development. Such delay may affect the composition of a stand by reducing the number of less cold-tolerant species.

High surface temperatures cause serious mortality to germinating seeds and seedlings on bare soil in the open. As long as soil remains moist, its high conductivity and heat capacity prevent an excessive rise in temperature. Dry surface soils in full sunlight, however, often reach 120 to 125 , which is the critical range for seedling injury, and may exceed 150 .

Studies in Idaho demonstrated that burned-over mineral surfaces reached slightly higher temperatures than unburned soil, but that the maximum value of 161 occurred on dry organic layers or duff. In those circumstances, only hardy species or protected individuals survive the first summer. In places where such injury is common, protected slopes, surface moisture, and even sparse vegetative cover become significant in survival of seedlings and the ultimate composition of forests.

Germination is inhibited on excessively acid or alkaline soils and by high concentrations of soluble salts. Such conditions are infrequent in most forest regions, however. Extreme acidity from oxidation of iron pyrite occasionally prevents both natural and artificial reforestation of spoil banks of strip mines.

Similarly, temporary alkalinity and high salt concentrations near heavy ash deposits hinder seedling development. Salt injury occasionally occurs in nurseries and plantations following high or irregular rates of application of commercial fertilizer. Saline soils along seacoasts and at the and margins of the forest sometimes create special problems for tree growth.

The soil reaction and other chemical properties usually affect the development, rather than germination, of seedlings. For instance, many conifers will germinate over the wide range of pH 2.0 to pH 11.0. Seedlings in general develop best between pH 4.5 and 6.0, although species differ widely in their adaptability to soil reaction. Trees sometimes occur naturally on soils that have a reaction higher or lower than the range considered favorable to them in nursery production.

These chemical factors exert major influences on seedling development and growth by making nutrients less or more available, by upsetting the nutrient balance within the plant, and by influencing the growth of disease-causing organisms. Factors, as yet unexplained, prevent the growth of some conifers and sensitive hardwoods on calcareous soils, but other species flourish under the same conditions. Several species are adversely affected even at reactions near pH 7.0. Perhaps too much attention has been given to soil reaction itself and too little to its associations with nutrient supply and microbial activity.

SOIL FERTILITY usually is thought to become important only after the initial establishment period. Small-seeded species, however, have little reserves and depend almost immediately on photosynthesis and on the inorganic nutrients they take up from the soil.

Differences in fertility among soils probably are important but are noticed only when the supply of one or more nutrients becomes critically low. Like light and temperature, fertility differences can affect establishment through their influence on the rapidity of development early in the season.

Pathogens and beneficial mycorhizal fungi are affected by soil moisture, temperature, reaction, fertility, and management.

Damping-off fungi kill many seedlings under natural conditions and can be catastrophic in forest nurseries. L. F. Roth and A. J. Riker studied the life history of two common damping-off fungi in sandy nursery soils in Wisconsin. They found that variation in soil reaction between pH 5.5 and 7.0 had little influence on the total damping-off of red pine, although Pythium was the more common above pH 6.0. In another study, Rhizoctonia was found to cause its greatest injury at pH 4.5. Temperature usually was important in determining severity of injury, but soil moisture determined which of the two fungal species predominated.

The use of acid soils and surface acidification have been the principal means of combating losses due to damping-off in nurseries. These methods are still useful although their limitations are now better understood. Information about associations between soil types and severity of seedling diseases is often useful in planning direct seeding and locating and managing forest-tree nurseries.