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PLANTATIONS BY AFFORESTATION

Bruce Zobel
North Carolina State University

Commonly, two terms are used relative to establishing plantations of forest trees. The term reforestation involves planting trees to replace those in areas that have been recently harvested. The other term used is afforestation, which involves planting trees on areas such as brush fields, grasslands, severely burned stands and forest stands where useful trees are not now present. This includes areas where plantations have been removed at a considerable time in the past and no care has been given so there have developed stands of vegetation in a “wild” condition. Afforestation relates to a conversion of land from other uses to forest plantations.

Generally, afforestation on such areas is difficult and each has special, and sometimes unique, problems.

In the past, and also into the future, much forest tree planting will be on areas currently not under forest management. This is the situation in areas over much of the world where abandoned farmland, grasslands such as in the llanos in Venezuela and Brazil, scrub trees and brushland in Brazil, the United States, Canada, Asia, South and Central America are now being planted. Huge regions of burned lands, such as in Indonesia, the US and elsewhere are having trees established on them.

A knowledge of species, and provenance within species is essential if afforestation is to be successful. Also, the environment and growing conditions must be known before afforestation is done.

Species can vary a great deal with source and environment under which they have developed. The environment where planting is to be done must be suitable for the species chosen. To know this, soil qualities must be right. For example, if the soil is basic with a high pH (of 8, or more), most pine species, eucalypts, and many other species should not be used. If the soil is acid (pH is lower than 6) one should grow species such as pines, acacias and most conifers. In the mid-range of pH about 7, most species will grow. Additionally, the chemical constituents of the soil must be known. If, for example, there is a high aluminum or low boron content in the soil, many species will neither survive nor grow well.

The environment and growing conditions must be known before afforestation is done.

The general environment of the area must be known. Averages of temperature, rainfall or other mean little. It is the extremes that are limiting in the establishment or growth of a species; often only one or two nights of low temperatures or 2 weeks of hot and dry weather will destroy a species, even though the average conditions over the year may appear to be satisfactory. Not knowing, or ignoring the importance of extremes, is the most common reason for failures of plantations. A good example is radiata pine (Monterey pine) and the best eucalypts which cannot withstand a short period of freezing weather. Some high elevation sources of spruce, fir and pine cannot grow if there is a period of high temperature if the area planted is at lower elevations. In some instances, a species like northern sources of loblolly pine will not grow well unless there is considerable difference between night and day temperatures while the southern sources do not require such differences. Tolerance to long dry (usually also hot) periods can be fatal to many species; for example, Eucalyptus grandis contains some clones which will die, or never return to normal growth, while other clones will apparently become dormant and then continue growth when rains come after an extended drought. Low elevation species from warm climates will not grow in high elevations. A prime example of this is radiata pine which is a marvelous species in its suitable environment but will freeze when planted where severe cold, snow or ice occurs.

Proper soil nutrients are critical, even when there is a suitable pH. The chemical constituents of the soil must be known; for example, if the aoil has high aluminum or low boron content, many species will never survive nor grow well. Several books have been written with details relative to soil properties including clays vs sandy reas, wet vs dry soils and other.

Tolerance to insects and diseases follow the same pattern. For example, loblolly pine along the Gulf Coast of the Southern US often is seriously damaged by fusiform rust when the environment is suitable for growth of the rust. Yet, it has been possible to develop certain clones of his pine that are not seriously attacked by the disease even when grown in the same environment.

One frequently hears that a species is resistant to diseases, insects r adverse environments. In truth, resistance is rarely found but different degrees of tolerance will be present. One needs to know if there is sufficient tolerance to pests or adverse environments before one uses a species in afforestation.

In afforestation, sometimes things that appear to be logical are incorrect. For example, if an area originally had mahogany growing on it, but then had been heavily cut or farmed, many people feel the same species, mahogany, should be established on it when a forest planting is done. Often this can result in disaster because the soil conditions and general environment of the sites to be planted have been so changed by man that they are no longer suitable for good growth of mahogany.

Proper preparation of the site and aftercare of the plantations are essential if there is to be success in afforestation planting.

In addition to having the correct species on the correct site, afforestation planting is only successful if the site is properly prepared for the planting operation. This is also true for later control of competing vegetation which will be evident after planting and can prevent the planted trees from developing correctly. Such care is true for all species, but especially so for some hardwoods like the eucalypts. If the early growth of a eucalypt planting is restricted by competition, it will never grow correctly even if freed from the competition. As one example, eucalypts cannot stand competition from grass; if this is ignored, survival and growth of any living eucalypt trees will be poor.

The better the site, the more competitive situations will be present. On the most suitable sites in the tropics it is necessary to remove the competing vegetation as much as 3 or 4 times the first year and a couple times the next year, if growth and tree form are to be satisfactory. In some instances, such as in part of Chile and Colombia, boron must be added and in the coastal area of the southern US, phosphorous is necessary for suitable growth of slash pine. Fertilization requirements are difficult to determine and costly; in some instances, use of the wrong fertilizer results in slower growth.

Needs for successful afforestation.

It is unfortunate that only a little research is being done relative to afforestation forestry; the bulk of the support is for reforestation. Reasons for lack of research support for afforestation are obvious because of the variable conditions involved and because many of the plantings are relatively small and their worth has not been proven. It is commonly believed that reforestation is more economic because it requires less site preparation, species to be used are known and the management methods such as fertilization and competition control have been developed.

Why are some of us so concerned about doing a better job of afforestation? It is clear that as time passes more afforestation will be needed. Some of the best sites now in forest tree plantations will be converted to agriculture. Forestry will be moved to environments that are more severe or marginal and that are often poorly understood. This is especially true on the vast acreage of denuded and super-severe sites now being afforested for environmental reasons rather than strictly for wood production. There are many horrible examples of areas where forestry was not considered important and the land has become totally unproductive for wood or environmental purposes such as water conservation. New varieties and species will need to be developed to grow on the new sites. Action is now needed if forestry will maintains its place as an important activity. Without good afforestation, forestry loses its importance and can become a disaster.

Submitted: Auggust 2006

Posted: 21 April 2007; V1