Overstory #59 - Choosing Species for Timber Production and Multiple Benefits

Introduction

Timber products can be a sustainable and high-value product from small farms, agroforests, and small-scale forestry projects. Like any other business venture or investment, careful planning can make a difference in terms of economic success or failure. This edition of The Overstory introduces the basic steps to choose species that match the needs, goals, and site conditions of a project prior to planting.

What species should we plant?

The most common question asked by those beginning a project with timber trees is: "What species should we plant?" There is no precise formula for choosing "the right" species. Instead, choosing species is a process reliant on personal knowledge, judgments, and experience, informed by literature reviews, the advice of other growers and resource professionals, and other information (Turnbull 1986). Knowledge of project goals, commercial requirements, planting site conditions, and the range of potentially suitable species is essential in the species selection process.

 The steps involved in choosing suitable species for a particular site and purpose are (after MacDicken 1994):

1. What products and services are desired? Determine the end-use requirements/products and set production objectives.
2. What type of environment does the planting site have? Describe the planting site using the best available sources.
3. What is known of similar environments? Review local experiences and relevant literature with other trees and crops.
4. What timber trees grow well under these conditions? Review local experiences, relevant literature, species trials from similar conditions.
5. What land-use practices are found under similar conditions? Review literature, observe local practices.
6. Select candidate list of species based on all available data. Assemble and analyze information on end uses, environment and species requirements.
7. Narrow down species list. Conduct test plantings on intended planting sites. Plant trees from a range of seed sources in either formal or informal experiments.

Defining desired end products

As with any other business venture, planting timber trees for commercial returns must begin with the end in mind: what products are desired? The desired timber products should be defined as accurately as possible, for example, "high-value native hardwoods for furniture," "small dimension saw timber for the local sawmill," or "finely-figured woods for local crafts market."

Because of the long-term nature and unpredictability of the timber market, growers may want to keep several options open for end-product use. Additionally, some trees may be used for multiple timber products, for example large logs for saw timber and smaller remnants for crafts. If so, each potential end-product should be defined, and species that offer the greatest flexibility in end-uses may be selected.

Different characteristics of trees will result in different timber products. An example of these for several types of wood is summarized below (after Evans 1992):

End Product: Solid wood products

Tree Characteristics: Large size, moderate to rapid growth, good form, ease of pruning
Property Requirements: Strength, stability, uniformity, good seasoning, working and finishing

End Product: Sheet products

Tree Characteristics: Very large size, good natural pruning, rapid occlusion (few knots)
Property Requirements: Figure, peeling or slicing quality, good bonding strength

End Product: Wood chips, pulp and paper

Tree Characteristics: Rapid growth, straight stems, early culmination, easy to grow, coppicing desirable
Property Requirements: Fiber length, light color, low extractives

End Product: Posts and poles (roundwood)

Tree Characteristics: Straight stems, strong apical dominance, few or thin branches, preferably self-pruning (without knots), little taper from top to bottom, bark should strip easily.

Property Requirements: Durable in contact with the ground or in water, capable of taking high cross-loads (high strength to diameter ratios for a given length), resistant to termites and wood-borers (Turnbull 1986).

New technologies for processing, wood preservation, and treatments may extend potential uses and market opportunities for species that do not meet specific criteria (Lemmens et al. 1995). Nevertheless, it is recommended to specify in advance the kind of wood products desired in order to ensure that the species will be able to meet the commercial objectives of the project.

Defining desired services

The desired services or multiple uses of the trees must also be clearly defined in advance for appropriate species selection. Suitable species must meet certain criteria, as well as be planned and managed appropriately, if they are to provide timber and serve a useful function in the farm system.

Some of the desirable characteristics for trees in certain agroforestry practices are outlined below:

Silvopastoral systems (trees and livestock)

• • Tolerant of livestock impacts (resistant to damage to root systems, bark, etc.)
  • Not poisonous/toxic to livestock
  • Have large crowns above livestock reach
  • Canopy allows acceptable light penetration for pasture forage

Windbreaks

• • Wind firm root systems
  • Bushy deep crown that allows some wind penetration
  • Wind strong, pliable branches (not brittle or easily breakable)
  • Delayed shedding of lower limbs
  • Branching to the ground (if single-species windbreak is used)
  • Salt-tolerant (if near coastal area)
  • Rapid growth (if early protection is required)
  • Long life

Crop shade trees (for understory intercropping)

• • Trees not excessively fast-growing or competitive with crops
  • No allelopathic effects on crops
  • Canopy allows acceptable light penetration to crops
  • Trees not damaged by cultivation or harvest of surrounding crops

Land rehabilitation or woodlots on marginal land

• • Tolerant of drought, poor soils, and neglect
  • Fast-growing
  • Deep-rooted
  • Nitrogen fixing and/or produces litter with high nutrient content (for soil improvement)

Defining the site characteristics

A description of the planting environment is essential in selecting species that will grow well on the site. The description of site characteristics should include elevation, slope, rainfall regime, maximum length of dry season, soil analysis, wind and temperature information, and so forth. Any other information that can be obtained about the site will also be valuable, including a survey of existing vegetation, a history of land use, etc.. The process of evaluating the project site can be as comprehensive as desired. Growers should allow sufficient time for this process—several months to a year is not excessive. The time consumed by a detailed site evaluation is rewarded by more informed planning decisions.

Determining similar environments

The more comprehensive the understanding of the project site, the easier it will be to limit species choices to the most promising species from similar environments. In some cases, for example in the Pacific Islands, the many varied climates, soils, and management capabilities can make the task of matching species even to similar local environments complicated. Sites separated by only a few miles away may have a very different soil type, wind direction, or rainfall pattern than the planting site being evaluated. Assistance may be available locally from extension agents and other resource professionals to help growers locate similar environments in the region. The successful species and practices from these environments can then be evaluated for their suitability for the project.

When considering exotic species, the most common method is to attempt use species that are successful in similar environments and latitudes elsewhere. This technique involves comparing the climate of the planting area with other equivalent climatic areas around the world. Then species can be evaluated based on their performance in species trials with similar conditions, or based on their performance in their native range (Turnbull 1986).

For certain species some of this work has already been done, and information is exchanged via data banks and other information on species (see Resources).

What timber trees might be suited to the conditions?

After determining the desired end-uses/services and the environmental conditions, review information about timber species. Local experiences, relevant literature, traditional practices, databases, and species trials from similar conditions can all be valuable sources of information. Species that potentially could perform well on the site and that meet the the end-use and management objectives of the project create a preliminary comprehensive list of potential species. This initial list may then be narrowed down by eliminating species based on practical considerations such as availability.

Understanding genetic variability

There is tremendous genetic variation within a single species. As a result, there are dramatic differences in growth rates and performance from different seed sources from a single species. These genetic variations can affect productivity as much as variation among species (Wadsworth 1997). For this reason, it is wise to test several different selections of a species to determine which is optimal for the project.

Testing and trials

The long-term nature of timber production usually precludes the possibility of testing a new species for a full rotation, from planting through to harvest and market. The most reliable information comes from species trials conducted in formal, designed experiments that examine all phases of growth (MacDicken 1994). However, since this is usually not feasible, even one or two years of informal species trials is invaluable. No amount of research and advice can substitute for test plantings on the intended planting site. The trial period typically involve planting a number of species, and when possible a range of seed sources (varieties and provenances) within each species.

The performance of these species can then be observed. Some early indicators for timber production include the following (after Wadsworth 1997):

• Early height growth is a good indicator of adaption of species to a site.
• Uniform growth of individual trees denotes favorable conditions.
• Self-pruning is a sign of a favorable site.
• Susceptibility to insects and diseases is minimal on sites to which trees are well-adapted.

The time and expense devoted to testing candidate species should be viewed as an investment. As with any other business venture, starting on a small scale and observing the results will aid in successful expansion to a larger scale planting. It is also an important form of risk-management, preventing growers from staking scarce time, land, and resources on species that do not perform as expected. One or two years of species trials will serve to eliminate more species from the list, and narrow the candidates down to those with the greatest possibility for success on the site.

References

Dawson, I. and J. Were. 1997. "Collecting germplasm from trees--some guidelines," Agroforestry Today, Vol 9, No 2, ICRAF House, United Nations Avenue, Gigiri, PO Box 30677, Nairobi, Kenya.

Evans, J. 1992. Plantation Forestry in the Tropics: tree planting for industrial, social, environmental, and agroforestry purposes. Clarendon Press, Oxford, UK.

Lemmens, R.H.M.J, I. Soerianegara, and W.C. Wong, eds. 1995. Plant Resources of South-East Asia (PROSEA) No. 5(2). Timber Trees: Minor Commercial Timbers. Backhuys Publishers, Leiden, The Netherlands.

MacDicken, K.G. 1994. Selection and Management of Nitrogen-Fixing Trees. Winrock International, Morrilton, Arkansas, USA.

Thaman, R.R. and W.A. Whistler. 1996. A Review of the Uses and Status of Trees and Forests in Land-Use Systems in Samoa, Tonga, Kiribati and Tuvalu. South Pacific Forestry Development Programme, Suva, Fiji.

Turnbull, J.W., ed. 1986. Multipurpose Australian Trees and Shrubs. Australian Centre for International Agricultural Research, Canberra, Australia.

Wadsworth, F.H. 1997. Forest Production for Tropical America. USDA Forest Service Agriculture Handbook 710, Washington, DC, USA.

Resources

There are many species references that can aid in the selection process. Here are some of our favorites:

Agroforestree database: a tree species reference and selection guide Authors: AS Salim, AJ Simons, A Waruhiu, C Orwa, C Anyango, 1998 Publisher: International Centre for Research in Agroforestry (ICRAF), Nairobi, Kenya Comments: A selection guide for agroforestry trees covering more than 300 species. Valuable for field workers and researchers who are engaged in activities involving trees suitable for agroforestry systems and technologies.

Plant Resources of South-East Asia (PROSEA) Handbooks Authors: various Publisher: PROSEA Foundation Comments: A valuable series on plant resources for Southeast Asia which is useful to all tropical regions. Order through: PROSEA Network Office, c/o Research and Development Centre for Biology (RDCB-LIPI), Jalan Ir. H. Juanda 22, P.O.Box 234, Bogor 16122, Indonesia. Tel: +62-251-322859, 370934; Fax: +62-251-370934.

FACT Sheets (formerly NFT Highlights) Authors: various Publisher: Winrock International Comments: For a concise summary of information about a multipurpose tree or shrub species, see the appropriate FACT Sheet at or order hardcopies from Winrock International. Many available in Spanish, French, Indonesian, Chinese, Vietnamese, and Khmer.

Related Editions to The Overstory

• The Overstory #61--Effects of Trees on Soils
• The Overstory #54--The Agroforester's Library, Part Two--Species References
• The Overstory #49--Traditional Pacific Island Agroforestry Systems
• The Overstory #43--Essentials of Good Planting Stock
• The Overstory #31--Tree Domestication
• The Overstory #19--Selected Tree Seed