Nitrogen Fixing Trees for Acid Soils - A Field Manual (Winrock, 1996, 110 p.) Selecting nitrogen fixing trees for acid soils (introduction...) 1. Collect information on the planting site 2. Determine what tree products and services are required 3. Make preliminary species selections 4. Check available research results to verify species selection 5. Determine If the selected species are native or naturalized to the planting site 6. Conduct field trials to determine how well exotic species will adapt to the site

Nitrogen Fixing Trees for Acid Soils - A Field Manual (Winrock, 1996, 110 p.)

Selecting nitrogen fixing trees for acid soils

Mark H. Powell and John C. Weber

The establishment of an agroforestry system in a site with acid soil begins with the identification of nitrogen fixing trees that will grow well under prevailing soil conditions. They should also provide farmers with the products and services they need. The selection process includes field visits, interviews with farmers, and careful research. The following steps are a guide:

1. Collect information on the climate, soils, and other characteristics of the planting site.
2. Determine what tree products and services are required.
3. Make preliminary species selections.
4. Check available research results to verify species selection.
5. Determine if the selected species are native or naturalized to the planting site.
6. If exotic species are selected, conduct field trials to determine how well they will adapt to the site.

Each of these steps will be described in more detail.

1. Collect information on the planting site

The first research priority is to collect as much information as possible on the climate, soils, and other characteristics of the planting site. A minimum set of information on climate and other site characteristics should include:

· Elevation
· Average annual rainfall
· Distribution of rainfall
· Average annual temperature
· Maximum and minimum temperatures
· Incidence of frost
· Number of months with less than 50 mm precipitation
· Topography, including degree and aspect (direction) of slope

Minimum soil information should include texture, depth, drainage, and pH. Information on soil phosphorus, aluminum, calcium, potassium, magnesium, and manganese would also be useful.

2. Determine what tree products and services are required

Careful research is necessary to determine what products and services farmers expect from the tree component of an agroforestry system. This is often best done through informal interviews with farmers and a study of local markets for tree products. In addition to their potential usefulness, tree species must also be acceptable within local cultures. Table 1 lists a number of nitrogen fixing trees that are tolerant of acid soils and gives some of their uses in agroforestry systems.

Table 1. Agroforestry uses of nitrogen fixing trees known to tolerate acid soils. The uses are living fence (LF), windbreak (WB), alley cropping (AC), fodder bank (FB), pasture improvement (Pl), home garden (HG), shade for perennial crops (SP), fuelwood and timber production (WT), improved fallow (IF), and ornamental (OR). "X" denotes that the species has actually been used for the purpose designated; "P" denotes that the species has potential for such a use.

Table 2a. Nitrogen fixing tree and shrub species that are tolerant of acid soils, grouped by adaptation to climate zones based on mean annual rainfall and mean annual temperature: species suitable for high-rainfall areas.

Table 2b. Nitrogen fixing tree and shrub species that are tolerant of acid soils, grouped by adaptation to climate zones based on mean annual rainfall and mean annual temperature: species suitable for medium and low-rainfall areas.

3. Make preliminary species selections

Basic information on climate, soil, and intended uses can provide guidelines for a preliminary selection of nitrogen fixing tree species for a particular site. Table 2 lists tree and shrub species that tolerate acid soils (pH 5.5 or less) according to their suitability to various climatic zones, defined in terms of mean annual temperature and rainfall. Species reported to grow well in soils with greater than 60 percent aluminum saturation are marked with asterisks.

Farmers may have enough experience with native species in the region to recommend trees that potentially fix nitrogen and are adapted to acid soils. This indigenous knowledge can be documented in informal interviews. Farmers should be encouraged to participate in species selection as fully as possible.

4. Check available research results to verify species selection

Once species are selected, collect as much information on them as possible to verify that they are the best choices available for the site. For example, the Nitrogen Fixing Tree Association (NFTA) publishes NFT Highlights short factsheets that provide a convenient reference for species selection. Table 2 provides information on which NFT Highlights are available in English, Spanish, Indonesian, Chinese, and Vietnamese. Highlights of particularly appropriate species are reprinted in Appendix A. The International Center for Research in Agroforestry (ICRAF) also provides information on tree and shrub species suitable for agroforestry through a computerized Multipurpose Tree Database and other information services.

5. Determine If the selected species are native or naturalized to the planting site

After preliminary species selection, an examination of the proposed planting site will determine whether the selected species are already native or naturalized to the site. If the species are already present, their physical appearance may give an indication of how well they are adapted to site conditions. Appearances may be deceptive, however. For example, poor specimens may reflect some aspect of past management such as repeated harvesting of the best-performing individuals. Farmers may also be able to provide information on past management and species performance. If the species identified through a preliminary selection process are not present at the planting site, then simple field trials should be conducted to help determine how they will perform.

6. Conduct field trials to determine how well exotic species will adapt to the site

Before introducing new tree or shrub species into an area, it is important to conduct simple screening trials. Such field testing is well within the capacity of rural development workers (extension agents, staff of non-governmental organizations, or Peace Corps Volunteers) or local community organizations (farmers', women's, or church groups). In A guide to establishing research and demonstration plantings with nitrogen fixing tree species, Macklin and colleagues (1989) recommend a simple low-input design for species screening in situations where land, labor, and technical expertise may be limited. Trials normally cover a fairly large number [5-10] of species planted in unreplicated plots. Replicated trials, however, provide more precise information and should be used if possible. The Oxford Forestry Institute, Winrock International, ICRAF, and other organizations publish field manuals for the design and establishment of such trials.

Selecting the trial site. Soil and climatic conditions at the trial site should be similar to those of the target planting area. If the planting area includes sites with a variety of growing conditions, then separate trials should be conducted for each type of site. Each individual trial should be established on a site that is as homogeneous as possible, so that differences in species
performance are not confused with differences in soil fertility, water availability, or other factors.

Designing the layout. Each plot within a trial consists of trees of one species planted inside one or more border rows of the same species. Trees in the border may be affected by conditions outside the plot, so their growth is not measured. Within the plot, trees should be spaced at intervals of 1 x 1 m or 2 x 2 m. The closer spacing is appropriate for trials that will be maintained for only a year or two. To generate useful results, a minimum of 16 trees should be measured. In a square plot of six rows with six trees in each row, there would be 20 trees in border rows and 16 trees to be measured (Figure 1). Larger plots with more trees are recommended for trials that will last longer than two years.

Figure 1. Sample design for an unreplicated screening trial with 12 species. Each block (plot) represents a different species. Within the plot trees should be spaced at intervals of 1 x 1m or 2 x 2m. The trees to be measured are in the black area. Border trees are in the light-gray area.

Selecting the trees to be tested. In addition to nitrogen fixing trees, it is important to include some indigenous trees and non-nitrogen fixing species in screening trials, especially any species that are already prominent in local forestry programs. A comparison with trees that are indigenous or have already been introduced provides a useful basis for evaluating new species.

For each species in a trial, include more than one provenance (seed source), if possible. Try to include at least two provenances from areas with acid soils because these are more likely to tolerate acid-soil conditions than provenances of the same species from other areas. For example, provenance trials of Gliricidia septum have shown that the Retalhuleu, Guatemala provenance is more tolerant of acid soils than other G. septum provenances. If a species screening trial were only to include less tolerant provenances of G. septum, on observer might conclude - incorrectly - that the species as a whole was not tolerant of acid soils.

If possible, seedlots from each provenance should comprise equal numbers of seed from at least 30 trees, all separated from each other by a distance of at least 100 meters. Wherever possible, seed should be included from trees that have already been naturalized in the area: such trees may have undergone some adaptation and selection that will make them more suitable than trees of the same species growing in a different location.

In general, begin with the best-adapted provenances available - for example, Retalhuleu provenance of G. septum. This approach will give you the best chance to maximize tree survival rates and performance, and it will save time and money.

Collecting data. Measure tree survival, height, and stem diameter at 6,12, and 24 months after planting out seedling-the precise schedule will depend on local growing conditions. Calculate mean height, diameter, and number of stems for each species after each measurement. Basal diameters are taken during the first year or until diameter at breast height can be taken.

Some species may be valued for products, such as fruits, in addition to their woody biomass. If possible, the output of these additional products should be assessed during the first two years. Data collection should include a visual evaluation of growth, phenology, and symptoms of any pests, pathogens, or nutrient deficiencies.

Reporting trial results. To make your trial results useful to people planting trees on other sites, provide a minimum set of information on the trial site, soil, and seed provenances. Information on the soil should include aluminum (A1), phosphorus (P), calcium (Ca), potassium (K), magnesium (Mg), and manganese (Mn) levels, if available. Information on seed sources should include location, soil pH, number of trees collected from, and date of collection.

Caution!

You may wish to assess the performance of some nitrogen fixing tree species that are new to your area. Remember that many introduced tree species have the potential to become weedy in new environments. They may invade pastures and agricultural land and crowd out the native vegetation. Any tree, particularly a thorny or non-fodder species, that begins to exhibit weediness (prolific seed production or rapid, heavy seedling establishment) should be watched carefully. Such species should be considered for eradication, especially if they have no clear potential for use.

Some species can also be hosts to pests and diseases. For example, in parts of the South Pacific, Erythrina species are hosts to a fruit-piercing moth (Othreis fullonia Clerck). The larvae of this species feed on Erythrina, and the adults cause series damage to important commercial fruit crops.