SECTION III. SIX STEPS TOWARD SUSTAINABILITY

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The commercial exploitation of non-timber resources is currently plagued by destructive harvesting, over-exploitation, and a basic disregard for the functional ecology of tropical plant populations. There are better ways to exploit a tropical forest. From an ecological standpoint, one of the most essential ingredients required to achieve a sustainable level of resource use is information: information about the density and distribution of resources within the forest, information about the population structure and productivity of these resources, and information about the ecological impact of differing harvest levels. An overall strategy for collecting this information, and for applying it in such a manner as to guarantee that the plant populations being exploited will maintain themselves in the forest over time, is presented and discussed in this section.

The different procedures described are sufficiently general that they can be applied to any class of non-timber resource (e.g. fruit and seeds, plant exudates, or vegetative tissue). Furthermore, their application allows flexibility so that management operations can be tailored to suit the specific ecological requirements of a particular site, species, or population. The procedures can be applied in forests that have already being heavily exploited for non-timber resources, as well as in more pristine, undisturbed habitats.

These guidelines do not comprise a single management technology or "package" that can be blindly applied without modification. The basic concept is to provide a constant flow of diagnostic information about the ecological response of the species or resource to varying degrees of exploitation. Sustainability is achieved through a continual process of adjustment in which any change in seedling establishment or population structure results in a corresponding change in harvest levels. The exact nature of this "fine tuning" process will depend on the site, the experience and judgment of local resource managers, the effectiveness of harvest controls, the precision of the diagnostic data collected, and, most importantly, the ecological behavior of the plant populations selected for exploitation.

As is shown in Figure 6, the complete process is composed of six basic operations or steps: (1) Species Selection, (2) Forest Inventory, (3) Yield Studies, (4) Regeneration Surveys, (5) Harvest Assessments, and (6) Harvest Adjustments. Taken together, these operations accomplish three fundamental management tasks. The species or resources to be exploited are first selected. Baseline data about the current density and productivity of these resource are then collected. Finally, the impact of harvesting the resources is monitored and harvest levels are adjusted as necessary to minimize this impact.

The actual sequence of operations is not fixed and can be adapted to a variety of different situations. Existing programs of resource exploitation, for example, have already selected the species to harvest. In such cases, management should start with forest inventory operations. There may also be situations in which a particular tract of land has only recently been made available for resource exploitation (e.g. an extractive reserve or community forest). In these situations, species selection should follow inventory operations so that the results from this fieldwork can be directly input into the selection process. There is no need to interrupt the harvesting and sale of resources during management operations. Data collection and monitoring can be conducted as background activities and a supplement to routine harvesting.

Figure 6. Flow chart of basic strategy for exploiting non-timber tropical forest plant resources on a sustained-yield basis. Complete process is composed of six steps: (1) Species Selection, (2) Forest Inventory, (3) Yield Studies, (4) Regeneration Surveys, (5) Harvest Assessments, and (6) serial Harvest Adjustments. See text for explanation of each management operation.

For ease of presentation, the following discussion focuses on the selection, inventory, and monitoring of a single species or resource. In most field situations, however, management operations will be concerned with the simultaneous exploitation of several different resources. The basic procedures employed are the same regardless of the number of species involved. Numerous species, for example, can be counted and measured in the same forest inventory.

SPECIES SELECTION
The decision of which plant resources to harvest will be based largely on economic concerns. Those resources possessing the highest current market price or the greatest potential for future market expansion will usually be chosen first. Social factors can also come into play here. Some forest resources may have a long history of extraction and traditional use in the region, and local people may have a strong cultural preference towards continuing to exploit these resources. Other resources (e.g. medicinals or plants of ceremonial importance) may be subject to certain taboos that prohibit commercial exploitation.

In addition to economic and social factors, a third set of criteria should also be considered--the overall potential of the resource to be managed on a sustained-yield basis. Some species, because of their reproductive biology, regeneration and growth strategies, or population structure, are inherently more able to withstand the continual perturbations of resource extraction than others. This fact is frequently overlooked.

The basic concept here is quite simple. Given a group of resources with similar economic profiles, why not select those that are the easiest to manage and have the highest potential for sustainable exploitation? Ideally, four basic pieces of information about a species' ecology should be input into the selection process:

• life cycle characteristics

• type of resource produced

• density and abundance in different forest types

• size-class distribution of populations

It would be convenient if all of this information could be easily looked up in a book somewhere. For the great majority of species, it can't. Compile as much information on these topics as possible from published sources, and spend time with local collectors to record their experiences and observations. Estimates of density and population structure will probably require a preliminary field reconnaissance of potential production areas. Try to coordinate these trips with the flowering or fruiting season of the species of interest so that specimens can be collected to document the exact taxonomic identity of the plant.

The life cycle characteristics of a species can either facilitate or severely complicate commercial harvesting. Species that fruit at unpredictable intervals and require specific animals for pollination and seed dispersal probably represent the worst case scenario. An annually fruiting species serviced by more common, generalist pollinators such as small insects or bees is much easier to work with. In terms of regeneration guilds, primary forest species adapted for growth and regeneration under a closed canopy will, in most cases, be preferable over fast-growing pioneer species that require the occurrence of large canopy gaps for seedling establishment.

The type of resource produced by a particular species can also have a major influence on its potential for sustainable exploitation. The harvest of bark, stem tissue, and roots almost always kill the tree. Managing these populations on a sustained-yield basis can be a difficult and expensive proposition. The harvest of latex, fruits, oil seeds and leaves, on the other hand, does not necessarily kill adult trees or drastically alter the size-class distribution of the population. Although the extraction of these resources is certainly not exempt from having an ecological impact (see Section II), these impacts are somewhat easier to avoid or correct.

A third key criteria for resource selection is the current density and distribution of the species. Abundant species which are obviously regenerating in the forest are considerably easier to manage than low-density scattered populations. The forest types within which a species occurs must also be taken into consideration. A resource may be extremely abundant in one forest type and completely absent in others. If this particular type of forest occupies only a very small area, or is inaccessible during certain times of the year (e.g. seasonally flooded forests), resource supply can become a problem.

Even more important than the overall abundance of plant resource is the size-class distribution of the individuals within its populations. A species may be the most abundant in the forest in terms of number of stems, but if all of these stems are of a similar size or if the population is characterized by a preponderance of large adult trees and exhibits no regeneration, the species is having trouble with seedling recruitment. If the establishment of new seedlings is a problem in the absence of harvesting, commercial levels of exploitation will undoubtedly be extremely difficult to maintain on a sustained-yield basis. If at all compatible with the economic and social criteria employed, the selection of species with a Type I population structure (i.e. with abundant natural regeneration) is strongly recommended.

To aid in the selection process, different expressions of important species characteristics are summarized in Table 5. The main categories and subdivisions have been adapted from the topics discussed in Sections I and II. There are admittedly a variety of different combinations and intermediate stages of the three possibilities listed. Some species may use both biotic and abiotic dispersal, different populations of the same species may exhibit a Type I size-class distribution in some habitats and a Type III in others, and some individuals within a single population may fruit annually while others are unpredictable in their phenology. These shortcomings notwithstanding, the information shown in Table 5 provides an ecological framework for comparing different forest resources which have been pre-selected using economic and social criteria.

Table 5 . Overall management potential of different non-timber forest resources based on their botanical characteristics, life strategy, productivity, and population structure. See Section I for further information on categories.

	........................Potential for Sustainable Management.....................
	Low	Medium	High
Resource Group	Bark, stem tissue, roots	Some resins, fruits and seeds	Latex, fruits and leaves
Yield/plant	Low	Medium	High
Species Characteristics:
Flowers	Few, large	Intermediate	Small, many
Fruits	Few, large	Intermediate	Small, many
Seed germination	Low viability	Intermediate	High viability
Sprouting capability	None	Low	High
Population Structure:
Size-class distribution	Type III curve	Type II curve	Type I curve
Tree density/hectare	0-5 adults	5-10 adults	10+ adults
Spatial distribution	Scattered	Clumped	Homogeneous
Regeneration Guild	Early Pioneer	Late Secondary	Primary
Flower/Fruit Phenology	Unpredictable	Supra-annual	Annual
Reproductive Biology:
Pollination	Biotic, with specialized vector	Biotic, with generalist vector	Abiotic
Pollinator Abundance	Rare; bats, hummingbirds	Intermediate; beetles, moths	Common; small insects
Seed Dispersal	Biotic, with specialized vector	Biotic, with generalist vector	Abiotic
Disperser Abundance	Rare; large birds, primates	Intermediate; small mammals	Common; bats, small birds

The easiest way to use the table as a selection tool is to assign a numerical value to each category. All parameters with low, medium, or high management potential, for example, could be recorded as 0, 1, or 2, respectively. Summing the total score for each species provides a rough "sustainability potential" index that can be used to compare and rank different species. All other factors being roughly equal (i.e. economic and social considerations), the species with the highest sustainability index should be selected.

To give an example of how this works, let's assume for the moment that we want to appraise the management potential of two hypothetical species. The first species, a small woody climber, produces bark from which a beautiful yellow dye is obtained. The species occasionally re-sprouts after harvesting. Its flowers are small, sweetly scented, produced at unpredictable intervals, and pollinated by small insects. The fruits are eaten and the seeds subsequently dispersed by birds. The species depends on canopy gaps for successful establishment. Natural populations exhibit a Type III size-class distribution and occur at densities of 3-5 individuals per hectare. The second species is a large, primary forest tree. It produces abundant quantities of fruit every year and sprouts copiously after cutting. The flowers are small and pollinated by wind. The seeds are dispersed by bats, birds, and a number of small ground-foraging mammals. The species occurs naturally in high-density populations of from 10 to 20 adult trees per hectare; the structure of these populations conforms to a Type I size-class distribution. Nothing is known about the seed germination or spatial distribution of either species.

To derive the sustainability index, the ecological data for each species are grouped into categories and then scored using a 0-2 point system and the management potential rankings shown in Table 5. The first species receives high management potential scores because of its flower and fruit characteristics (2 points each) and abundance of pollinators and dispersers (2 points each), and medium scores because of its low sprouting capability (1 point), biotic, generalist pollination and dispersal (1 point each), and late secondary regeneration guild (1 point). Low management potential scores are assigned to the species because of its resource group, yield per plant, Type III size-class distribution, low population density, and unpredictable phenology (0 points). Summing all of the points yields an index value of 12 for the first species.

The second species receives high management potential scores in every category except fruit characteristics and seed dispersal and exhibits a significantly higher sustainability index (26 points). There is little question that this species has the higher potential for being managed on a sustained-yield basis.

FOREST INVENTORY
Density and size-class structure data are the most fundamental pieces of information required for management. Just as foresters need to know how many cubic meters of mahogany (Swietenia spp.) occur in a particular forest, the management of non-timber resources also relies on estimates of the distribution and abundance of different species. These estimates can only be obtained through a quantitative forest inventory. Inventories also provide the baseline data necessary to monitor the impact of harvesting. Without some knowledge of initial density and size-class structure, the population could slowly go extinct with each successive harvest and never be noticed.

Forest inventories are time-consuming, somewhat costly, and extremely tedious to conduct. It is a good idea, therefore, to do a little planning before initiating this fieldwork. In particular, several questions concerning the resources that have been selected for exploitation (or that are already being exploited) need to be addressed:

1. Where exactly does the species/resource of interest occur in greatest abundance? Try to map its distribution as precisely as possible using references to physical or cultural features (e.g. mountains, rivers, villages, etc.), established political boundaries (e.g. which sub-district or province) or geographic coordinates (e.g. from Global Positioning System readings).
   

2. Is the species limited to a certain forest type, or is it more or less evenly distributed throughout the region?
   

3. Is the resource of interest produced by only one species or several species? What is the exact taxonomic identity of these plants?
   

4. In what manner, for how long, and by whom has the resource been exploited? Are some collecting areas more heavily exploited than others? Has the resource been planted, selectively favored, or otherwise managed by local communities?

5. Are there any good maps, aerial photos or satellite images of the region?
   

6. Has the region ever been inventoried before, and if so, for what type of resource (e.g. timber surveys, botanical exploration, mining or geo-chemical reconnaissance)?

Questions 1 to 3 attempt to define the location and identity of the species. The latter part of this can be more difficult than it sounds. Given the high diversity and limited botanical exploration of tropical forests, many important fruit trees, rattans, and medicinal plants have yet to be identified. A specimen collected recently in Peruvian Amazonia of one of the most commonly used trees for house construction turned out to be a species new to science. Without a scientific name, it is very difficult to find the information that may be available about a plant. A whole literature can be overlooked by failing to realize that the rambai trees (see Table 4) in the forest are really Baccaurea motleyana.

Question 4 assesses the resource's history of exploitation. The structure of populations which have been exploited commercially for hundreds of years is usually very different from those which have been subjected to only periodic, subsistence use. If no type of management activity has been conducted, it is very likely that the exploited species occurs at a lower density in the forest. Extensive planting, on the other hand, may have notably increased the local abundance of the resource.

Questions 5 and 6 provide some idea about how difficult or expensive the inventory will be to conduct. Doing inventory work in a region with no maps requires quite a bit more planning than that conducted in more familiar terrain. Needless to say, if a general forest inventory or timber survey has already been conducted in the region--and the resource of interest is a tree--every attempt should be made to get a copy of the results.

As much information as possible should be compiled for each of these six questions. Potentially useful sources include both published and unpublished "gray" literature (e.g. local government documents, internal reports, memoranda, maps, etc.) about the region and species of interest. A review of plant specimens at the nearest herbarium can provide information on the distribution, habitat, and flowering and fruiting phenology of different species. Informal interviews with local collectors are always extremely enlightening. Local export statistics, although notoriously unreliable and covering only a small percentage of the useful flora found in tropical forests, can sometimes provide a good historical overview of the pattern and intensity of resource use.

Armed with this preliminary information, the next step is to actually design and conduct the inventory. It is strongly recommended that a professional forester or inventory specialist be involved at this stage. Although the mechanics of designing such an inventory are beyond the scope of this primer, a few general comments about the nature of the data that need to be collected are warranted:

• The inventory should provide a reasonably precise estimate of the total number of harvestable trees per hectare (i.e. the resource density) in different forest types. For fruit and oil seed species, this means the total number of adult trees. For latex-producing species, medicinal plants and rattans, some juvenile trees may also need to be included.
  

• The inventory should provide data on the current population structure or size-class distribution of adult trees. Collecting these data requires that the diameter (DBH) of all stems be measured. Height measurements can be substituted in the case of herbaceous plants, small understory palms or woody shrubs.
  

• The inventory should provide a preliminary assessment of the regeneration status of the species. Does the species appear to be maintaining itself in the forest? Are there a sufficient number of small trees to replace the inevitable death of adult trees? To begin answering these questions, smaller, non-productive individuals must also be counted and measured in the inventory.
  

• How small should the smallest individuals be that are included in the inventory? This depends on the size of the species and its abundance in the forest. The lower size limit for rattan or a stemless palm will be different from that required for a large canopy tree. For tree species that occur at a relatively low density in the forest, a minimum diameter limit of 10 cm DBH is recommended; more abundant species will probably require a slightly higher diameter cut-off. A smaller minimum diameter increases the amount of information obtained from the inventory, but it also increases the time and expense of fieldwork because of the greater number of stems that have to be counted.

The results from the inventory should be separated into different forest types prior to analysis. The data from each habitat are then compiled into size-class histograms showing the number of individuals in different diameter or height classes (see Figures 3 and 4). For most large canopy trees, grouping the data into 10 cm diameter classes will produce a reasonable depiction of population structure. A 5 cm diameter class interval may be warranted for understory trees; the use of 50 cm height classes is frequently appropriate for shrubs and small palms. As a general rule, the histograms should contain from 8 to 12 size classes. Dividing the overall range in size of the data (i.e. the diameter or height of the largest individual minus that of the smallest one) by 10 provides a quick estimate of an appropriate size-class interval.

The inventory lays the foundation on which a program of sustainable forest use can be developed. We now know how many trees we have to work with, where they are located, and whether they are regenerating or not. Based on the visual analysis of the size-class histograms, we also now have an extremely important point of reference against which to assess the ecological impact of forest exploitation.

YIELD STUDIES
Given an understanding of the density and size-class distribution of a forest species, the next question that needs to be addressed is "How much of the desired resource do natural populations of the species produce?" Suppose 250 kilograms of seed are harvested from a mixed dipterocarp forest. Is this level of harvest sustainable? Well, that depends. How many seeds does the population produce? Is this only 10% of the total population seed production, or were 95% of all seeds removed? It makes a difference. Just as foresters (theoretically) use growth data to avoid cutting timber faster than it is produced in the forest, the sustained-yield management of non-timber resources also requires information about the productive capacity of the species being exploited. This information is obtained through yield studies.

The basic objective here is to obtain a reasonable estimate of the total quantity of resource produced by a species in different habitats or forest types. Given that larger plants are invariably more productive than smaller plants, of particular interest is the relationship between productivity and plant size. A simple yield study designed to collect these data can be conducted in three steps:

1. As it is rarely feasible, or even warranted, to measure the yield from all of the individuals in a population, a representative sample of plants of differing size is first selected from each forest type. Selection should be limited to healthy, undamaged trees; if possible, a minimum of three individuals per size class should be chosen from each habitat. The sample plants should be permanently labeled with paint to facilitate their re-location in the field. In addition to their use in the yield studies, these individuals will later form part of the monitoring system used to assess the impact of harvesting.
   

2. The productivity of each sample plant is then carefully measured. The exact methodology employed to make these measurements will necessarily vary with the type of resource. Counting fruits is different from counting leaves or measuring stem growth (e.g. for rattan), and latex which drips slowly into a cup is different from crystallized resin or "damar" which has to be chipped off the trunk. Probably the easiest way to obtain meaningful production data is to enlist the help of local collectors and train them how to weigh, count, or measure the quantity of resource actually collected from each tree during harvest season. For plant exudates and many vegetative structures, this procedure provides a sufficiently precise estimate of size-specific productivity. In the case of fruits and seeds, these harvest data must be supplemented with visual or, even better, quantitative estimates of the amount of marketable material that was left unharvested.

3. Finally, the data collected from each sample tree are plotted to construct a simple scatterplot or yield curve for each forest type showing the relationship between plant size and yield. An example of this type of graph is presented in Figure 7, which shows the size-specific rate of fruit production for Myrciaria dubia. In this species, there is an exponential increase in fruit production with increasing diameter, the largest individuals producing over 3,300 fruits in some years. Yield curves like this are important because they can be used to predict the quantity of resource produced by any size plant--regardless of whether a plant of that exact diameter or height was actually measured in the yield studies. The values can be read straight off the graph.

Figure 7. Annual fruit production as related to tree size for Myrciaria dubia plants growing in the lowlands of Peruvian Amazonia. Two years of fruit production data are shown.

An additional point of interest in Figure 7 is the variation in fruit production exhibited by M. dubia from year to year. This type of behavior is almost standard practice with tropical plants, and applies to the production of latex,

resins, leaves, and bark as well as to that of fruits and seeds. Temperature, rainfall, sunlight, soil nutrients, pollination, competition, and a multitude of other ecological factors are not constant from one year to the next--and the plants reflect this. To account for this variability, yield studies should be repeated every few years using the same group of sample plants. This task can be greatly simplified by having trained local people collect the data as part of their routine harvest activities.

The results from the inventory provide information about how many productive trees of each size class occur in different forest types. The yield studies will tell us how much of the desired resource each one of these trees produces. Combining these two data sets will generate satisfactory estimates of: (1) how much the entire forest can produce, (2) what size plants are responsible for the largest percentage of this production, and (3) which forest types provide the highest yields.

These estimates are of incalculable practical value. Specific production areas can now be delineated in different forest types, access routes and collection centers can be established, and the costs and benefits of different harvest scenarios can be evaluated in detail. At this point, the resource in question is no longer simply being extracted from the forest--it is now being managed.

REGENERATION SURVEYS
The baseline data collected in the forest inventory and yield studies provide an estimate of the total harvestable yield from the forest. From the discussions presented in Sections I and II, however, it is clear that not all of this material can be harvested from the forest for very long. What we really want to know is the sustainable harvest yield from the forest. How much of this resource can we harvest year after year without damaging the long-term stability of the plant populations being exploited? Answering this question requires information about the ecological impact of differing harvesting levels.

The first signal that a plant population is being subjected to an overly intensive level of harvest is usually manifested in the size-class distribution of that population. For most species and resources, the effects of over-exploitation are most clearly visible in the seedling and small sapling stages. Harvesting may kill a large number of adult trees (e.g. rattan, gharu, or palm hearts), may lower individual tree vigor to the point that flower and fruit production is affected (e.g. leaf or bark harvest, or the tapping of plant exudates), or may remove an excessive number of seeds from the forest. From a population standpoint, the net result of these activities is the same--all reduce the rate at which new seedlings are established in the population. This impact can be detected, and hopefully avoided, by periodically monitoring the density of seedlings and saplings in the populations being exploited.The basic procedure used to conduct these regeneration surveys is as follows:

1. A network of permanent regeneration plots is first established throughout the forest. The exact number of plots that are used will depend on the current abundance of seedlings and saplings in different forest types. High-density populations will require a smaller number of plots; scattered low-density populations will require a more intensive sample. Each plot should be permanently marked and its exact location mapped or described in detail to facilitate re-location. The services of an experienced forester or ecologist would be extremely helpful in laying out this plot network.
   

2. Within each plot, the total number of seedling and saplings of the desired species less than or equal to 10 cm DBH (or whatever the minimum diameter limit used in the original forest inventory) are counted and recorded. For ease of data collection, these plants can simply be tallied into height classes and it is not necessary to actually measure each individual. The use of four 50 centimeter height classes (e.g. 0 - 50 cm, 50 - 100 cm, 100 - 150 cm and 150 - 200 cm) and one 1 - 10 cm DBH diameter class is appropriate for most tree species. A larger class interval (e.g. 100 cm) may be necessary for tree populations exhibiting a reduced number of seedlings and saplings; smaller classes will be required for understory trees and shrubs (e.g. note 2.0 cm basal diameter classes used for M. dubia in Figure 7).

3. The plot results are grouped by forest type and averaged. These summary data are then added to the size-class histograms constructed from the inventory results to provide a complete picture of population structure from seedlings to large adult trees. An example of a composite histogram containing height classes for seedlings and saplings and diameter classes for juveniles and adult trees is shown in Figure 8. The data were collected in West Kalimantan from a natural population of Shorea atrinervosa. The transition between height and diameter classes was defined based on the observation that most S. atrinervosa saplings start to obtain a diameter (DBH) of 1.0 cm at heights slightly greater than 200 cm.
   

4. The regeneration plots are periodically re-inventoried to monitor fluctuations in the number of seedlings and saplings recruited into each population. An interval of every five years is probably sufficient for most species. The occasional observations of collectors who may have passed through the plots during harvesting are also useful for monitoring the smaller size classes.

The final size-structure histograms produced using the regeneration survey data (e.g. Figure 8) are important management tools. In essence, the seedling and sapling densities shown for each populations are a demographic "yardstick" with which to measure the actual long-term impact of harvesting. To use a medical analogy, these data are the vital signs by which to assess the health or infirmity of the population.

Figure 8. Size-class histogram for Shorea atrinervosa population illustrating the use of both height and diameter classes. Data from regeneration plots have been grouped into four 50 cm height classes and one 1.0 - 10.0 cm diameter (DBH) class. Inventory results are divided into eight 10 cm (DBH) diameter classes. Numbers shown along x-axis represent the upper size limit of each class. Note compressed, logarithmic scaling of y-axis due to the large range in values (e.g. from 3 to 250,000).

HARVEST ASSESSMENTS
Harvest assessments are an additional type of monitoring activity used to gauge the ecological impact of resource harvest. These are primarily visual appraisals of the behavior and condition of adult trees that are conducted concurrently with harvest activities. In many cases, these quick assessments can detect a problem with reproduction or growth before it becomes serious enough to actually reduce the rate of seedling recruitment.

The sample plants selected and marked for the yield studies are perfect subjects for these observations. They represent a range of different size classes and they are stratified by forest type. During the regular harvest period, each of these individuals should be carefully inspected and observations recorded on several different aspects of the plant and its immediate environment:

• Does the overall vigor of the plant appear to be good? Are there yellow leaves or obvious wounds caused by harvesting? Is there any evidence of insect pests or fungal pathogens?

• If inspections are made during the period of flowering and fruiting, pay special attention to the number of fallen flowers and immature fruits under the crown of the tree. A simple visual estimate of few, moderate, or excessive is sufficient in most cases. Collect a sample of seeds (e.g. 20 - 50) and review each one for the presence of insect predators. Record the percentage that are predated.
  

• Are the seedlings in the vicinity being trampled or otherwise damaged by collectors?

In addition to these basic observations, the results from the periodic yield studies should also be used to monitor the health of adult trees. Careful comparison of the data collected from each tree over several measurement periods will, in many cases, be sufficient to distinguish between normal variability and an actual reduction in size-specific productivity.

HARVEST ADJUSTMENTS
The monitoring operations are used to appraise the sustainability of current harvest levels (see Figure 6). The seedling and sapling densities recorded in the original regeneration survey represent the threshold values by which sustainability is measured. As long as densities remain above this threshold value--and no major problems are detected in the harvest assessments--there is a high probability that the current level of exploitation can be sustained. If, however, seedling and sapling densities are found to drop below this value, immediate steps should be taken to reduce the intensity of harvest. The effectiveness of this harvest reduction will be verified during the next regeneration survey. Further reductions in harvest levels may be warranted if seedling and sapling densities fail to stabilize, or drop even lower, during the five-year period.

Specific problems encountered during the harvest assessments (e.g. loss of vigor, increased seed predation, or drop in productivity) should result in similar harvest adjustments. If the problem is limited to only one or two individuals, the harvest of these trees should be completely suspended until subsequent assessments indicate that the situation has improved. Physical impacts such as trampling or wounding may require that changes in the pattern, as well as the intensity, of harvesting be implemented.

The general mechanics of this adjustment process are shown graphically in Figure 9. The left side of the figure depicts the initial structure of the population at TIME 0, immediately following the first inventory of the regeneration plots. The right side shows the structure of the population five years later at TIME 1. The threshold values for seedlings and saplings are shown as dotted, horizontal lines. The first four size classes shown in each histogram represent 50 cm height classes, the remainder are 10 cm DBH classes.

Figure 9. Diagrammatic effect of harvest intensity on the size-class structure of a plant population to illustrate the basic strategy behind periodic harvest adjustments. TIME 0 represents size-class distribution immediately following inventory of regeneration plots; TIME 1 shows population structure 5 years later after annual harvesting. A. Incipient stages of over-exploitation with decrease in seedling (size class 1) and sapling (size class 2) numbers below threshold values (shown as dotted, horizontal lines). B. Under-exploitation of resource. C. Sustained-yield harvesting.

The uppermost histogram, Figure 9A, illustrates the incipient stages of resource over-exploitation. By TIME 1, seedling and sapling densities have dropped well below their threshold values. Harvest levels in this population should, as a result, be immediately decreased before the reduction in numbers exhibited by the smaller size classes is passed on to the intermediate and large size classes.

Figure 9B, on the other hand, depicts a hypothetical example of under-exploitation. The population in question has actually increased its level of seedling establishment in response to harvesting, a behavior that suggests that additional quantities of resource could be safely harvested from the forest. That exploitation might, in fact, enhance regeneration is not an entirely unrealistic scenario. Seed collections could reduce the competition and mortality rate among seedlings, or limit food supplies to the point that many frugivores and seed predators were driven from the site. The reduction in canopy cover caused by the harvest of leaves or stems might also improve light conditions in the understory for seedling establishment. Whatever the ultimate cause may be, an increase in seedling and sapling densities over a five-year period is a good indication that the population can withstand a greater intensity of exploitation.

Finally, the histogram shown in Figure 9C illustrates a steady-state or sustainable harvest level. The existing intensity of resource extraction has little effect on the number of seedlings and saplings recruited into the population, and, unless conditions change drastically, this level of exploitation should be able to be maintained almost indefinitely. From an ecological perspective, this situation represents a verifiable example of sustainable resource exploitation.

In actual practice, achieving a sustainable yield in this manner will probably involve a considerable number of harvest adjustments. There is frequently a lag time in a population's response to disturbance, and after several cycles of apparently stable results from the regeneration surveys, the population may exhibit a drastic fluctuation in seedling and sapling densities. The important thing is that these fluctuations do not go unnoticed. By gradually lowering, or even raising in some cases, the intensity of resource extraction, the level of seedling establishment should eventually approximate the threshold value established for the population.

A key variable in all of this is control over the actual intensity or level of exploitation. How do you go about reducing the level of harvest a certain percentage? Two different procedures can be used to make these adjustments:

1. The first method regulates the number or size of the plants being exploited. For example, if a 10% reduction in resource extraction is desired, the baseline data from the inventory and yield studies can be used to determine the exact number of trees from each size class required to achieve this objective. These trees should be marked with bright paint or flagging and left unharvested to regenerate. A different set of "seed" trees can be chosen every two to three years to ensure a more even distribution of regeneration throughout the site.
   

2. The second method limits the total area from which the resources are extracted. Using this procedure, the management area is divided into ten parcels or production units, each parcel containing more or less the same number of adult trees. Harvesting only nine of these parcels would produce roughly a 10% reduction in harvest level. The parcels should be rotated so that a different one is left unharvested each year.

For populations that have never been exploited before, a good first approximation is to extract no more than 80% of the total harvestable yield during the first collection cycle. Harvest levels can later be increased or decreased as necessary based on the results from the first regeneration survey.

A NOTE ABOUT REMEDIAL TREATMENT
The six steps outlined in this section provide a simple and effective method for achieving a sustainable harvest of non-timber forest products. The hallmark of this inherently passive form of management is that the intensity of human intervention is adjusted to account for the ecological dynamics of the plant populations being exploited--instead of the other way around. In some situations, however, a more intensive or active form of resource management may be warranted. If, for example, seedling densities continue to drop in spite of drastic harvest reductions, or productivity declines, or trees start to die, some form of remedial treatment should be initiated as soon as possible. A few potential courses of action are listed below:

• Enrichment planting is a relatively easy method for enhancing the seedling abundance of a valuable species. Dayak communities in West Kalimantan use enrichment planting to maintain commercial densities of durian, rattan, and illipe nut in their managed forests. The açai stands managed by the forest farmers in eastern Amazonia are similarly enriched by planting mango and cacao seedlings. In both cases, the practice has proven quite successful. An additional advantage of enrichment planting is that the planting stock can be specifically selected from the most desirable individuals in the population (e.g. the most high-yielding, the most vigorous, or those with the most tasty fruit), with the result that the overall genetic composition of the population is gradually improved over time.
  

• Selective weeding can be used to increase the survival and growth of young plants or to stimulate the productivity of adult trees. Also known as "cleaning" or "underbrushing", this operation decreases the competition at ground level by removing the seedlings and saplings of undesirable species. Light levels in the understory may also be increased somewhat following treatment. Selective weeding is most commonly applied around the base of especially high-value or productive trees prior to harvest season to facilitate the collection of fruits or seeds.
  

• Cutting and removing woody vines from the crowns of adult trees can frequently increase their productivity. This practice can also have a positive effect on seedling establishment by opening up the canopy a little and allowing more sunlight into the understory.

Depending on the nature and severity of the management problem, these operations can be applied either singly or in combination. Regeneration surveys and yield studies should be conducted annually for several years following any type of treatment to closely monitor changes in population structure and/or yield. A pre-treatment schedule of monitoring activities can be resumed as soon as the population exhibits a return to normal, baseline conditions.

LITERATURE
Several basic texts on forest inventory techniques are available to interested readers:

1. Avery, T.E. 1983. Forest Measurements. 3rd edition. McGraw-Hill, New York.
   

2. D. Alder and T.J. Synnott. 1992. A Manual of Permanent Plot Procedures for Tropical Rainforest. Tropical Forestry Paper No. 25, Commonwealth Forestry Institute.
   

3. FAO. 1981. Manual of Forest Inventory. FAO Forestry Working Paper No. 27, Rome.
   

4. Joint Forest Management Support Program. 1992. Field Methods Manual. Vol. I. Society for the Promotion of Wastelands Development, New Delhi.
   

5. Loetsch, F. and K.E. Haller. 1973. Forest Inventory. Vols. I and II. BLV Verlagsgesellschaft, Munich.
   

6. Philip, M.S. 1983. Measuring Trees and Forests. Forestry Division, Univ. Dar Es. Salaam.

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