The complex inter-relationships and ecological processes described in Section I exist in a very delicate balance. This balance is easily disrupted by human intervention, and land-use practices that at first seem benign can later have a severe effect on the structure and dynamics of forest tree populations. Almost any type of resource exploitation conducted in tropical forests will have an impact. It may not be immediately visible to the untrained eye--but it is definitely occurring.
In general, the overall ecological impact of forest utilization depends on the floristic composition of the forest, the nature and intensity of harvesting, and the particular species or type of resource under exploitation. Sporadic collection of a few fruits may have little effect on the long-term stability of the tree populations being exploited. Intensive annual harvesting of a valuable oil seed, on the other hand, can gradually eliminate the species from the forest. The felling of adult trees can produce a similar result in a much shorter period of time.
In spite of all the discussion surrounding the sustainable use of tropical forests, surprisingly little has been written about the ecological impacts of non-timber resource extraction. The purpose of this section, therefore, is to summarize what is currently know about the topic, and to assess the potential long-term impacts of harvesting different types of plant resources.
Given that the initial impact of resource harvesting is largely determined
by the specific type of plant tissue or compound extracted, the following analysis
employs an ecological, rather than a commodity, approach. From a marketing or
use perspective, fruits, nuts and oil seeds are completely different products.
The act of harvesting these products, however, produces a similar ecological
impact--it removes seeds from the forest, and hence reduces the total number
of seedlings that can potentially be recruited into the plant populations being
exploited.
Using this approach, the enormous variety of non-timber plant resources produced by tropical forests can be grouped into three basic categories: (1) fruits and seeds, (2) plant exudates, and (3) vegetative structures. This classification scheme is illustrated in Table 4. Specific examples of each resource group and type of plant product from Amazonia and Southeast Asia are also included. All species listed are primarily natural forest species, but some of the plants may also be cultivated. The listing in Table 4 is far from exhaustive.
Table 4. Selected examples of non-timber tropical forest products grouped by resource category and plant part. Local nomenclature and use information taken from personal observations.
FRUITS AND SEEDS
To isolate the specific ecological consequences of collecting fruits, nuts,
and oil seeds from a tropical forest, it is important to first separate out
the negative effects of destructive harvesting. In many tropical regions, an
increasingly common practice is to simply cut down a tree to harvest its fruit.
This damaging, short-sighted, and wasteful practice can have a drastic impact
on the distribution and abundance of fruit resources within a forest.
In Peruvian Amazonia, for example, the female trees of the aguaje palm are frequently felled by commercial collectors. After very few of these "harvest" cuts, the forest is left with a preponderance of barren male trees. Over time, the species disappears completely from the forest. Destructive harvesting has also seriously reduced the abundance of the ungurahui palm, the babassu palm, and a wide variety of other important Amazonian fruit trees. No program of commercial fruit extraction will ever be sustainable as long as harvesting involves an ax.
Even in the absence of destructive harvesting, the collection of commercial quantities of fruits and seeds can still have a significant ecological impact. The manifold difficulties experienced by tropical trees during the germination and seedling establishment stages were described in Section I. Periodic fruit harvests can make the process of seedling recruitment even more problematic.
In terms of simple demographics, if a tree population produces 1,000 seeds and 95% of the new seedlings produced from these seeds die during the first year, the population has still recruited 50 new individuals. If, on the other hand, intensive fruit harvesting removes all but 100 of these seeds from the site prior to germination, the maximum number of seedlings that can be recruited into the population is reduced to only 5. This ten-fold shortfall in recruitment can cause a notable change in the structure of the population.
In reality, this example is probably overly optimistic. First, it is assumed that all of the seeds produced are deposited precisely in the appropriate site for germination and early growth (i.e. the regeneration niche of the species). Second, there is always the possibility that the fruits and seeds left in the forest after harvesting will experience a mortality rate higher than 95%. Commercial collectors are, in effect, competitors with forest frugivores, and their activities reduce the total supply of food resources available to ground-foraging animals. In response to decreased fruit densities, frugivores might be forced to increase their foraging to obtain sufficient food. The net result would be an increase in the total percentage of fruits and seeds destroyed.
Rather than increasing their search for a constantly diminishing food source, it is also possible that some frugivores will simply migrate to more isolated tracts of forest in response to commercial fruit collection. This response could have a serious impact on seedling establishment for those species whose seeds require cleaning by animals to germinate. Additionally, some of the frugivores that migrate off-site may have played an important role in seed dispersal. Without a dispersal agent, a higher proportion of the fruits and seeds produced by these species will fall directly under the crown of the parent tree where they are more easily harvested by collectors, more easily encountered by seed predators, and more susceptible to the effects of competition from other seedlings.
All of these factors interact in a synergistic fashion to inhibit the recruitment of new individuals into a plant population. Over time, this lack of recruitment will modify the size-class distribution of the population being harvested. If commercial fruit collection continues uncontrolled, the target species can be eliminated from the forest. This process of gradual population disintegration is illustrated in Figure 4. The size-class intervals depicted are 10.0 cm DBH; note the change of scale in the latter three time periods to compensate for the decrease in total population size.
As is shown at TIME 0, the hypothetical population initially displays the Type I size-class distribution of a shade-tolerant canopy tree with abundant regeneration. After several decades of fruit collection, however, the structure of the population has been notably changed (TIME 1). The infrequency of seedling establishment has caused a reduction in the smaller size classes; the greater number of stems in the intermediate size classes reflects the growth of saplings that were established prior to exploitation. The structure of the population at this point conforms to a Type II distribution. By TIME 2, the population has been even further degraded by the chronic lack of regeneration. There are intermediate size classes that contain no individuals at all, and it appears that the existing level of saplings and poles is insufficient to re-stock these classes. Finally, the size-class histogram shown for TIME 3 represents the culmination of a long process of over-exploitation. The population consists of only large, old adult trees, none of which are regenerating (Type III population structure). In the absence of remedial action, it is only a matter of time before this tree species becomes locally extinct. At no point during this process is there any dramatic visual evidence (e.g. dead or dying trees) that anything is wrong. The forest still contains adult trees of the desired species which continue to produce fruit--and harvesting will probably continue unabated.
Figure 4. Hypothetical example of change in population structure experienced by a forest tree species in response to intensive collection of fruits, nuts, or oil seeds. Size-class intervals are 10 cm DBH; each time period represents approximately 30 years. Note change of scale in the size-class histograms for latter three time periods.
The example shown in Figure 4 represents an extreme case of uncontrolled over-exploitation, and does not necessarily imply that every level of harvesting involving fruits and seeds leads directly to species extinction. Some species and populations will be more susceptible to over-exploitation than others. The key ecological parameters to be considered here have to do with (1) the initial density or size of the population, (2) the actual intensity of fruit or seed collections, (3) the degree to which the plant depends on animals for pollination and dispersal, and (4) the specific regeneration and growth requirements of the species being exploited.
As a general rule, forest species that occur in high densities, exhibit abundant regeneration, and are pollinated by either "generalist" animals (e.g. small insects) or wind are able to tolerate more intensive harvesting. Conversely, low density populations that display a Type II or Type III size distribution and possess an obligate relationship with a specific pollinator or seed disperser will have a much lower sustainable yield of fruit and will be much more prone to over-exploitation. It is worth noting that most of the forest populations of Brazil nut that are currently being so intensively exploited in Amazonia exhibit all of these latter characteristics.
Finally, in addition to its impact on seedling establishment, population structure, and the foraging behavior of local animal populations, harvesting commercial quantities of fruits, nuts, and oil seeds can also affect the genetic composition of the tree population being exploited. In this case, the important question is not so much how many fruits or seeds are harvested, but rather which ones.
Tropical tree populations usually exhibit a high degree of genetic variability.
A single population of forest trees, for example, will usually contain several
individuals that produce large succulent fruits, a great number of individuals
that produce fruits of intermediate size or quality, and a few individuals that
produce fruits that, from a commercial standpoint, are inferior because of their
small size, bitter taste, or poor appearance. If this population is subjected
to intensive fruit collection, the "inferior" trees will undoubtedly
be the ones whose fruits and seeds are left in the forest to regenerate. Over
time, the selective removal of only the best fruit types will result in a population
dominated by trees of marginal economic value. This process, although more subtle
and occurring over a longer period of time, is identical to the "high-grading"
or "creaming" of the best tropical timbers that occurs in many logging
operations.
PLANT EXUDATES
When properly conducted, the tapping of latexes, resins, and gums does not disturb
the forest canopy, kill the exploited tree, or remove its seeds from the site.
In theory, this activity probably comes the closest to conforming to the ideal
of sustainable non-timber forest product extraction. In actual practice, the
exploitation of plant exudates can be very destructive.
In West Malaysia, oleo-resin is collected from Dipterocarpus trees by chopping large holes into the trunk and then building a fire inside to maintain the flow of resin. This sequence of "boxing and firing" is usually repeated several times, and a large tree may be "boxed" at two or three places along the trunk. This process severely weakens the vigor of the tree. Internal resources which might have been allocated to basic ecological functions such as fruit production and growth are spent on resin production and the formation of callous tissue to heal the wounds. The damaged trees are almost certainly not replacing themselves in the forest.
The collection of gharu (Aquilaria spp.) in most parts of Southeast Asia is accomplished by felling the tree. As there are no external signs to indicate whether a tree contains this valuable resin, collectors frequently fell every Aquilaria tree they find. Once a tree containing gharu has been felled, collectors use axes and knives to hack out the blackened heartwood. The uncontrolled exploitation of this exudate, together with the wasteful trial and error method of searching for it, has virtually eliminated Aquilaria trees from all but the most remote and inaccessible forest areas.
The destructive harvest of plant exudates is not limited to either resinous trees or Southeast Asia. Couma macrocarpa is a valuable latex and fruit-producing tree in many parts of Amazonia. The species produces copious amounts of creamy latex which is used in the manufacture of chewing gum; the latex is also occasionally used as an adulterant in Para rubber (Hevea brasiliensis). Although the species can be tapped repeatedly as easily as rubber and exploited every year for its fruits, opportunistic collectors have felled an incalculable number of Couma trees to quickly drain them of their latex.
Rubber, chicle (Manilkara zapota), and jelutong (Dyera costulata) are common examples of plant exudates that are tapped in a non-destructive fashion. It is tempting to assume, therefore, that the exploitation of these resources will be automatically sustainable in the long term. It should be remember, however, that maintaining a continual supply of latex is contingent on these species being able to replace themselves in the forest. There is currently a large number of tappable rubber trees growing in Amazonian forests. These trees will eventually die. Are any provisions being made to insure the recruitment of a second or third generation of Hevea trees?
It is useful in this context to briefly examine the physiology of rubber production by Hevea trees. Rubber latex is manufactured in special cells using stored carbohydrates. In addition to rubber, the latex contains proteins, sugars, tannins, alkaloids, and mineral salts. Although the exact biological function of this rich concoction is unknown, biochemically it is very expensive for the tree to produce. The abundant production of rubber latex by Hevea trees is an abnormal response to injury--a tapped tree produces hundreds of times more latex than it would have formed had it not been tapped. The net result is that commercial tapping regimes cause the tree to divert a considerable proportion of the resources normally used for growth and reproduction to the production of rubber. This diversion of resources can cause a measurable reduction in the growth of rubber trees subjected to commercial tapping regimes.
As is shown in Figure 5, tapping reduced the diameter increment of plantation-grown rubber trees in Southeast Asia by as much as fifty percent over a five year period. In this experiment, the sample trees were growing in an open environment with abundant light, water, and nutrients, and free of competition from other plants. The growth of wild Hevea trees in a forest environment would undoubtedly be even more severely affected.
Figure 5. Diameter growth exhibited by plantation-grown Heveatrees when left untapped or subjected to two tapping intensities.
It is highly possible that this reduction in vegetative growth would also
be accompanied by a reduction in seed output. Several studies have shown that
loss of vigor caused by disease, seed predation, or herbivory can reduce total
fruit production by increasing the rate of fruit abortion. There is no reason
to assume that repeated tapping will not produce a similar response. The physiological
demands of producing a continuing supply of latex are in conflict with the ecological
imperative of producing seeds.
VEGETATIVE STRUCTURES
The final category of non-timber forest resources contains a diverse assemblage
of different plant tissues used for fiber, building materials, medicines, fish
poisons, and foods (see Table 4). The actual plant part exploited may be either
the root, stem, leaf, bark, or apical bud. Although the
origin and use of these products are very different, their harvest produces
a similar ecological impact. The plant species will either be killed during
the collection process, or, in a limited number of cases, it will survive harvesting
and later regenerate the vegetative structure(s) removed.
There are numerous examples of plant resources that are killed or fatally wounded by the harvest of vegetative structures. The current situation with rattan in Southeast Asia provides a particularly useful illustration of the deleterious effects of harvesting commercial quantities of stem fiber.
After locating a suitable specimen in the forest, rattan is harvested by cutting the plant at the base and then pulling the entire spiny stem and leaves out of the forest canopy by repeated strong tugs. Given that a large cane may be over 100 meters long, this is a particularly arduous task. Once on the ground, the spiny leaves and sheath around the stem are removed with a knife and the stem is bundled for transport out of the forest.
The actual impact of harvesting depends on the specific growth form or type of rattan that was cut. Large cane rattans usually possess a single stem that does not re-sprout after cutting. Harvesting kills these individuals. Smaller cane rattans, however, are typically multi-stemmed and can re-sprout after cutting if sufficient time is allowed between harvests. As might be expected given this trait, intensive and uncontrolled harvesting has drastically reduced the abundance of solitary rattans in many localities (e.g. the Philippines, Indonesia and Malaysia). Unfortunately, the rising demand for small-diameter canes has also caused the over-exploitation of many multi-stemmed species, collectors cutting them too young or too close to the ground to permit re-sprouting.
The commercial collection of other types of vegetative structures can also cause an ecological impact, although this impact is frequently less notable than that produced by the extraction of stem tissue. Harvesting leaf fibers may have a negligible effect on the plant population being exploited if: (1) individual plants are not killed in the process, (2) a sufficient number of healthy leaves are left on each plant to photosynthesize, (3) the reproductive structures and apical bud are not damaged, and (4) sufficient time is allowed between successive harvests for the plant to produce new leaves. The periodic collection of leaves from the chambira palm in Peruvian Amazonia, for example, has little effect on the vigor of exploited trees, and the species appears to be maintaining itself well in local forests under current levels of exploitation.
The collection of roots and bark tissue usually kills or fatally weakens the exploited tree species. The impact of this selective mortality can become quite severe under high levels of exploitation. In the early 1930s, for example, a significant export trade in barbasco was developed in many parts of Amazonia. Commercial collectors began digging up, and not replacing, every barbasco plant they could find. The species was quickly depleted in the wild and is now produced almost exclusively in plantations.
Apical buds represent the final category of non-timber forest products harvested from vegetative structures, with palm hearts being the most important and well-known example. In Amazonia, two forest palms are the preferred source of this delicacy, Euterpe oleraceaand Euterpe precatoria. Euterpe oleraceais a slender, multi-stemmed palm that is widely distributed in the seasonally flooded forests of eastern Amazonia; the species forms extensive oligarchic forests along the floodplain of the Amazon estuary. Euterpe precatoriagrows in a similar habitat in western Amazonia, but is a solitary or single-stemmed palm. These differing growth forms play a major role in determining the overall ecological impact of harvesting.
In a single-stemmed palm species, harvesting the "heart" or apical
meristem necessarily kills the tree. This is exactly what happens when palm
hearts are extracted from E. precatoria. The establishment of a palm
heart canning factory in Iquitos, Peru during the mid-1980s was sufficient impetus
to destroy almost every population of this species in a wide radius around Iquitos.
The factory was eventually forced to close due to the scarcity of palms.
The vast stands of E. oleraceafound in eastern Amazonia are also exploited quite heavily for palm hearts. Fortunately, the multi-stemmed growth form possessed by E. oleraceaenables the species to sprout back after cutting, and this ecological factor has greatly facilitated its survival in the region. In an exemplary demonstration of forest management capabilities, local collectors on Onças Island near Belem, Brazil have developed an innovative system for harvesting palm hearts on a sustained-yield basis using weeding and pruning techniques to take advantage of E. oleracea's unique ability to sprout from the stump after cutting.
The initiative of the forest farmers on Onças Island in the Amazon
represent an appropriate point of closure for this section. The exploitation
of almost any type of non-timber forest resource produces a measurable impact
on the structure and dynamics of tropical tree populations. There are typically
two responses to this impact. One is to completely ignore that it is occurring,
the other is to implement appropriate management activities that will minimize
the intensity of this impact. The former course of action, or lack thereof,
inevitably leads to forest degradation and resource depletion; the latter may
ultimately produce a sustainable form of land-use.
The first two sections in this primer have been concerned with the ecology
of tropical plant populations--how these populations are structured, how they
function, and what happens to them when they are subjected to commercial resource
exploitation. The unfortunate conclusion to be drawn from this material is that
much of the current exploitation of non-timber tropical forest products is not
being conducted on a sustainable basis. There are, however, ways to change this.
With this objective in mind, the overall focus of the primer now shifts to issues
of a more applied or practical nature. How can a knowledge of plant population
dynamics be incorporated into a program of sustainable resource exploitation?
What can be done to monitor the ecological impact of harvesting? What types
of information are needed to minimize these impacts while maximizing the long-term
economic returns from forest exploitation? These types of questions are addressed
in Section III.
LITERATURE
A comprehensive treatment of the ecological impacts of harvesting non-timber
tropical forest products has yet to be written. The following articles, however,
will provide the interested reader with a good start on this important topic:
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:
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:
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 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:
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:
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
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:
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:
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:
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:
There are basically three major conclusions to be drawn from this report. The
first concerns the ecology of tropical forest plants. Natural populations of
most species occur at low densities in the forest, most require the services
of animals to pollinate their flowers and disperse their seeds, and most have
a very hard time getting their seedlings established in the understory. Plant
populations with these characteristics exist in a very delicate balance with
their environment.
The second conclusion of importance relates to the ecological impact of harvesting
non-timber resources from these plant populations. Although the exploitation
of some plant parts (e.g. fruits, seeds and latexes) is less damaging than others
(e.g. bark, stems, or roots), almost any form of resource harvest produces an
impact on the structure and function of tropical plant populations. If nothing
is done to mitigate these impacts, continued harvesting will deplete the resource.
This process is accelerated by destructive harvesting.
The final conclusion is a challenge. There are ways to exploit the non-timber
resources produced by tropical plant populations with a minimum of ecological
damage. Doing so, however, requires management. Baseline data about the size-class
structure and yield characteristics of the population must be collected, regeneration
surveys must be conducted, harvest levels must be periodically adjusted, and,
in some cases, remedial treatments such as enrichment planting or weeding must
be initiated. Although quite a bit more involved than simply picking up fruit
or tapping rubber trees, these management procedures will produce a sustainable
form of resource utilization.
There are convincing ecological reasons to implement the management strategies outlined in this report. If practiced on a sustainable basis, the exploitation of non-timber forest products provides a unique way to use species-rich tropical forest for profit and still conserve most of the biological diversity and ecosystem functions (e.g. protect soil fertility, prevent erosion, control run-off, regulate climate) of the forest. No other form of land-use practiced in the tropics has the potential to do this.
The apical bud of a plant is the primary growing point or meristem located at the apex of the stem.
Biotic implies an active role by animals; abiotic means either wind or water.
Babassu palms (Orbignya phalerata) grow along the southern and eastern fringe of Brazilian Amazonia; oligarchic forests of this species occupy almost 29 million hectares. The fruits of babassu provide a variety of subsistence and commercial products. The seeds contain an oil useful for cooking, soap-making and burning. Flour, animal feed, medicines and beverages are produced from the fruit.
The roots of the barbasco plant (Lonchocarpus spp.) contain rotenone, an extremely potent natural insecticide.
Brazil nuts (Bertholletia excelsa) are produced by an enormous forest tree (up to 50 meters tall and over a meter in diameter) which is widely distributed throughout the upland forests of Colombia, Venezuela, Peru, Bolivia and Brazil. The commercial exploitation of this species was initiated over 150 years ago. During the past thirty years, the annual harvest of Brazil nuts has fluctuated between 40,000 and 100,000 metric tons.
Cartographic information is essential for planning and implementing a forest inventory. Especially useful are large-scale maps (scale from 1:10,000 to 1:100,000), soil or geologic survey maps, standard aerial photographs (scale from 1:6,000 to 1:12,000; preferably as overlapping stereopairs), and multi-spectral satellite images. Sketch maps drawn by experienced collectors can also be very helpful.
Chambira (Astrocaryum spp.) is a large, spiny palm found in many regions of Amazonia. Its leaf fibers are widely used for cordage and weaving material. The ubiquitous Amazonian "jikra" (woven sling bag) is made from chambira fibers.
Several species of Dipterocarpus trees produce an oleo-resin or "damar" which has long been valued as a source of varnish, caulking, and, more recently, as a base for perfumes.
In most cases, these fallen flowers will be aborted or unpollinated reproductive structures. A drastic increase in the quantity of fallen flowers encountered beneath a tree could indicate a lack of pollinators or resource limitations.
To illustrate the first method, let's say we start with a productivity or harvest yield of 100,000 fruits per hectare. A 10% reduction in this is equal to 10,000 fruits. How many trees need to be taken out of production to save 10,000 fruits? This result can be calculated from the yield curves for the species. In the case of Myrciaria dubia (Figure 6), the answer is three 12 cm diameter trees, twenty 6 cm diameter trees, or a variety of other tree size-tree number combinations.
The first species is patterned after Fibraurea tinctoria, a common dye-producing plant in Southeast Asia. The roots and stems of this liana are also used medicinally to treat dysentery and eye diseases.
Global Positioning System (GPS) devices receive signals from a special network of satellites and use these readings to calculate the precise geographic coordinates of a given location. This simple, portable, and relatively inexpensive technology has numerous applications in forestry and resource management.
A herbarium is basically a botanical museum containing dried specimens of different plant species. These specimens show the leaves, flowers and/or fruits of the plant, its scientific name, and the date and locality of collection. Supplemental information on local uses, nomenclature, or the ecology of the plant are also frequently included. Careful comparison with these specimens can be used to identify a species.
Life Cycle Characteristics: To give credit where credit is due, tropical ecologists and botanists have amassed an enormous quantity of information on the ecology of certain forest species. Detailed information about the botany, phenology, and reproductive biology of many non-timber forest resources can, in fact, be gleaned from published sources.
Myrciaria dubia is a small shrub commonly found along the banks of rivers and ox-bow lakes in western Amazonia. Its fruits contain one of the highest concentrations of vitamin C (2,000-2,990 mg ascorbic acid/100 g of fruit) of any species known to science. There is a considerable local demand for the fruit which is used to prepare juices, ice creams and liqueurs.
Maintaining the density of a pioneer species within a closed forest environment is more difficult and entails a larger ecological impact than that of a primary forest species. The management of these species also carries with it a subtle motivation to fell trees and increase the areal extent of canopy gaps within the forest. Given the present objective, i.e. selecting species that can be exploited in primary forest with minimal ecological impact, the selection of pioneer species is usually not recommended.
Rattans are climbing, spiny palms that occur in the mixed dipterocarp forests of Southeast Asia. The largest concentration of species occurs in Peninsular Malaysia and Borneo where at least 104 and 151 species, respectively, have been identified to date. The stem fibers of about 20 of these palm species are widely sought after as a source of cane for manufacturing furniture, woven mats, baskets and other types of wickerwork.
Given the abundance and scattered distribution of the individuals to be measured, the regeneration plots used in tropical forestry are usually of small size (e.g. 25 to 100 square meters) and of either a circular or square configuration.
The second species is based on Brosimum alicastrum (see Figure 2). In addition to its edible fruits, the seeds of this species are exceptionally rich in protein, the leaves provide a palatable forage, and several parts of the tree are used medicinally.
Shorea atrinervosa is a widely distributed canopy tree in northern Borneo and Sumatra. It is a common component of mixed dipterocarp forest and can obtain high densities in selected habitats. The seeds produced by the tree, a type of illipe nut, contain an edible oil.
The ungurahui palm (Jessenia bataua) forms dense populations in seasonal swamp forest; it is common throughout Amazonia. Its fruits are used to make a nutritious beverage and are also the source of a high-quality edible oil.
