SECTION II. IMPACT OF HARVESTING

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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.

Resource Category	Plant Part	..........................................Example................................................
		South America	Southeast Asia
Fruits and Seeds	Fruits Nut.Seed Oil seed	Aquaje (Mauritia flexuosa) Caiimito (Pouteria spp.) Uvos (Spondias mombin) Almendro (Caryocar spp.) Castana (Bertolletia excelsa) Ungurahui (Jessenia bataua) Babassu (Orbignya phalerata)	Durian (Durio ziberthinus) Rambai (Baccaurea mothleyana) Rambutan (Nephelium lappaceum) Petai (Parkia speciosa) Melinjau (Gnetum gnemom) Tengkawang (Shorea spp.) Kemiri (Aleurites moluccana)
Plant Exudates	Latex Resin Floral nectar	Shiringa (Hevea brasiliensis) Balata (Monilkara bidentata) Copal (Protium, Docryodes)	Gutta percha (Palaquium spp.) Jelutong (Dyera costulata) Damar (Dipterocarpus spp.) Gharu (Aquilaria spp.) Aren (Arenga pinnata)
Veetative Structures	Stem Leaf Root Bark Apical bud	Pona (Socratea, Iriartea) Chambira (Astrocaryum spp.) Barbasco (Lonchocarpus spp.) Chuchuhuasa (Maytenus spp.) Açai (Euterpe oleracea)	Rattan (Calamus, Korthalsia) Pandan (Pandanus spp.) Tuba (Derris spp.) Medang (Litsea spp.) Aren (Arenga pinnata)

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:

1. Conelly, W.T. 1985. Copal and rattan collecting in the Philippines. Economic Botany 39:39-46.
   

2. Kahn, F. 1988. Ecology of economically important palms in Peruvian Amazonia. Advances in Economic Botany :42-49.
   

3. Peluso, N.L. 1983. Networking in the commons: a tragedy for rattan? Indonesia 35:95-100.
   

4. Peters, C.M. 1990. Plenty of fruit but no free lunch. Garden 14:8-13.
   

5. Siebert, S.F. and J.M. Belsky. 1985. Forest product trade in a lowland Filipino village. Economic Botany 39:522-533.
   

6. Vazquez, R. and A.H. Gentry. 1989. Use and mis-use of forest harvested fruits in the Iquitos area. Conservation Biology 3:350-361.

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