THE ECOLOGY OF TROPICAL TREES AND FORESTS:
A CRASH COURSE

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In a very basic sense, tropical forest trees are really no different than any other type of plant. Their seeds germinate, they grow, they flower, they produce fruit, and eventually they die. What makes tropical trees unique--and the object of so much scientific inquiry and public concern--is the amazingly complex and diverse ways in which they accomplish these routine life functions. Put a large number of these trees together, each species germinating, growing, flowering, and fruiting in a different way, at different times and with varying degrees of success, and you have some idea about why we understand so little about the ecology of tropical forests. What we do know at this point, however, is that most tropical forests exhibit several ecological characteristics that make sustainable harvesting more elusive than it might first appear.

TREE DENSITY AND ABUNDANCE
One of the most fundamental and well-known characteristics of tropical forests is the large number of species that grows in them. To illustrate this point specifically for trees, floristic data collected from small tracts of tropical forest in Amazonia and Southeast Asia are presented in Table 1. Although there is quite a bit of variability in the estimates from different sites, it is clear that the tropical forests in both regions are extremely diverse and may contain from 100 to almost 300 different tree species in a single hectare. To put this in perspective, a mixed hardwood forest in the northeastern United States contains about 17 tree species per hectare (one hectare=10,000m² or 2.47 acres).

Table 1. Total number of tree species larger than 10.0 cm in diameter (DBH) within small plots of tropical forest in Amazonia and Southeast Asia.

Site	Soils/Habitat	Area	No. Species
Amazonia
Yanamono, Peru	Alluvial sediment	1.0 ha.	283
Mishana, Peru	White sand	1.0 ha.	275
Rio Xingu, Brazil	Upland forest	1.0 ha.	162
Breves, Brazil	Upland forest	1.0 ha.	157
Para, Brazil	Flooded forest	1.0 ha.	103
Southeast Asia
Gunung Mulu, Sarawak	Alluvial sediment	1.0 ha.	225
Gunung Mulu, Sarawak	Red-yellow clay	1.0 ha.	215
Wanariset, Kalimantan	Upland forest	1.0 ha.	180
Andalau, Brunei	Upland forest	1.0 ha.	140
Raya-Pasi, Kalimantan	Upland forest	1.0 ha.	135

From a management standpoint, the high species diversity exhibited by tropical forests is a two-edged sword. On the positive side, the large number of different plant species present frequently implies that there is an equally large assortment of useful plant resources available. It has been estimated, for example, that one out of every six species found in the lowland forests of Southeast Asia produces an edible fruit, nut, oil seed, medicine, latex, gum or other non-timber forest resource. In most of the tropical areas where this has been studied, "resource richness" is a direct consequence of high species diversity.

Unfortunately, an additional consequence of high species diversity is that the individuals of a given species usually occur at extremely low densities. There is a limit to the total number of trees per hectare that can be packed into a tropical forest. If you have a large number of species, each species can only be represented by a few individuals.

This trend of high species diversity coupled with low species density has been repeatedly documented in tropical forest inventories. The results from two such inventories, one conducted in Brazilian Amazonia and the other from Pasoh Reserve on the Malay Peninsula, are shown graphically in Figure 1. As the histogram clearly documents, the great majority of the tree species at both sites are represented by only one or two individuals. Less than ten percent of the species had populations with more than four trees per hectare.

Figure 1. Densities of different tree species within 3.0 hectare tracts of tropical forest in Amazonia and Southeast Asia.

The basic problem here is straightforward. Tropical forests contain a lot of different species. Although many of these species produce valuable non-timber products, most of the trees of interest are scattered throughout the forest at very low densities. Low density resources are difficult for collectors to locate, require long travel times, produce a low yield per unit area, and are extremely susceptible to over-harvesting.

It is important to note that there are numerous exceptions to the rule of high species diversity in tropical forests. Although the fact is seldom mentioned in much of the current literature, dense aggregations of a single tree species are known to occur in habitats where severe flooding, shallow soils, or frequent disturbance preclude the formation of species-rich forest. These oligarchic forests (Gr. oligo= few, archic= dominated or ruled by) have been reported from almost every region of the wet tropics. The extensive stands of aguaje palm found in the lowlands of Amazonia are notable examples of this type of forest.

In terms of tree density and yield, oligarchic forests rival many of the commercial plantations that have been established in the tropics. Given their wide distribution, productivity, and relative ease of harvest, these unique plant communities would seem to have great potential for supporting a program of sustainable resource exploitation.

FLOWERING, FRUITING, AND REPRODUCTIVE DYNAMICS
The different ways in which tropical trees produce their flowers and fruits can be a major stumbling block to resource exploitation. This unavoidable fact of life is perhaps most obvious in those cases where the actual resource of interest is a fruit or a seed, and where the seasonality and magnitude of fruit production have a direct impact on harvesting. It should be remembered, however, that fruits contain the seeds necessary for a species to regenerate and maintain itself in the forest. Irrespective of the species or type of resource being exploited, plant reproduction is a key issue in sustainability.

PHENOLOGY OF FLOWERING AND FRUITING
Phenology refers to the timing or seasonality of specific biological events (e.g. leaf fall, growth or, as in this case, the production of flowers and fruits.) Detailed studies of flowering and fruiting phenology have been conducted in almost every area of the tropics. The results from these studies have shown that tropical trees vary greatly in terms of the timing, duration and intensity of flowering and fruiting. Different species may produce flowers annually, supra-annually, i.e. with an interval of several years between flowering events, or even several times a year. The periodicity of fruit production exhibits similarly complex patterns. Very few tropical forest species produce reliable fruit crops during a well-defined, predictable season each year.

One of the most interesting examples of supra-annual flowering and fruiting is exhibited by the Dipterocarpaceae, a large family of dominant canopy trees in Southeast Asia. At irregular intervals of from two to ten years, numerous dipterocarp species will more or less simultaneously start to flower within the forest. This mass floweringphenomena is usually followed by an extremely abundant level of fruit production known as mast fruiting.In an especially intense mast year, almost every dipterocarp and up to 80% of all canopy trees may burst into flower.

The exploitation and management of tropical forests would be considerably easier if the production of flowers and fruits occurred on a more predictable basis. In fact, it is hard to imagine a more difficult management situation than one in which the key species produce fruit at irregular intervals of from two to ten years. This, however, is exactly the framework within which the illipe nut trade in northwestern Borneo is forced to operate.

POLLINATION
The low density and scattered distribution of individuals in most tropical tree populations represents a dilemma for pollination. How do you move pollen effectively from the flowers of one tree to another when the distance between these individuals may be in excess of 100 meters? A brief look at Table 2, which lists the principal pollinators of twelve commercially important tropical trees, suggests the answer to this question.

Table 2. Pollinators of a selected number of tropical plant resources. Animals listed represent principal pollinators; the flowers of each species may also be visited by other animals.

Species	Common Name	Use	Pollinator
Shorea spp.	Illipe nut	oil seed	thrips
Hevea brasiliensis	Rubber	latex	thrips/midges
Theobroma cacao	Cacao	oil seed	midges
Mangifera indica	Mango	edible fruit	flies
Artocarpus heterophyllus	Jackfruit	edible fruit	flies/beetles
Orbignya martiana	Babassu	oil seed	beetles
Bactris gasipaes	Peach palm	edible fruit	beetles
Bertholettia excelsa	Brazil nut	edible seed	bees
Euterpe oleracea	Açai	edible fruit	bees
Ceiba pentandra	Kapok	fiber	bats
Durio zibethinus	Durian	edible fruit	bats
Parkia speciosa	Petai	edible fruit	bats

Tropical trees have evolved relationships with a variety of animals, ranging from tiny thrips and midges to bees and large bats, to shuttle pollen between trees. These relationships can be quite specific, with one type of insect being solely responsible for pollinating the flowers of a particular forest species. It is important to emphasize that these plant-animal interactions are not simply isolated anecdotes from tropical forest folklore. The large majority of tropical trees rely exclusively on animals to transfer their pollen. One study conducted within a small tract of lowland forest in Costa Rica found that 139 (96.4%) of the 143 tree species surveyed were pollinated by animals.

There is an important lesson to be gained from these findings: no pollinators, no fruits; no fruits, no seedlings; no fruits or seedlings, no products or profit. Any serious program of commercial resource exploitation conducted in tropical forests must necessarily include measures to insure adequate pollination. In some cases, this requires a greater appreciation of the fact that land-use practices far away from the immediate harvest site can be extremely disruptive to populations of wide-ranging animals such as bats or even bees. The current situation with the nectivorous bat, Eonycteris spelaea,provides a dramatic, yet unfortunate, example of this.

Eonycterisbats are apparently the exclusive pollinators of durian trees in Peninsular Malaysia. These bats, however, feed preferentially on the flowers of Sonneratia alba,a coastal mangrove which occurs in dense groves and produces a few large flowers continually throughout the year. In order to forage on this reliable food source, the bats fly 20 to 40 kilometers from their roost each night. During this journey to the coast, any durian trees that they may encounter are pollinated almost as a dietary afterthought. Any attempt to maintain a viable population of these important pollinators in Malaysia must inevitably address the fact that the principal food source of Eonycteris spelaeais currently being decimated by coastal development.

SEED DISPERSAL
The importance of animals in the reproductive biology of tropical trees does not end after pollination. Once fruits and seeds have been successfully nurtured to maturity, the next problem faced by flowering plants is what to do with these offspring. Given the incredible variety of different fruit types produced by tropical trees, many of them extremely rich in protein, starch, or sugar and quite "costly" for the plant to produce, it is clear that these fruits have not evolved to simply drop to the ground beneath the parent tree. It seems more likely that these fruits have been specifically designed to be eaten--to be eaten, and the intact seeds within the fruit effectively dispersed to a new location.

Seed dispersal offers at least three ecological benefits to a plant. A dispersed seed has a greater probability of escaping the excessive crowding and mortality that invariably occurs under the crown of the parent tree. Dispersal may also allow a seed to spread its species into new habitats. Finally, some types of dispersal may position a seed in the precise site required for successful germination and growth.

These benefits are not mutually exclusive, and all three may occur depending on the plant species, the dispersal agent and the immediate environment. There is, however, an ecological cost associated with seed dispersal, especially when animals are involved. During the process of handling, transporting, or feeding on fruits, frugivores may destroy a large proportion of the seeds inside.

The large number of tropical trees with animal-dispersed fruits suggests that the actual costs of using these dispersal agents are greatly outweighed by the potential benefits. Research conducted in the tropical forests of Central and South America indicate that over 90% of the canopy trees produce fruit adapted for consumption, and subsequent dispersal, by animals; related studies in Southeast Asia have produced similar results. Clearly, bats, birds, primates, peccaries, fish and a wide assortment of other vertebrates are responsible for moving an enormous quantity of seeds around in a tropical forest. These animals may either remove fruits directly from the tree, or they may forage on fruits which have already fallen to the ground.

The presence of fruit-eating animals in a tropical forest can be a problem for commercial collectors. These animals damage or consume large quantities of fruit, and their activities quickly become a nuisance when the species being eaten is an economically important one. In those cases where collectors and frugivores are actively competing for the harvest of the same species, the animals invariably get there first. It is not surprising that a common solution to this problem has been simply to eliminate the animals from the forest.

It is worth remembering, however, that forest frugivores play an important role in dispersing the seeds of many commercial tree species. The seeds of some species, in fact, will not even germinate without first being cleaned by animals. The distribution and abundance of the seedlings produced by a forest species are frequently controlled by the action of dispersal agents, and, like it or not, the great majority of these dispersers are animals. Failing to conserve viable populations of these animals would be a serious management error.

REGENERATION AND GROWTH
Safe arrival from the parent tree to the forest floor does not, by any means, guarantee that a seed will germinate and become established in the understory. The seed must avoid being eaten, it must encounter the appropriate light, moisture, and nutrient conditions for germination, and it must be able to germinate and grow faster than the seeds of all the other species that are trying to establish themselves on the same spot.

There is a high probability that a seed will come in contact with an animal during the lapse between dispersal and germination. In most cases, this encounter proves fatal for the seed. In terms of total numbers, seed predation is unquestionably one of the most severe sources of mortality during the life of a plant. Over 98% of the seeds of some forest species are lost to predation; rodents, beetles, ants, and weevils are the most frequently cited seed destroyers.

Even assuming that a seed has successfully germinated and a new seedling has put down roots and unfurled its first new leaves, there is still very little chance that the plant will become established on that site. The first year of life for a seedling is plagued with problems. To begin with, light levels in the forest understory are usually so low (1 to 2% that of full sun) that it is difficult for the seedling to grow. Added to this is a very high probability that it will be browsed on by an animal, outcompeted by its neighbors, smashed by a falling branch, attacked by fungal pathogens, ripped out of the soil by a rolling rock, or wilted by wildly fluctuating moisture levels.

A graphic example of the severe mortality experienced by tropical tree seedlings during their first year is provided by the three survivorship curves shown in Figure 2. Brosimum alicastrumis a widely distributed canopy tree in Central and South America; Shorea curtisiiand Shorea multifloraare both common components of mixed dipterocarp forest in Southeast Asia.

Figure 2. Seedling survivorship curves for Shorea curtisii,S. multifloraandBrosimum alicastrum.All three species are primary forest trees.

As is illustrated in this figure, seedling survivorship by these three species after 12 months ranges from a high of 22% for S. curtisiito a low of only 3% for B. alicastrum.Five months after germination over 50% of the seedlings of each species have already died. Taking into account seed predation and lack of germination, less than 0.1% of the seeds produced by B. alicastrumbecome established seedlings. Only a very small fraction of these seedlings (approximately 1 in 1.5 million) will ever make it to the canopy and produce fruit. Data such as these, which are by no means atypical of tropical trees growing in primary forest, provide a convincing demonstration of how difficult it is for a tree population to maintain itself in the forest--even in the absence of any type of resource harvesting.

CANOPY GAPS
The excessive seedling mortality that characterizes the regeneration of many forest species raises an important management question. Where are the few seedlings located that actually survive? What type of site provides the necessary conditions for seed germination, and also allows the new seedling to express an optimal rate of growth relative to its neighbors? The specific combination of environmental conditions which describe such a site may be thought of as the regeneration nicheof a species. To a large extent, the area and distribution of these niches are what regulate the number of seedlings that become established in the forest.

Extensive research on seedling establishment has shown that the regeneration niches of a large percentage of tropical trees have one feature in common--they are all in some way related to the sporadic occurrence of treefalls. Tropical trees may blow down, get struck by lightning, or simply die from old age and fall down. Each of these events produces an opening or "gap" in the forest canopy that allows direct sunlight to enter the understory. In addition to the increased sunlight, canopy gaps also exhibit lower humidity, higher temperatures, and higher soil moisture levels than those found under a closed canopy.

These abrupt changes in the understory environment have a notable effect on most of the seedlings in the vicinity of a canopy gap. In smaller gaps, many of the more shade-tolerant seedlings which have managed to survive under a closed canopy will display a significant increase in growth in response to the higher light levels. Numerous canopy species exhibit this behavior. Their seeds germinate in the shade and the young seedlings grow until they have produced about two or three leaves. The seedlings then appear to enter a state of suppression in which they exhibit little or no height growth. There are only two possible outcomes to this physiological condition. The seedlings slowly die over time, or they are "released" by the occurrence of a canopy gap overhead.

In larger gaps, the drastic increase in light level and soil temperature may trigger the germination and growth of light-demanding species. The seedlings of these species grow extremely fast under sunny conditions, and large gaps are soon swamped with these aggressive, "weedy" plants. Any shade-tolerant species which may occupy the site are soon outcompeted. Classic examples of light-demanding species include Ochroma lagopus(balsa wood) and Cecropiain the American tropics, and many of the over 250 species of Macarangafound in Southeast Asia.

Canopy gaps play a critical role in the establishment and growth of tropical trees; up to 75% of the canopy trees in some areas require the occurrence of a treefall for seedling establishment. The problem is that there is absolutely no way to predict exactly when and where a canopy gap will be created. There is also no guarantee that the desired species will actually colonize a gap should it occur.

REGENERATION GUILDS
Tropical trees have evolved an incredible variety of different life strategies to pollinate their flowers, to disperse their seeds, and to enhance the establishment of new seedlings into their populations. It has long been noted, however, that there are certain similarities and patterns in these strategies which allow tropical species to be grouped into distinct ecological categories. Light tolerance (i.e. shade-tolerant or light-demanding), for example, is one of the most frequent ecological characteristics used to group species. The reason for grouping species is not to obscure the inherent diversity of different strategies found within a tropical forest. Rather, these categories provide a useful tool for more rapidly understanding the basic ecological requirements of a forest species.

For the purpose of this report, it is useful to define three groups or "guilds" of forest species based on their regeneration characteristics, growth rate and life span. These three regeneration guilds are: (1) primary, "climax", or mature forest species, (2) early pioneer or "secondary" species, and (3) late secondary species. In spite of the names applied to these different groups, all three types can occur in mature tropical forest. A schematic listing of the basic ecological characteristics of each guild is presented in Table 3.

Table 3. Basic ecological characteristics of early pioneer, late secondary, and primary tropical forest species

Character	Early Pioneer	Late Secondary	Primary
Distributiion	very wide	very wide	usually restricted: many endemics
Seed dormancy	well-developed	slight to moderate	none
Seed or fruit size	small	small to intermediate	large
Seed dispersal	birds, bats, wind	mainly wind, but also mammals	mammals, birds
Shade tolerance	very intolerant	intolerant	seedlings very tolerant, later intolerant
Gap size required	large	intermediate	small
Seedling abundance	very scarce	usually scarce	abundant
Growth rate	very fast	fast	slow to very slow
Wood density	light	light to medium	very hard
Life span	10 to 25 years	40 to 100 years, sometimes more	100+ years

Primary tree species germinate in the shade and can survive in the understory for a considerable length of time until a canopy gap opens overhead. Their seeds are usually large and few, with abundant seed reserves and little or no dormancy. These species, as a group, are highly shade-tolerant and possess a photosynthetic system adapted for growth under very shady conditions. Growth is relatively slow and wood density, as a result, can be extremely high. Primary trees may live for several hundred years and attain heights of over 60 meters. Many valuable tropical hardwoods fall into this guild, together with several important fruit and oil-seed trees. Many understory palms and herbs are also primary species.

Early pioneers frequently persist for long periods in the soil as dormant seeds. Their seeds, which are small and produced in abundance, require the stimulus of a large gap opening (i.e. increased soil temperature or light intensity) to germinate. After germination, these species exhibit high maximum rates of photosynthesis and growth. Wood density is correspondingly light. Early pioneer tree species mature rapidly, reproduce early, and die young (circa 10 to 25 years).

Late secondary plants represent an intermediate guild between primary and early pioneer species. These species are typically light-demanding, but their seeds do not exhibit the stringent dormancy of early pioneers and smaller gap sizes are required for germination. Seed dispersal into gaps is facilitated by wind, birds, bats, or ground mammals. Late secondary species exhibit the fast growth and maximal photosynthesis of many pioneer species, but they grow to a much larger size and persist for longer periods in the canopy. Wood density is variable, but usually lower than that of primary species.

Every plant does not fall neatly into one of these groups. There are varying degrees of shade tolerance and a complete spectrum of responses to varying degrees of light, soil moisture, and competition from other species. Even within a single species, the genetic make-up of different individuals can cause great variability in growth rate, wood density, fruit production, seed germination and seedling establishment. A true understanding of the ecological behavior of a species can only be gained through careful observation and detailed study. Such an effort, however, is highly warranted. Sustainable resource use hinges on a species' ability to continually establish new seedlings while being subjected to repeated and intensive harvesting. A basic knowledge of its regeneration and growth requirements can greatly facilitate this process.

POPULATION STRUCTURE
The ultimate criteria by which the life strategy of a species must be measured is its effectiveness in "recruiting" new individuals into its population. The more effective this strategy, the longer the population will be able to maintain itself in the forest. One method of measuring this success is to monitor the frequency and abundance of seedling establishment over a period of several decades, and to record the resultant increase or decrease in population size over time. Fortunately, it is not always necessary to conduct this laborious and time-consuming procedure. In many cases, the recruitment history of a particular species is reflected by the size distribution of individuals within its population. A rapid appraisal of population structure can frequently provide information about whether or not a species is regenerating itself in the forest.

Population structure data have long been used by foresters and ecologists to study the regeneration dynamics of forest species. The results from these studies have shown that the structure of most tree populations can be described by a limited number of size-class or diameter distributions. Three of the most common types of population structure are shown in Figure 3.

Figure 3. Three idealized size-class distributions exhibited by tropical tree populations. Size-class intervals are 10 cm DBH.

The Type I size-class structure displays a greater number of small trees than large trees and an almost constant reduction in the number of trees from one size class to the next. This type of population structure is characteristic of shade-tolerant, primary species which maintain a more or less constant rate of seedling establishment. In these populations, it is pretty safe to assume that the death of an adult tree will, at some point, be replaced by individuals growing up from the smaller size classes. A Type I structure is thought by many authors to represent the ideal of a stable, self-maintaining population. This is the type of structure that you strive to preserve in natural populations of non-timber forest resources.

A Type II size-class structure is characteristic of populations which experience sporadic or irregular seedling establishment. The actual level of regeneration may be sufficient to maintain the population, but its infrequency of occurrence causes notable "peaks" and "valleys" in the size-class distribution as the new seedlings grow into larger size classes. This type of distribution is common among late secondary species that depend on canopy gaps for regeneration. It may also reflect a population whose regeneration has been temporarily interrupted through excessive harvesting of fruits or seeds, direct physical damage to seedlings (e.g. trampling by collectors), or lack of pollinators or dispersal agents.

The final size-class distribution, Type III, reflects a species whose regeneration is severely limited for some reason. Most of the individuals in these populations are more or less the same size, and although many of them may be producing flowers and fruits, no seedlings have been successfully established. Type III population structures are frequently encountered among light-demanding, early pioneer species which require large canopy gaps for regeneration. In the absence of such a disturbance, these species may temporarilydisappear from the forest--the former population represented only by the seeds lying dormant in the soil. A Type III distribution is not restricted to early pioneer species. Populations of late secondary or primary species can also exhibit this pattern if seedling establishment is interrupted for a long enough period of time. Unless conditions change, these populations will permanentlydisappear from the forest.

Although the three size-class distributions correlate well with the three different regeneration guilds, it is important to remember that the population structure of a species is extremely dynamic and sensitive to changes in the level of regeneration. A Type I distribution can easily change into a Type II if existing rates of seedling recruitment are reduced. Further constraints on regeneration may drive the population to a Type III distribution. Given this behavior, it is most useful to view the three structural types as a single sequence through which a population passes on its way to extinction. The occurrence of a Type III structure in a population of shade-tolerant, primary trees, for example, is a sure sign that something is wrong.

Although necessarily brief, the preceding discussion has attempted to show that tropical forest species display a variety of different ecological characteristics that can make sustainable harvesting a very difficult objective to achieve. The major problem areas are:

• the high diversity and low population density of plant species

• the irregularity of flowering and fruiting

• the importance of animals for pollination and seed dispersal

• the high mortality and low success rate during seedling establishment

• the sensitivity of population structure to changes in the level of natural regeneration

These five characteristics are immutable facts of plant life in the tropics. As will be shown in Section II, ignoring them can cause serious, in some cases irreparable, damage to the plant populations being exploited.

LITERATURE
Several excellent textbooks are available to readers desiring further information about the ecology of tropical trees and forests. A few examples are cited below:

1. Gentry, A. (ed.) 1990. Four Neotropical Rainforests.Yale University Press, New Haven

2. Jacobs, M. 1988. The Tropical Rain Forest: A First Encounter. Springer-Verlag, Berlin.

3. Longman, K.A. and J. Jenik. 1987. Tropical Forest and Its Environment. 2nd edition. Longman Ltd., London.

4. Richards, P.W. 1952. The Tropical Rain Forest.Cambridge University Press, Cambridge.

5. Whitmore, T.C. 1984. Tropical Forests of the Far East.Clarendon Press, Oxford.

6. Whitmore, T.C. 1990. An Introduction to Tropical Rain Forests.Clarendon Press, Oxford

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