Forests are dynamic units, which consist of individuals at all stages of their life cycles. We can think of forests as mosaics, containing a variety of patches in different phases of restoration. Forests have evolved their own system of regeneration, which is termed succession. Succession, a change in the composition of the forest over time, occurs because openings (“gaps”) continually appear in forests, and because the seeds and seedlings of different species have varied requirements for germination and growth. When an opening does appear, certain plants establish themselves in them and form a pioneer forest, only to be replaced later by other species adapted to the changing conditions, and eventually resulting in a stable forest composition – the climax forest. This process is described below.
Natural disturbances play a great role in forest succession. Trees die of old age, or are struck by lightning, or are blown over by wind, or are knocked down by other falling trees. When this occurs, gaps appear in the forest canopy, which alters the environment for all of the plants surrounding the open area, be it large or small. The frequency of and degree to which catastrophic events occur depends on the forest. In much of the Amazon, and on Borneo, there are few catastrophic events, and therefore forest regeneration is usually confined to small gaps where one or a few trees have died. In the flood (varzea) forests in Peru, there are “stripes” with different patterns of species, each of which represents a stage of succession. Because these areas are subject to annual flooding, heavy siltation, and annual changes in the river course, a climax forest is never attained – only a series of pioneer forests of varying ages. On the other hand, in Papua-New Guinea, where volcanic activity and earthquakes are fairly frequent and where there has been a long tradition of shifting cultivation, there are large areas of regenerating forest and therefore a fairly elaborate mosaic of pioneer and climax forest.
Tropical rainforests go through several stages during regeneration. A small gap, such as that caused by the death of a small tree, or the loss of a limb, will not alter much in the forest. Limbs from other trees will fill in a gap and their shade will prevent the growth of most seedlings on the forest floor, except for those which do not require much light. A somewhat larger gap changes the physical state of that area of the forest. There will be more light, heat, and wind on the forest floor where a gap forms. The forest floor of the gap will become hotter and drier than previously, although more rain will reach the ground (but it will be dried quickly by the sun). The temperature here can be as much as 10o C higher than under the canopy. Even the wave lengths of light reaching the forest floor are altered. Normally canopy trees absorb “red” (long) wavelengths and the forest floor receives only about 2% of the photosynthetically-active wavelengths (440-700 nm) in small spots or flecks. But in a gap, more of the light reaching the forest floor will be in the red wavelength range (660-770 nm). Under these circumstances, the slower-growing and shade-tolerant seedlings in the understory cannot survive, and are gradually replaced by seedlings of fast-growing and light-tolerant species (the so-called pioneer species). Pioneer species are characterized most importantly by a requirement for strong light for seed germination and seedling establishment, and also, in general, aggressiveness, tolerance to dessication, rapid growth, early reproduction, efficient seed dispersal mechanisms, and small seeds with long dormancy periods. (The fig, Ficus insipida, for instance, has a photosynthetic rate six and one-half times greater than that of any other known species, although it will probably not hold that record when other tropical pioneer species are examined. And it must live in bright sunlight [Allen, 1996].). When a gap forms and light strikes the forest floor, their seeds, which have been dormant for a long time, are able to sprout and grow rapidly to fill in the gap. At this early successional stage, the extent of seedling sprouting and survival is determined mainly by factors in the environment – competition, nutrient availability, temperature, degree of shade, etc. Then those seedlings which survive begin to influence their own environment as they grow – by producing shade, using soil organic matter, and producing new types of habitats. At the same time, epiphytes and climbing plants begin to colonize the growing young trees.
The early pioneer trees are often low and short-lived, and may be replaced by longer-lived, taller pioneer species which form a higher canopy forest. Meanwhile, the seedlings of climax species remain undeveloped in the shade of the pioneer trees, as they do not do well under gap conditions of high temperatures and high light intensities. Moreover, specimens of climax species in gaps appear to be susceptible to attack by boring insects. The pioneer species will eventually, however, be replaced by climax species since, as pioneer seedlings require light, they cannot survive and reproduce under the newly-formed (pioneer) canopy. Climax species in general have large seeds with substantial nutrient reserves (so they can wait), have shade-tolerant seedlings, are slow-growing relative to the pioneer species, and are self-perpetuating, since once the pioneer trees form a canopy, the climax species’ shade-tolerant seedlings can grow in their shade. As large pioneer trees die, conditions are conducive for the small trees of the climax species to grow rapidly and take their place. (While climax species’ seedlings are shade-requiring, older specimens actively seek light.) Once established, a climax forest can reproduce itself endlessly, since it provides shade for its seedlings, and the large trees have attained the canopy. This series of events is what Whitmore (1998) calls a “shifting mosaic steady state.” (Although pioneer and climax species are defined here as having quite different characteristics, in truth all of these species lie on a continuum and it is not always easy to define these terms.) However, decade-long studies on Nicaraguan forest after very large gaps were created by a hurricane indicated that, when very extensive gaps form in tropical forests, pioneer species do not succeed in repressing the growth of other species. This is apparently because there are relatively few seeds or seedlings of pioneer species in the forest, and so they are not able to “flood the market” nor to capture the lion’s share of available space and light, as they do when smaller gaps occur (Vandermeer, et al., 2000).
There is a very large genetic pool, and incredible diversity among the organisms in rainforests. There are species which are adapted to almost every condition which may occur during and after forest perturbation. For example, there are fast-growing species, species which require sun, those which require shade, those with rapidly-germinating seeds, others with seeds with long dormancy periods – in short, whatever happens in a forest, there will be species which can exploit the situation and thrive. However, diversity in a secondary (successional) forest tends to be lower, at least at first, than in primary forest. Palm diversity, for one, is very low in secondary forests.
How rapidly do large forest trees reach their destination in the canopy? Little is known of this topic so crucial to forest management programs and to our ability to restore degraded tropical forests. Recently, however, Clark and Clark (2001) found that most trees do not grow constantly or at maximum rates – there are too many obstacles and too much competition. Some trees even decreased in height while striving for the canopy when they were damaged or died back partially due to adverse conditions. To attain even half of their final stature might require 35 to 85 years.
Rainforests are especially vulnerable ecosystems. The seeds of the woody species are fragile, many forest plant seeds cannot tolerate sun and thus cannot disperse across cleared land, the soil is fragile and easily eroded by heavy rainfall, and many species have a very limited distribution and will be decimated by removal of even small areas of forest. Rainforest regeneration is slow, and the forest may indeed never regenerate, at least to its original condition. The ancient Cambodian capital city of Angkor was deserted in 1431, and while the forest around it has regrown, after almost 600 years it is still different from the original climax forest in this area. The forests in the areas of Guatemala where the Maya lived and which they abandoned as long as 1200 years ago are less diverse than other forests nearby; in fact, many of the tree species in these forests are those which had been used and presumably planted by the Maya.
Man-made disturbances have been much more dramatic, for the most part, than natural ones. During the El Niño of 1982-1983, there was a great drought in Southeast Asia and many fires – both man-made and natural – occurred. El Niño is a recurring climatic pattern during which warmer water from the western Pacific region moves eastward into the cooler eastern Pacific. Normally the temperature differential of the water between the western and eastern portions of the Pacific sets up trade winds which blow strongly from east to west, causing ocean currents to flow westward. In El Niño years, because warmer water pushes into cooler regions, the temperature differential is less and the trade winds weaken. Because ocean and atmosphere are so closely linked, heat and water exchanges are altered, which influences rainfall patterns in the tropics and in temperate zones as well. During the aforementioned El Niño period, three million hectares of forest were destroyed in Kalimantan (Indonesian Borneo), one third of which had been recently logged and where much flammable dead dry wood had been left. Another third was primary forest which had been desiccated and killed by the drought. Elsewhere, settlers moved in along logging roads and set fire to what remained of the forest after logging. Thus, the drying effects of El Niño were exacerbated by human activities. The huge gaps which are formed by logging and fires are not amenable to normal regeneration processes, and in many cases the resulting forest (if any) will be vastly different from the previous one. Hunting by settlers and loggers leads to the loss of animals which are the major seed dispersers and which are therefore vital in tree reproduction. In Sabah (North Borneo) much land, previously forested, is still grassland after a drought and an ensuing fire in 1915. (Whitmore, 1998.)