Chapter 3 Primary Forestsand Their Productivity

Chapter 3 Primary Forestsand Their Productivity Primary forests are those that have existed without sig- Compared to other forests, primary forests generally nificant human or other disturbances for longer than the achieve a relatively stable equilibrium between gross lifespan of mature trees (60 to 80 years is used by FAO) procluction and respiration, low gross production relative (Anon. 1982c). In such relatively stable forests, func- to standing biomass, high biomass relative to energy tional relationships evolve that display preferences, flow, low net production (yield), weblike rather than tolerances, capacities, and interdependencies among linear food chains, large volumes of organic matter, organisms not otherwise evident. Such forests are self- intrabiotic inorganic nutrients, high diversity, well-orga- sustaining and evince both ecological and economic nized stratification and spatial heterogeneity, narrow values useful to society. In present times, during a small niche specialization, large organisms, long and complex fraction of the last 2 million years, primary forests are life cycles, closed mineral cycles, slow nutrient ex_ believed to have approached their maximum extent change rates with the environment, quality versus quan_ (Whitmore 1982). Many are worth preserving in perpetu- tity production, developed internal symbiosis, and good ity. As a unique source of information on the relations nutrient conservation and stability (Odum 1969). between forests and their environments, primary forests and their dynamics merit thorough study by foresters. Valid as these concepts may be, the traditional vision of They afford important points of reference for assessing the virgin forest as being "unharmed" is unrealistic (Spurr silvicultural departures, and Barnes 1980). Disturbances (or even major disasters) are natural and frequent relative to the lifespan of most This chapter describes those characteristics of primary forest trees. Instability of the forest is inevitable, even in forests that are significant to productivity. There has been the Amazon, where climatic change and tribal interven- a long history of ecological studies of such forests tion may have occurred within the lifespan of the oldest throughout the Tropics; some of the most thorough were trees. Caution must be exercised in classifying tropical those of Jones (1956) in west Africa and Schulz (1960) in forests as primary because of the possibility of past hu- Suriname. More recently, the ecology of wet forests in man intervention (Catinot 1974). Puerto Rico (Odum and Pigeon 1970), the Amazon (Fittkau and Klinge 1973), the Far East (Sutton and others The following description of ecosystems has been 1983, Whitmore 1984), and other regions have been adapted from Odum (1972). The structural components studied, include the climatic regime, inorganic substances in- volved in mineral cycles, organic compounds, au- The physical environment of forest ecosystems deter- totrophic (self-nourishing) forms of life, and heterotrophic mines the pattern and the rate of change, and it may set forms (nourished from others). Heterotrophic life forms limits on how far ecosystem development can go (Odum include phagotrophs, or macroconsumers, which are 1969). But the ecosystem itself may in turn modify the largely animals that ingest other organisms, and sapro- physical environment. Succession culminates in a maxi- trophs, which are microorganisms that decompose mum degree of homeostasis in which the ecosystem's organic matter. The living and nonliving parts of ecosys- organic responses tend to compensate for environmental terns are so interwoven that they are hard to separate. changes. The primary forest provides maximum self- protection from disturbances. It also attains, relative to Moist tropical ecosystems are so complex that their per- available energy, maximum or near-maximum biomass formance cannot be studied by traditional one-problem, and interaction among organisms. Interaction may in- one-solution investigations. To understand them, it may clude competition, in which one of the organisms in- be necessary to use investigative systems involving math- volved is suppressed; commensalism, in which one is ematical modeling and cybernetics (Odum 1972). benefitted, but none is suppressed; and mutualism, in which more than one organism benefits (Longman and Much recent ecological research may appear peripheral Jenik 1974). to forest productivity. It concerns information primarily of scientific interest, such as soil microbiota and understory Primary forests exhibit both resilience, the capacity to plants, leaf-area indexes, litterfall, biomass, and gross absorb changes and still persist, and stability, the capac- productivity in terms only vaguely indicative of useful ity to return to an equilibrium state after a temporary wood volume or growth (Harper 1977). Whereas forest- disturbance (Holling 1973). ers have studied forests chiefly to ascertain how much 63 Chapter 3 usable wood they might produce, ecologists are more Forest Structure broadly interested in how forests function as systems. Forest structure is concerned with the sizes, relative Ioca- These broader insights are at least partly of use to the tions, and types of life forms. The description presented forester. Ecologists have shown the added costs to society here applies chiefly to moist forests (fig. 3-1 ). A summary of misuse or destruction of forests, and they are explaining of structural contrasts between moist and dry, tropical phenomena that heretofore were neither well understood and subtropical primary forests appears in table 3-2 nor studied in depth by foresters or anyone else. (Murphy and Lugo 1986). The primary forest, an ecological ideal, is not an ideal for Rain forests, as defined by Baur (1964b), are generally at production to satisfy all human needs. Evolution tends to least 25 m tall. Semi-evergreen forests range up to 40 m; favor individual species more than the forest as a whole evergreen seasonal forests, to 50 m; and equatorial rain (Harper 1977). It would be surprising if activity that fa- forests from 40 to 60 m. A tall tree reported from a Ma- vored one individual against another also maximized the laysian tropical forest, a Koompassia excelsa, attained performance of the population as a whole as a producer 84 m (Foxworthy 1927, cited by Longman and Jenik satisfying human wants. Thus, production of the entire 1974). Tropical forests tend to be shorter in dry areas and population requires compensation for some of the evolu- at high altitudes (fig. 3-2). In Venezuela, mean height of tionary advancements favoring individual fitness. An trees 10 cm in diameter at breast height and larger at example may be the need to reduce forest density to elevations up to 3,000 m above sea level was 10 m; accelerate growth rates of those trees selected for a fu- above that elevation, it was 5 m (Veillon 1965). ture timber crop. Such modifications may start with primary forests but, once applied, have removed the forest Forest Density. Forest density is measured by the num- under such management from the category of primary, ber and size of the trees, stand basal area, or stocking (volume per unit of land area). In 1980, the primary and old secondary forests of 76 tropical countries worldwide had an estimated area of Number and Size of Trees. The number of trees per more than 6 million km 2 (Anon. 1982a; table 3-1 ). They unit of forest area that have attained a certain d.b.h, is a _ constituted about 76 percent of all potentially productive significant measure of forest density (Schulz 1960). In forests in these countries. In tropical America, the corre- primary moist tropical forests, the number of trees per sponding percentage was 87 percent. Between 1980 and unit of area is fairly uniform (table 3-3; Heinsdijk 1957, 1990, some 2.4 million ha of closed broadleaf forests Wyatt-Smith 1949b). were logged in tropical America (Anon. 1993b) suggest- ing that the area of undisturbed forest in the region in 1990 was about 450 million ha. Figure 3-1 .--The canopy of moist forests at low elevations is formed by broad tree crowns, as illustrated by this forest in Panama. 64 Primary Forests and Their Productivity ,:: .... i:=3:::::::,: .ii: ::i I :_:iill i} i :Patchy:_':::::. i:!i_i'{// , , :. ! '}, : i::::2--7:!:::if::::.:::: .................. :- ..:...:::/,., Figure 3-2.wOn shallow soils, such as on these limestone hills in Puerto Rico, the forests are short, the .....'.: trees having low, broad crowns. ::}}'::i ::i_ i :._::,::i_: '7:'.:'7 ': i_: : :,_:_: _ of tree sizes. The de Liocourt quotient, or ratio "q" (Amo- bi 1973, Harper 1977), is described in the previous chapter. If these ratios are similar throughout the range of diameter classes, the stand is considered "balanced," The number of trees 10 cm in d.b.h or more per unit of because mortality in each class apparently is being offset area varies by site. For seasonal forests, the mean is by a compensating number of trees entering the class about 500 trees per hectare; for submontane forests, it is from below. If the smaller classes exceed, in number of up to 1,000 trees per hectare (Baur 1964a). In Venezu- trees, those immediately larger by a greater ratio than ela, the number varied with the altitude (table 3-4; that for the upper end of the d.b.h, range, the stand is Veillon 1965). considered to be "positive," in that ingrowth is apparently more than adequate for replacement. A converse In primary forests, most of the trees are small (fig. 3-3). trend, termed "negative," suggeststhat regeneration may An extensive sampling of the forests of the Amazon be deficient. (Heinsdijk 1957) revealed that about half the trees are consistently in the smallest class (table 3-5). In higher latitudes, this proportion is lower (Baur 1964a). Stand tables for tropical moist forests show consistency, x_:, Dawkins (1959) found great uniformity in tree numbers by sizes in primary tropical high forests and constructed a "pantropical" stand table (table 3-6). Great numbers of trees start out, but only a few reach large size.

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