Vol. 133, No. 5 The American Naturalist May 1989 MONODOMINANT AND SPECIES-RICH FORESTS OF THE HUMID TROPICS: CAUSES FOR THEIR CO-OCCURRENCE Wildlife Conservation International, New York Zoological Society, Bronx, New York 10460; *Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48824 Submitted December 9, 1987; Revised August 2, 1988; Accepted November 9, 1988 Tropical forests are among the most diverse plant communities worldwide. This long-standing generalization (Richards 1952) has been reconfirmed by recent reviews (Gentry 1982; Leigh 1982; Whitmore 1984). Nevertheless, the occurrence of large areas of old-growth tropical forest dominated by one or a few species has been noted in several major surveys of tropical vegetation (Richards 1952; Letouzey 1970; Whitmore 1984). Such single-species-dominant forests may cover hundreds of square kilometers and occur adjacent to significantly more-diverse forest types. Single-species-dominant (hereafter, monodominant) forests of the humid trop- ics are often evergreen and are found on a variety of substrates. In this paper we consider only evergreen forests on well-drained soils. Species described as nearly exclusive dominants in their respective forests in equatorial Africa include Gil- bertiodendron dewevrei (De Wild.) Leonard (Lebrun and Gilbert 1954; Gerard 1960), Cynometra alexandri C. H. Wright (Eggeling 1947; Hamilton 1981), and Julbernardia seretii (De Wild.) Troupin (Gerard 1960). Monodominant forests have also been reported in the American tropics (most notably forests dominated by Mora excelsa Benth.; Beard 1946; Richards 1952; Rankin 1978), in Southeast Asia (Zon 1915; Laan 1927; Koopman and Verhoef 1938; Whitmore 1984; Rai and Proctor 1986), and in tropical Australia (Connell and Lowman 1989). In many of these monodominant forests, the principal species accounts for more than 80% of canopy-level trees. The juxtaposition of monodominant forests with adjacent species-rich, mixed forests has led to the suggestion that comparisons of these two systems might be useful (Janzen 1977; Connell 1978). Differences in the structure and composition of the two forests may indicate processes leading to monodominance in the one case and, by extension, mechanisms maintaining species diversity in the other case. We compare a widespread monodominant forest of central Africa, domi- nated by Gilbertiodendron dewevrei, with an adjacent mixed forest that is consid- erably more diverse. Forests dominated by G. dewevrei (hereafter, Gilbertioden- dron forests) are most extensive on the plateau surrounding the central basin of Am. Nat. 1989. Vol. 133, pp. 613-633. 0 1989 by The University of Chicago. 0003-014718913305-0009S022M)M) All rights reserved. 614 THE AMERICAN NATURALIST TABLE l Mechanism Proposed Evidence Change in substrate Change in forest diversity andlor species composition with change in quality soil quality Succession Monodominance is temporary: dominant canopy species shows poor recruitment in the understory Mortality and herbiv- Monodominance is stable: dominant species' regeneration is abun- ory gradient dant, with reduced seed predation and herbivory relative to other species Disturbance regime Greater abundance of tree falls and shade-intolerant species in mixed forest the Zaire River, but they also occur in smaller patches in forests of the central basin itself (Gauthier et al. 1977) and in Central African Republic, Cameroon, and Gabon (Lebrun and Gilbert 1954; Letouzey 1970). This forest type has been mentioned in earlier studies of central-African vegetation (Louis 1947; Wambeke and Evrard 1954; Devred 1958; Frankart 1960; Jongen et al. 1960; Pecrot and Leonard 1960). It has been the subject of a detailed floristic study (Gerard 1960), but not in comparison with adjacent mixed forests. The study reported here examines the composition, structure, and physical environment of Gilbertiodendron forest and of a co-occurring mixed forest in the Ituri region of Zaire. We also include an overview of the literature on upland monodominant forests of the humid tropics worldwide to enable pantropic com- parisons and to elicit global patterns consistent with our specific central-African case. PROPOSED MECHANISMS FOR MAINTENANCE OF DIVERSITY Mechanisms proposed as important in maintaining tropical forest diversity, or implicated in cases of low diversity, were used to derive a list of expected patterns in forest composition, structure, and physical environment (table 1). These were verified in the field, where species-rich and species-poor forests co-occur. Con- firmation of an expected pattern was interpreted as support for the importance of one or more of the mechanisms. Substrate The possible importance of substrate in determining species richness in tropical forests has long been recognized (Davis and Richards 1933, 1934; Lemee 1961; Richards 1961; Ashton 1971). Chemical fertility and soil moisture are two factors often considered important. Soils of low nutrient status have been associated with forests of low species diversity or with those dominated by relatively few species (Richards 1952; Ashton 197 1; Brunig 1973; Janzen 1974). Gradients of increasing diversity have been correlated with increasing rainfall (Gentry 1982) or soil mois- ture (Lemee 1961; Hall and Swain 1981), but flooded forests and mangrove forests MONODOMINANT TROPICAL FORESTS 615 are monodominant in a number of tropical regions (Steenis 1958; Chapman 1976; Janzen 1977). We exclude these from consideration here, however, because the conditions for forest regeneration are different and may be inappropriate for direct comparison with our primary example of Gilbertiodendron forest. If the physical environment, specifically substrate quality, determines the diver- sity of forests in a given area, then spatial shifts in floristic diversity should accompany changes in the underlying soils. Furthermore, if a change in forest type is due to a pronounced change in the physical environment, it is likely that some tree species would be characteristic of just one forest type, either because conditions exclude them from the other or because the competitive balance is so altered that they become rare (Goldberg 1985). Succession It has been hypothesized that species richness might result, in part, from the specialization by tree species to variations in available resources (Ashton 1977). For instance, variation in tree-fall openings may offer an opportunity for speciali- zation to different regeneration niches (Hartshorn 1978; Denslow 1980; Orians 1982; Pickett 1983). Accordingly, forests of low diversity might provide only a limited array of such resources (tree falls) or constitute a successional stage in which specialized species had not yet invaded or reached maturity. Connell et al. (1984) have suggested that tropical forest diversity is promoted by "compensatory mechanisms" that favor survival and reproduction of rarer species over more-common ones. For instance, chemical interference or competi- tion among conspecific juveniles might lead to reduced recruitment of common species, thus allowing coexistence with rarer species. If the greater diversity of the mixed forest relative to the monodominant forest is due to compensatory mechanisms, then either such mechanisms are absent from the monodominant forest or, if the monodominant forest is a sera1 community, the effects of these processes are not yet evident. If monodominance is a successional stage, then a more diverse community would be presaged in the understory. The dominant species would be poorly represented in the smaller-size classes, and the invasion of specialized canopy species would be apparent by their presence in the understory. We would expect this pattern to be pronounced in the vicinity of mixed-forest boundaries, where propagules of other species are most available. Predation Interactions across trophic levels-in particular, predation and herbivory- have been shown to promote diversity in producer assemblages. This has been repeatedly demonstrated in rocky-intertidal communities (Connell 1961, 1978; Paine 1980, 1984; Dayton 1984). Janzen (1970) and Connell (1971) have hy- pothesized a similar effect for predation on seeds and seedlings in tropical forests. This has received some confirmation for specific cases (Boucher 1981; Clark and Clark 1984). If predation on juveniles (or any other factor affecting seed survival and estab- lishment) maintains the greater tree-species diversity of mixed forest relative to 616 THE AMERICAN NATURALIST monodominant forest, we would expect higher seed and seedling mortality in mixed forest and poor subcanopy representation of the canopy species. Con- versely, if the importance of a single species in monodominant forest is a result of freedom from seed predation and herbivory, then we would expect abundant regeneration of the dominant in the understory and low seed and seedling mortal- ity, at least relative to that of the mixed-forest canopy species. Disturbance Recently, species richness has been recognized as a possible function of non- equilibrium conditions (Connell 1978; Huston 1979). Wind-throw disturbances have been implicated in the creation of nonequilibrium conditions in tropical- forest zones (Connell 1978; Hubbell 1979). Whitmore (1978) has suggested that wind-throws give rise to a mosaic of successional stages and consequently con- tribute to rain-forest diversity. Hubbell and Foster (1983) have illustrated how disturbance, in combination with chance immigration,