Tropical seasonal forests

When most people think of a tropical forest they envision a jungle that is hot, humid, dark, noisy, and filled with strange, possibly dangerous animals and plants. In fact, many different types of forests can be found in zones surrounding the equator, and these forests vary in their physical characteristics (average rainfall and soil types) and biological characteristics (plant and animal species). Although all forest types exhibit some degree of temporal variation, tropical rainforests are characterized by less severe cyclic variations compared to seasonal forests (Table 1). In tropical seasonal forests (Figure 1), temporal patterns of temperature and rainfall, as well as plant and animal activities, are correlated within and between years, requiring organisms to shift their responses to changing conditions, to escape harsh conditions, or to minimize potential threats to survival and reproduction with intermediate traits.

For example, the root systems of plants inhabiting seasonal environments must have the capacity to regulate water and nutrient absorption in both wet and dry conditions. Animals also must accommodate to seasonal changes since resources required for survival and reproduction, particularly food and mating sites, are not continuously available. If a population of a given species is sufficiently diverse, evolutionary processes are expected to favor traits minimizing negative consequences to life history (patterns of mortality and reproduction) occasioned by variability in resource dispersion and quality over time and space.

Scientists utilize a variety of research methods to evaluate the nature of variability in tropical seasonal forests and the effects of these variations on plant and animal populations. One of these techniques utilizes a multiyear series of rainfall data yielding an unbroken sequence of points, each representing total monthly rainfall. A quantitative test, time-series analysis, is used to produce a model summarizing past, current, and estimated future rainfall events. This statistical approach describes features of the rainfall series that may be similar within and between years and that may serve as predictable environmental stimuli eliciting compensatory physiological and behavioral responses in organisms.

Thus, individuals in populations located in tropical seasonal regimes survive or die depending upon the success or failure of their phenotypic traits. In the real world, however, reoccurring cycles of rainfall or other environmental stimuli will not be perfectly correlated over time within and between cycles, exposing organisms to some degree of unpredictability and risk. Since the response of plants and animals inhabiting tropical seasonal forests will not reliably fit many challenges presented by changing conditions, the survival and reproductive success of individuals in populations will depend upon phenotypic flexibility. In tropical seasonal forests, then, developmental, physiological, and behavioral features are expected to represent “best-of-a-bad-job,” rather than optimal, responses.

Forests, their canopies, and other characteristics are fundamental to the processes, functions, stability, maintenance, and persistence of ecosystems. Locally and globally, the physical and biological elements of forests filter and cool the atmosphere, absorb and deflect sound and light, provide refuge for animals and plants, and buffer the potentially damaging effects of temperature (desiccation), rainfall (erosion), and wind (tornadoes may destroy forests and extirpate species). Detritus (falling leaves) and decomposing trees produce humus, the primary building block of soil. Trees also produce large quantities of oxygen and absorb carbon dioxide, processes critical for regulation of the negative consequences of global climate and other anthropogenic effects.

The varieties of tree species grouped into different tropical seasonal forest types (habitats) are determined by the range of local (food) and global (climate) conditions in given environments that change across time and space. Local (niche) and global factors responsible for habitat characteristics may be classified as physical (abiotic) or biological (biotic). Abiotic factors include all inorganic properties of forest landscapes such as altitude, geological and geographic formations, and waterways. Biotic factors include all organic features of forest landscapes, for example, distribution, abundance, type, and composition of plant and animal species.

Central American Tropical Dry Forests

The tropical dry forests of Central America can be used as prototypes for tropical seasonal forests worldwide. These landscapes are characterized by clearly defined wet and dry seasons of about six months duration each and by two easily distinguishable habitat types, deciduous and riparian (riverine). Gordon Frankie reported that Costa Rican riparian and deciduous forests exhibit flower and fruit activity primarily during the dry season, November through April. In the deciduous forest, leaf fall is synchronized for most trees during the early to mid-dry season. In both habitats, soil, altitudinal gradients, and local disturbances (e.g., tree-fall gaps) are important determinants of plant growth, distribution, and abundance patterns. Climate, particularly rainfall, is the most significant predictor of seasonality.

Most trees in the riparian forest retain their leaves throughout the year, displaying a phenological (tree activity) pattern similar to wet forest sites in Costa Rica. Riparian habitat, with higher humidity and greater proportion of evergreen vegetation, is most likely characterized by a higher level of primary productivity compared to deciduous habitat, though few empirical studies exist to test this assumption. Riparian habitat is also likely to exhibit greater complexity and stability resulting in greater resilience when confronted with environmental perturbations (for example, drought and hurricanes).

Fruiting Patterns

For more than 30 years, Theodore Fleming, a biologist at the University of Miami, has conducted research on plant-animal interactions in many Latin American tropical seasonal forests, including those in Costa Rica. During that time, he has served as a faculty member and administrator with the Organization for Tropical Studies (OTS), an internationally recognized consortium of universities in the United States and Costa Rica whose primary goals are to facilitate scientific initiatives in tropical ecosystems and to train future tropical ecologists. Comparing fruiting patterns over time and space in New World (Central and South America) and Old World (Asia and Africa) tropics, Fleming and his colleagues demonstrated that seasonal unpredictability of fruiting patterns is greater in Old World forests. This finding has important implications for differential physical (soil composition and temperature) and biological (species composition, abundance, survival, and reproductive strategies) features. The differential environmental patterns documented by Fleming's group are expected to have significant consequences for patterns of mortality and survivorship of individuals, populations, and species inhabiting tropical forests west and east of the Atlantic Ocean.

The presence of temporal variation in tropical seasonal forests is generally problematic from the organisms' point of view, since their biological processes, from molecular and cellular to phenotypic levels, must be sufficiently flexible to endure both wet and dry conditions. Although plants and animals experience disadvantages associated with inhabiting tropical seasonal forests, a major advantage is that time-varying patterns associated with seasonality yield environmental predictability, providing clear cues and signals about changing conditions (for example, changes in sunlight, humidity, or rainfall patterns). The discrete stimuli signaling wet and dry seasons in time-varying environments modify physiological and behavioral processes of organisms, allowing adjustments to the stress of different temporal features (for example, abundance and distribution of nutrient and water resources).

Ecologists studying plants and animals inhabiting tropical seasonal forests measure patterns of mortality to evaluate the success of organisms' responses to temporally varying regimes. On the whole, these studies show that many organisms display a trade-off between adult mortality and mortality of immature age-sex classes, and adult survivorship is commonly favored in seasonal forests if survival and reproduction exhibit certain features (extended life span). Ecological studies have shown that this life-history trade-off results from uncertainty and risk associated with reproducing in seasonally changing habitats in which, compared to wet tropical forests, cues and signals predicting the initiation and termination of seasons are highly variable. These challenges may stress developmental, physiological, and behavioral processes influencing lifetime reproductive success of individuals in a population. Statistical analyses of time-varying features are predictable on average, but the onset, duration, and termination of temporal patterns within and between years are usually difficult for the perceptual mechanisms of organisms to assess. Ambiguity and error resulting from between-cycle variability usually leads to significant mortality of one or more age-sex classes in a population.

Modes of Reproduction

All other things being equal, plant and animal populations inhabiting seasonal environments may respond to cycle variability with one of six modes of reproduction, and it is important to remember that each life-history strategy entails both costs and benefits for the organisms displaying it. One mode is characterized by brood parasitism whereby some bird species in tropical seasonal forests are parasitized by other species of birds that lay clutches in nests of the parasitized (host) species. The parasitic species destroys some proportion of the host's clutch, resulting in partial or complete loss of the host's seasonal reproductive investment. Parasite-host associations are highly specialized with respect to the species involved, and parasites gain an advantage over their hosts by laying their eggs prior to hatching of host eggs. Though this life-history pattern appears to be a reproductive dead-end for hosts, the lifetime reproductive benefits to hosts must have been higher than alternative patterns available over evolutionary time. In New World (neotropical) tropical seasonal forests, ground cuckoos (Tapera naevia, Dromococcyx pavoninus, D. phasianellus) are common brood parasites.

A second reproductive pattern is displayed when plants and animals specialize to the most common type or subtype of tropical seasonal forests, such as riparian or deciduous habitat. Specialists are usually characterized by a diet limited in food types and a tolerance for a narrow range of environmental conditions. Of approximately 230 primate species, my research suggests that 18 species are extremely specialized. Of these 18, however, most are exclusive to rainforests, two prefer wet forests (Hylobates hoolock, H. syndactylus), while the geographic distributions of five species are limited to or include large areas of seasonal forests (Propithecus diadema, Nasalis larvatus, Presbytis pileata, P. potenziani, Gorilla gorilla). It is interesting to note that the diets of the previous seven species include significant proportions of leaves, an observation worthy of further study since a folivorous diet is often thought to buffer primates from environmental stressors because, all other things being equal, leaves are continuously available. The neotropical plant Tabebuia chrysantha (araguaney) displays a life-history mode specialized to optimize lifetime reproductive success in deciduous habitat, and, like other specialist plants, produces flowers and fruits in dry season, the period most beneficial to the survival of new growth. A specialist strategy may be beneficial when conditions favor the specialized traits; however, specialists may be highly vulnerable if environmental stimuli change significantly (changes in rainfall patterns incompatible with a root system's particular functions).

In a third case, some species exhibit a phenotypic compromise or mix of traits advantageous to an intermediate fit across changing conditions (across both deciduous and riparian habitats in Central American environments). These generalist phenotypes are the most common ones displayed by plants and animals in tropical seasonal environments. All other things being equal, generalists thrive in a wide range of environmental conditions and utilize a variety of resources (for example, food items, resting sites). Compared to other life-history modes, generalist strategies may seem intuitively to be least costly to lifetime reproductive success; however, generalists may be spread too thin in time and space to respond successfully to many stressful environmental events, such as climate change or habitat destruction. Recent scientific evidence supports this proposition. The mantled howler monkey (Alouatta palliata) inhabits a broad range of forest types, including tropical seasonal forests, throughout the forests of Middle America and the Pacific coast of northern South America. Mantled howlers prefer a diet of flowers, fruit, and new leaves. Kathryn Stoner and other investigators have shown that mature leaves serve not only as a fallback food when preferred items are unavailable to howler monkeys but also as a buffer against environmental stress if distribution, abundance, or quality of preferred plant dispersion changes in time or space. The Alouatta genus is extremely successful biogeographically as the most widely distributed neotropical primate genus. However, the most well-known and widely ranging generalist species among mammals is the omnivorous Homo sapiens.

A fourth reproductive pattern displayed by some tropical seasonal forest species entails specialization on one of the environment's morphs (for example, dry or wet conditions). A number of these taxa are migrating species such as cockatoos (Calyptorhynchus spp.) that move to and from rainforests and tropical seasonal forests to maintain a relatively unchanging exposure to a tolerable range of humidity. This strategy permits the species to escape the negative effects of the least favorable suite of conditions to the more favorable suite of conditions (favorable for breeding or preferred food items).

A fifth life-history strategy observed in tropical seasonal forests is mast fruiting, whereby individuals of a given plant species coordinate flower and fruit production, usually in accordance with the highest availability of pollinators. In the tropical seasonal forests of Central America, Manilkara spp. display mast fruiting. A sixth life-history pattern observed by a relatively few tropical seasonal forest plant species such as Pithecellobium saman entails the production of flowers and fruit biannually. This mode of reproductive allocation may represent a lifetime energy-conservation strategy for some large trees. For each of the six life-history modes, the evolutionary and ecological strategies characteristic of specific populations depend upon a suite of factors. Among the most important factors are ancestral and derived traits of a species, as well as differential life-history metrics describing populations, such as generation time (T), rate of intrinsic increase (r), and net reproductive rate (R₀).

Threats to Tropical Seasonal Forests

The tropical seasonal forest biome buffers the tropical rainforest biome and the tropical savanna biome. Thus, tropical seasonal forests are of critical importance for the preservation of biodiversity in tropical zones worldwide. Habitat destruction and other anthropogenic effects threaten evolved (for example, cooperative) and by-product (for example, competitive) associations within and among species. Local (patch or niche) and global (climate) effects interact, potentially inducing ecosystem perturbation and instability, in some cases leading to a cascade of network changes, including loss of species and ecosystem collapse.

Climate change and other factors varying in frequency, rate, duration, intensity, and quality contaminate, modify, and extirpate biogeochemical environmental components and attendant processes in tropical seasonal forests. The severity of anthropogenic outcomes continues to increase, with the potential to drive inherently resilient ecosystems beyond their capacities to accommodate and adapt to changing regimes. Jean Carlos Santos and his colleagues studied the research and conservation status of the Brazilian Caatinga, a tropical seasonal forest. These investigators showed that tropical seasonal forests are typically resilient and, thus, of particular scientific interest for a general understanding of factors associated with developmental, physiological, and phenotypic as well as biogeochemical buffering in complex systems. Santos's group highlighted the ecological, social, and economic importance of tropical seasonal forests. However, using tropical dry forests as an assay, these authors found that only 1 percent of dry forests in Central and South America, and only 5 percent of these forests worldwide, are protected.

These Brazilian scientists also found that tropical seasonal forests have been relatively ignored in the scientific literature, with tropical humid forests receiving the most research and conservation focus. Tropical seasonal forests are frequently located in areas having few academic resources, are poorly represented in the scientific literature, and are rarely highlighted as hot spots. Tropical seasonal forests require attention from professional, nonprofessional, and student conservationists, who can apply their knowledge, skills, and labor in ongoing initiatives from community to international levels in order to understand, address, and manage crises attendant to changes in tropical ecosystems. As two high-profile ecologists stated in the early 1990s: “On one point there is no argument: tropical forests are, indeed, in trouble.” The worldwide status of tropical seasonal forests deserves treatment equal to that received by humid tropical ecosystems.