Seasonally Dry Tropical Forests

Neotropical seasonally dry forests are found from northwestern Mexico to northern Argentina and southwestern Brazil in separate areas of varying size (fig.1-1). Their different variants have not always been considered the same vegetation type (e.g., Hueck 1978) or biogeographic unit (e.g., Cabrera and Willink 1980), but recent work has helped to define the extent, distribution, and phytogeography of seasonally dry tropical forest (SDTF) as a coherent biome with a wide Neotropical distribution (Prado and Gibbs 1993; Pennington et al. 2000; Pennington, Lewis et al. 2006). This unified interpretation is important both for biogeographic inference and for setting conservation priorities in Neotropical SDTF, which is the most threatened tropical forest type in the world (Miles et al. 2006).

In the previous chapter much attention was given to the study of the present status of the seasonally dry tropical forest (SDTF) in the Neotropics. Nevertheless, a broader picture of this ecosystem requires consideration of the historical events governing it. Herein, we address the major biogeographical hypotheses concerning the colonization of the SDTF in South America.

Seasonally dry tropical forests (SDTFs) are considered one of the most endangered tropical ecosystems (Janzen 1988c). High degrees of degradation are reported, not only for the Neotropics, but also in the old tropics (Miles et al.2006). Causes and consequences of such degradation are known on a limited basis, and much needs to be learned in terms of gaining a full understanding of what controls environmental deterioration trends in these ecosystems and their impact on ecosystem services. Furthermore, current knowledge on the extent and degree of fragmentation of tropical dry forests is constrained because of the low priority for conservation within governmental and nongovernmental funding agencies. In general, the perception that tropical forests do not exist outside of the Amazon basin, or that high priority should be given to tropical rain forests, has limited the current body of scientific literature (Sánchez-Azofeifa et al. 2005).

Knowledge of biodiversity is key to sustainable management. For tropical dry forests, information on soil animal diversity is sparse compared with tropical wet forests, and considerably less than for grasslands and deserts. Nevertheless, given the rapid transformation of dry tropical forests, primarily due to land use change (Janzen 1988a), it is important to examine the uniqueness of its faunal diversity and functioning as well as its vulnerability to increasing rates of global changes (Sala et al. 2000).

The primary objective of this chapter is to provide a preliminary overview of insect diversity of Central American seasonally dry tropical forests by comparing the proportions of species that either are restricted to dry forests versus wet forests or occur in both. One of the major impediments to evaluating insect diversity, especially in the tropics, is that the majority of species are still undescribed. However, over the past 20 years an intensive inventory has been carried out in Costa Rica, and several taxonomists have dedicated considerable effort towards describing this fauna (Hanson 2004). Although the proportion of the country occupied by dry forests is relatively small (about 15 percent), Costa Rica provides some of the best data presently available; additionally, the diversity of its entomofauna is considerable.

Seasonally dry tropical forests (SDTFs)—with a mean annual temperature greater than 17 degrees Celsius, rainfall ranging from 250 to 2000 millimeters annually and highly seasonal, and an annual ratio of potential evapotranspiration to precipitation of less than 1 (Holdridge 1967)—represent a unique combination of challenges for the living biota contained within them. The harsh abiotic factors create an environment that is hot, with little water, and that generally has an extremely variable but often sparse resource base. It is likely that both intra- and interspecific competition are greater in the severe environments encountered in SDTFs. As a consequence of these abiotic and biotic factors, many species that we think of as ‘tropical’ cannot survive in the dry forest. Despite the biological, cultural, and long-standing economic interest in dry forests, these habitats and the mammals inhabiting them remain poorly known.

Primary productivity in seasonally dry tropical forests (SDTFs) is controlled largely by the amount and timing of rainfall. Because water availability determines leaf production as well as photosynthesis, both interannual and within-rainy-season precipitation constrains and controls ecosystem productivity and nutrient dynamics. Subsequently, soil water availability also regulates organic matter decomposition, fine-root production, and micro-bial dynamics; hence the timing for and the amount of available nutrients are strongly coupled to the seasonal and annual variations in rainfall. The second important driver for SDTF productivity and nutrient dynamics is land use change (seeChap.​ 10). In many areas secondary succession starts on abandoned agricultural fields; thus, secondary forests with different recovery times are becoming a prevalent feature in the dry tropics (Miles et al. 2006). As a result, research in secondary forests has significantly increased (Lawrence and Foster 2002; Campo and Vázquez-Yanes 2004; Urquiza-Haas et al. 2007).

Seasonality in the presence and production of leaves is the defining characteristic of seasonally dry tropical forest (SDTF) ecosystems and has important implications for their functioning. Temporal patterns of shoot activity influence photosynthetic carbon gain and thus affect competition between tree phenological types (Givnish 2002), the seasonal rhythms of tree-herbivore interactions (Leigh 1999; Dirzo and Boege 2009), and the annual ecosystem carbon uptake and energy fluxes (Kucharik et al. 2006). The impact of leaf phenology on tree carbon return is associated with its effect on the length of leaf exposure to herbivore and pathogen damage, the timing of leaf loss for water balance, and the energy investment in leaf construction (Franco et al. 2005). The timing and the length of leaf presence in tropical forests has also been suggested to play a key role in maintaining tree diversity by regulating the abundance of pest pressures (Leigh et al. 2004).

Water availability is one of the most important factors controlling species distribution in terrestrial ecosystems (Holdridge 1967). Organisms have developed remarkable physiological adaptations to withstand, avoid, or tolerate water limitations. It is crucial to recognize and understand these adaptations, and there is an ample literature addressing this subject. However, very few of these studies have focused their attention on seasonally dry tropical forest (SDTF) species, and fewer still have examined the study of water dynamics at the ecosystem level. This is the aim of this chapter.

Seasonally dry tropical forests (SDTFs) were recognized as the most endangered major tropical ecosystem in 1988 because of their high deforestation rates (Janzen 1988c). Maass (1995) analyzed the effect of conversion of SDTF to agriculture and pastures on ecosystem processes. He concluded that the main consequences of these conversions were (1) reduction in species diversity, (2) reduction in soil vegetative cover, (3) disruption of the water cycle, (4) changes in nutrient status, and (5) losses of nutrients from ecosystems through different pathways.

Over the last two decades several studies have shown that plant species of contrasting life-forms ranging from small herbs to large trees may experience a decline in reproductive success following habitat fragmentation and population disruption (Bawa 1990; Aizen and Feinsinger 1994; Aguilar et al. 2006). Such outcome has been shown for many plants throughout the tropics, particularly trees, where human activities have resulted in elevated rates of habitat fragmentation and degradation (Ghazoul and Shaanker 2004; Quesada and Stoner 2004; Quesada et al. 2004). Because almost 90 percent of angiosperms (i.e., flowering plants) depend on animals for effective pollination and sexual reproduction (Buchmann and Nabhan 1996), it is of central concern to understand the capacity of pollinators for transferring pollen among individuals and its consequences on plant reproduction in newly created anthropogenic landscapes.

The Mesoamerican region is blessed with tremendous biological richness, a high level of species endemism, and a diverse cultural heritage. Yet in 2000, only 30 percent of the region’s forest cover remained. Overall, 37.4 percent of the land area of Mesoamerica is used for agriculture (CCAD 2002); much of this agriculture is concentrated in the more seasonal areas previously occupied by seasonally dry tropical forest (SDTF). Pasture is the predominant agricultural land use in Central America, constituting 61.1 percent of all agricultural land in 2000 (FAO 2005). Rice, sugarcane, maize, and beans constitute other major agricultural products grown in dry forest zones of Mesoamerica (Donald 2004; Harvey, Alpizar et al. 2005).

Fragmentation and habitat destruction of tropical forests is nowhere more apparent than in the seasonally dry tropical forests (SDTFs) of Central America (Janzen 1988b; chap.​ 1). In Central America, old-growth tropical dry forest had been reduced to less than 20 percent of its original extent by the mid 1980s (Trejo and Dirzo 2000), largely as a result of disproportionately high human population density and intensive agricultural activity within this habitat zone (Murphy and Lugo 1986a). Although rates of deforestation in Central America peaked in the twentieth century, palynology data indicate that humans have been using fre to manipulate forest cover in Central American SDTF for thousands of years (Janzen 1988b; Piperno 2006).

Seasonally dry tropical forests (SDTFs) are preferred habitats for people who live in the Neotropics. The reasons for this are straightforward. There are fewer big trees in dry forests than in moist or wet forests, and the land is easier to clear for farming. It is also easier to burn the felled trees and slash after clearing, because of the seasonal drought. Once the tree cover has been removed, the underlying soils are frequently more fertile than those found in regions of higher rainfall, because nutrient leaching is less extreme. Finally, the pronounced dry season keeps pest levels down in agricultural fields and reduces the incidence of mosquito-borne and fungal pathogens that cause health problems. It is not surprising that in many Central American countries the population density in dry forest areas is three to four times higher than that found in wet forest areas (Tosi and Voertman 1964).

Human populations depend for their survival and well-being on benefits derived from ecosystems, also called ecosystem services (MA 2003). The nature, magnitude, and reliability of the services provided by a certain ecosystem depend on its particular characteristics, the human group that interacts with it, and the nature of their interaction (MA 2003; Maass et al. 2005). The ability of an ecosystem to provide services depends on its processes—that is, the interactions between its physical and biotic components—and the rates and variability over time and space of such processes and components (Kremen 2005). The benefits people obtain from ecosystems also depend on demographic, economic, political, cultural, scientific, and technological characteristics of the human groups that interact with the ecosystem (MA 2003; Castillo et al. 2005). Human groups determine which services they demand, extract, or expect from ecosystems and thus drive decisions about how to manage them (MA 2003; Bennet and Balvanera 2007). Given the tight relationship between the delivery of ecosystem services and human well-being, the long-term maintenance of the capacity of ecosystems to provide services is essential to ensure a promising future for humanity. Indeed, technical and social interventions need to be designed to foster the maintenance of the services to ensure human well-being (MA 2003).

Recent emphasis in research on Latin American tropical forests dominates our understanding of how seasonally dry tropical forests (SDTFs) may respond to climatic change and also of how changes in land use and fire incidence may interact with this response. As a consequence, our analysis focuses mainly on how Latin American SDTF may interact with climate over the twenty-first century and the wider context within which this interaction may occur, especially with respect to rain forest. In Latin America, SDTFs generally occur where rainfall is less than 1600 millimeters per year and where the dry season is substantial, lasting at least 4 to 6 months during which precipitation is generally less than 100 millimeters per month (Gentry 1995). Apart from rainfall, SDTFs are also associated with specific edaphic factors, notably nutrient-rich soils (Ratter et al. 1973; Furley 1992; Vargas et al. 2008). In contrast, Neotropical savannas, while often occurring under identical climatic conditions, are found on nutrient-poor soils that are usually high in aluminum and sometimes seasonally flooded (fig. 16-1). Savannas and SDTFs are further distinguished ecologically by deciduousness, structure, and fire resistance. Savanna trees are frequently evergreen, whereas most SDTF species are deciduous or semideciduous. Savannas are open, grass-rich vegetation, whereas SDTFs have a closed canopy with few understory grasses. Without human intervention, although SDTF trees may occasionally possess fire adaptations (e.g., thick corky bark), they lack the widespread fire-resistant features above and below ground that are characteristic of the woody flora of Neotropical savannas, which frequently burn during the dry season because of the presence of flammable C4 grasses (Taiz and Zeiger 2006). While SDTF, savanna, and rain forest have complex relationships and perhaps represent points on a continuum of vegetation types, there are in general clear ecological differences between savanna and SDTF that must be taken into account when considering the effects of climate on tropical forests.

In the mid 1990s the first global synthesis of our knowledge of the biology of seasonally dry tropical forests (SDTFs) was published (Bullock et al. 1995). The motivation for that synthesis was the fact that vast areas of tropical dry forests of the world were poorly studied and yet they represent one of the most threatened ecosystems of the world. These systems provide a vast treasure of biological information on the adaptive modes of organisms to an environment that is not thermally limited but where water availability varies greatly within and between seasons. It was hoped that revealing the knowledge that had accrued would be a stimulus to further research on these unique ecosystems as well as new efforts for their conservation.