The Role of Disturbance in Dry Tropical Forest Landscapes

Disturbance can be defined as 'any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment'. This definition requires that the spatial and temporal scales of the system and disturbance be determined. Disturbances are typically characterized by their size, spatial distribution, frequency or return time, predictability, and magnitude (which includes both intensity and severity). These disturbance attributes set the parameters for the suite of species, both plant and animal, that can persist within a given system. As such, an understanding of seasonally dry tropical forests in Asia requires an understanding of disturbance within the region. However, disturbances are relatively poorly understood in dry tropical forests, partly because of the weak seasonality in temperature and high tree species diversity of these forests relative to most forest systems of the world. There are about 1,048,700 km{sup 2} of dry tropical forests worldwide and that only 3% of this land is in conservation status. In other words, 97% of the world's seasonally dry tropical forest is at risk of human disturbance. About half of this forest occurs in South America, where most of the conservation lands are located. Satellite imagery based on MODIS (Moderate Resolution Imaging Spectroradiometer) data shows that only about 3.8% of the world's dry tropical forests are in Australia and South east Asia. The susceptibility of these forests to human disturbances is of great concern and is largely unstudied. Because natural disturbance regimes shape the ecosystem structure and are in many ways integral to these forest systems, it is critical to know how natural disturbance affects dry forest in order to understand the effects of human activities on these forests. Even basic information about disturbances in dry tropical forests is only recently available. Therefore this chapter brings together much of the available information from dry tropical forest throughout the world with the goal of developing an understanding of the role of disturbance in Asian dry forests. Most ecologists now recognize that disturbances, rather than being catastrophic agents of destruction, are a normal, perhaps even an integral, part of long-term system dynamics. The composition, structure, organization, and development and trophic dynamics of most forest systems are the products of disturbances. As an example, the forest composition for two disturbances in the Anaikatty Hills of Western Ghats were compared, where the low disturbance was from past logging followed by cutting and illicit felling and grazing and the high disturbance was due to human presence, past logging, and fuelwood collection. They found higher species richness and Shannon-Wiener diversity index for the low-disturbance forest (98 and 3.9, respectively) compared to the high-disturbance stand (45 and 2.71, respectively) as well as significant differences in mean basal area of trees, density of seedlings, number of species, density and diversity of shrubs, and number of species and diversity of herbs. Some ecological systems contain species that have evolved in response to disturbances. Adaptations typical of dry tropical forest plants are drought tolerance, seed dispersal mechanisms, and the ability to sprout subsequent to disturbance. In contrast, evidence was found that human disturbance in Kakamega Forest of western Kenya has significantly reduced allelic richness and heterozygosity, increased inbreeding, and slightly reduced gene flow in Prunus africana in the past century