Seasonally Dry Tropical Forests. - book reviews

Ecology, Jan, 1997 by Kamal Bawa

Tropical dry forest, characterized by Dan Janzen as the "most endangered tropical ecosystem," once covered vast tracts of the earth, but now occupies only a very small fraction of its original area. The species richness of dry forests is not as high as that of humid forests, but these forests are an important reservoir of biodiversity, and a significant source of goods and services to human societies. The relatively simple structure of dry forests makes them more suitable to unravel ecological patterns and processes than their wet counterparts, and indeed, some of the most significant insights into ecosystem structure and function in the tropics have had their basis in research conducted in dry rather than in wet forests. However, despite the biological importance of these forests, their intense and prolonged use by humans, and their rapidly diminishing size, not a single book exclusively devoted to the ecology of the dry forests has been available until the publication of this volume. This book, edited by Stephen Bullock, Harold Mooney, and Ernesto Medina, represents the output of a symposium held at the Estacion de Biologia Chamela, Jalisco, Mexico. The date of the symposium is not specified, but most papers seem to be current, as indicated by their list of references.

The papers can be grouped into four broad categories. The first category includes reviews with emphasis on the structure and general features of dry forests in different parts of the world; the other categories are ecosystem processes, including species interactions, conversion, and ethnobotany. Most papers deal with Central American forests, acknowledged by the editors as the focal region of the book. However, there are some chapters from Asia and Africa. Moreover, references to work in all tropical regions of the world are scattered throughout most chapters.

Perspective on the structure and composition of dry forests are provided for Central America and the Caribbean (Murphy and Lugo), Brazilian caatinga (Sampaio), Africa (Menaut, Lepage and Abbadie) and Thailand (Rundel and Boonpragob). Diversity of life forms is a separate topic in the chapter by Medina. These chapters highlight several features: tropical dry forests are less diverse than wet forests; total biomass of trees and net annual primary productivity are also lower, but the diversity of plant life forms is greater; and the human impact is intense.

Complementing these studies of contemporary forests, Graham and Dilcher describe changes in the composition of the plant communities in the Americas based on the fossil pollen record. Dry forest elements first appear in Mid to Late Eocene, increase in diversity through Mid Tertiary, and can be recognized as a part of a distinct community in the Miocene/Pliocene.

The global perspective is provided by Gentry. On the basis of 28 sample plots established throughout the world, Gentry concluded that although dry forests are less diverse than wet forests, diversity in dry forests, beyond a certain threshold of precipitation, does not increase with an increase in annual rainfall. Gentry also noted that although tropical dry forests share many genera and families with wet forests, species overlap is minimal. Furthermore, various dry forests differ in their species composition as well as in levels of endemism. He also pointed out that centers of endemism are not equatorial but near the tropics. Thus, efforts to conserve dry forests cannot be restricted to a single geographical region.

A number of chapters cover ecosystem processes such as primary production (Martinez-Yrizar, nutrient cycling (Jaramillo and Sanford), nitrogen trace gas emissions (Matson and Vitousek), soils (Cuevas) and drought responses (Holbrook, Whitbeck, and Mooney). Some of the common themes include comparisons with wet forests, responses to the pronounced dry season, and a general lack of information about basic processes. However, the authors and editors do not take advantage of the opportunity to address a common set of conceptual issues concerning ecosystem processes.

Species interactions is the topic of two chapters: plant reproduction (Bullock) and plant-herbivore interactions (Dirzo and Dominguez). Bullock's masterful review of various aspects of reproduction, including phenology, pollination, seed dispersal, and mating patterns brings together a wealth of information. This is by far the most comprehensive chapter in the book, with the longest list of citations. The chapter on plant-herbivore interactions is a useful documentation of community and population level patterns and the consequences of herbivory.

Animal diversity is the subject of only one chapter on vertebrates by Ceballos; the editors comment on a lack of information on invertebrates.

Maass explores the causes and consequences of conversion of tropical dry forests into other types of land uses and, like others, pleads for more research on sustainable land use in areas occupied by tropical dry forests. Much of the tropical dry forest has been converted to pasture and agriculture. Thus, unless the economic consequences of conversion on other land uses, particularly agriculture, are fully explored, pleas for sustainable land use are not likely to receive much attention.