Introduction

Tropical forests have received considerable attention recently, stimulated in part by the high rate at which they are being modified or completely destroyed. Emphasis has been given to their potential effect on the global carbon balance and on biogenic emissions (García-Méndez et al., 1991; Matson & Vitousek, Chapter 16). Most studies have focused on tropical forests growing in wet or humid climates which account for 58% of tropical forests (Brown & Lugo, 1982), while seasonal forests growing in drier climates, which represent the remaining 42%, have been very little studied (Murphy & Lugo, 1986a). Tropical dry forests represented nearly 20% of the total biomass of forests in the world in the early 1970s (Persson, 1974). But due to deforestation, and extensive use of the area for intensive agriculture, pastures and shifting cultivation among other uses, the present area is considerably reduced.

Constraints on soil biological activity and their effect on ecosystem production, soil organic matter formation and nutrient cycling have been little studied in dry forests (Anderson & Flannagan, 1989). Most of the work on this topic has been carried out in India with considerably less information available from dry forests of the neotropics.

In this review I will discuss how water availability primarily constrains soil biological activity in tropical dry forests, in order to highlight its importance in the dynamics of organic matter and nutrients in the ecosystem.

Climatic characteristics of tropical dry forests

Strong rainfall seasonality is the overriding macro-determinant in dry forests. Seasonality and, most important, the duration and intensity of dry and wet periods, exert a strong influence on the biological activity of both the above- and below-ground parts of the ecosystem.