Introduction

The production and emission of trace gases from tropical forests are of interest for calculating global budgets, for determining local ecosystem mass balances, and for the insight they provide into ecosystem processes. Globally, tropical dry forests are widespread and functionally different from other biomes, and hence need to be characterized directly. More-over, the extent and intensity of human modification of dry forests (in comparison to other tropical biomes: Murphy & Lugo, 1986) raises the possibility that land use change in this biome could be globally significant in the concentration or distribution of one or more gases.

At the ecosystem level, loss to the atmosphere can be a major pathway of transfer of nitrogen and sulfur from terrestrial ecosystems (Likens et al., 1977; Bowden, 1986), and hence a possible determinant of the long-term fertility and potential productivity of terrestrial ecosystems. Since gaseous emissions are not normally included in watershed-level measurements of nutrient inputs and outputs, they can provide a useful complement to watershed studies. Finally, on the process level the trace gases emitted from terrestrial ecosystems represent either end products or by-products of fundamental metabolic processes that occur within terrestrial ecosystems. Their magnitude, timing and regulation provide insight into the operation of those processes even where emissions are not quantitatively significant in either global or local element budgets (Matson, Vitousek & Schimel, 1989).

None of these reasons is specific to tropical dry forests; all apply to any biome with a large areal extent and significant human modification. However, the dry forest biome is particularly interesting for three reasons. (1) It occupies an intermediate position on the great moisture gradient that underlies variation among tropical ecosystems (Medina, Chapter 9).