Root production is an important process in the dynamic carbon and nutrient cycle of terrestrial ecosystems. Root productivity of plant community can be affected by temperature, rainfall and availability of nutrients. In dry forests, root density within the soil system depends on seasonal resources availability, age and soil properties (Pavon and Briones, 1998). In tropical dry forests experience seasonal drought. This seasonality is more pronounced in regard to dry and wet periods than to seasonal fluctuations in temperature (Castellanos,et al.,. 1991). Due to the lack of information on how biology of these forests respond to the seasonality of rainfall (Murphy and Lugo, 1986),our study focuses on understanding of root production by plants in the Guánica Dry Forest as a function of water availability. The Guánica Dry Forest is located in south western Puerto Rico (Fig 1). The rainfall recorded here over the period of 70 years varied from 500 mm to 1500 mm. The soils of the Guánica Dry Forest normally exhibits a deficit of water, ten out of twelve months of the year and seasonal climatic conditions indicate runoff only during the months of September and October (Lugo, et.al., 1978). Considering the great variation in water availability we hypothesized that the fine root density (<2mm)of plants in the Guánica Dry Forest will reflect the seasonal patterns of rainfall, with an increase in root density during the rainy season.