Plants in the Guanica Forest (latitude 18.degree. N) grew under the following climatic conditions: annual temperature, 25.8.degree. C; annual rainfall, 929 mm; annual pan evaporation, 2074 mm; actual evapotranspiration, 722 mm; yearly runoff, 205 mm; and mean solar radiation, 4366 kcal/m2 per day. The climate was seasonal with most of the runoff between Sept.-Oct. Soils normally exhibited water deficits during 10 mo. of the year. Shallow soils with frequent limestone outcropping compounded the water supply problem of plants. Four upland plant associations were studied. The scrub forest had a basal area (BA) of 4.2 m2/ha and a complexity index (CI) of 2 calculated for trees with dbh [diameter at breast height] > 5 cm. Its shallow soils were very sandy or rocky. Succulent plants were common and the canopy was open. The deciduous forest had a BA of 8 m2/ha and a CI of 20. Its soils were deeper and were 64% sand and 33% silt. A taller and denser canopy developed in this forest. A mahogany plantation grew in semi-evergreen forest soils that were 1 m deep and rich in clay and silt. An ecotonal forest (between deciduous and semi-evergreen) had a BA of 10.7 m2/ha and a CI of 20. The canopy of the ecotonal forest was relatively closed with a leaf area index (LAI) that varied seasonally between 1.2-4.2. Plant photosynthesis, respiration, transpiration, litter fall and loose litter standing crop responded to moisture availability. Net daytime photosynthesis increased linearly with soil moisture availability (r [correlation coefficient] = 0.76) and averaged 2.0 g/m2 lead area per day for non-succulent plants. Night-time respiration rates increased with decreasing soil moisture availability (r = 0.65) and averaged 0.36 g/m2 leaf area per day for non-succulent plants and 0.1 g/m2 photosynthetic area per day for succulent plants. Tree trunks respired during the dry season at a rate of 2.64 g/m2 surface area per day. Soil respiration was 4.74 g/m2 per day. Transpiration of succulent plants was 64 g H2O/m2 photosynthetic area/day, while for non-succulent plants it was 576 g H2O/m2 leaf area per day. Water use efficiency was 6.2 for succulent plants and 1.8 for non-succulent plants. Succulent plants also exhibited night-time or 24 h C-uptake and water loss during early morning or late afternoon. Leaf fall and standing crop of loose litter increased with decreasing soil moisture availability. Mean litter-fall production for the scrub, deciduous forest and mahogany plantation in g/m2 per day was 0.28, 0.79 and 1.48, respectively. Mean litter fall and mean standing crop of loose litter for the forest as a whole were 0.66 g/m2 per day and 715 g/m2, respectively. Mean annual litter turnover was 0.34. Gross primary productivity averaged 6 g/m2 per day and correlated linearly with soil moisture availability (r = 0.87). Soil moisture availability (a function of shallow soils and low rainfall) determines forest productivity, growth characteristics, water loss and physiognomy.