El Bosque Estatal de Susúa is located on a belt of ultramafic rock that cuts through the southwestern corner of Puerto Rico.  The Susúa landscape comprises 2 north-south oriented ridges that are bisected by a valley in which flows the Rio Loco.  Susúa is in a climatic transition zone between the humid mountains of the Central Cordillera to the north and the dry coastal forests to the south.  The mean annual temperature is 23.9°C and the average annual precipitation is 1413 mm.  A strong rain shadow exists from Puerto Rico’s orographic weather pattern such that, in a dry year, the northern half of Susúa received 39% more rain than the southern half.  Soils are either serpentinite outcrop or oxisols (Inceptic Hapludox or Anionic Acrudox), with rod depths ranging from 22 cm to > 80 cm.  Vegetation on the ridges and slopes resembles dry forest vegetation, while the valleys consist of gallery forest.  Fifty plots (100 m² each) were established in 2002 in order to measure soil properties and leaf chemistry.  Sixty-nine species of woody plants (dbh ≥ 2.0 cm) are present in the plots, 13% of which are endemic to Puerto Rico.  The biomass of Susúa is variable, averaging 63 Mg/ha on the slopes, 130 Mg/ha on the ridges, and 188 Mg/ha along the rivers.  Standing biomass is significantly higher in plots in the north as compared to the south (p<0.04) and is also significantly higher in plots on deep or alluvial soils as compared to shallow and moderate depth soils (p<0.0001).  Soil chemistry suggests high levels of exchangeable magnesium (median=3577 mg/L), with a median Mg:Ca ratio of 2.8:1.  Water use efficiency, as measured by δ¹³C of leaf tissue, is significantly higher in the shallow and moderate soil depths as compared to deep and alluvial soils (p<0.0001).