Water Availability and Soil Chemistry of Ultramafic Soils in Susua Forest, Puerto Rico.
Jamie L. Horvath, Univ of Pennsylvania, 240 S 33rd St, Philadelphia, PA 19104 and Arthur Johnson, Dept of Earth & Env Science, Univ of Pennsylvania, Philadelphia, PA 19104.
El Bosque Estatal de Sus˙a is located on a belt of ultramafic rock that cuts through the southwestern corner of Puerto Rico. The Sus˙a landscape comprises 2 north-south oriented ridges that are bisected by a valley in which flows the Rio Loco. Sus˙a 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 Sus˙a 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 Sus˙a 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).