Structure and floristic diversity of the vegetation on an ultramafic hill in Surigao del Sur, Philippines

The forest on ultramafic hill in Surigao del Sur is, indeed, unique in terms of floristic diversity and soil metallic elements. Although the ultramafic plot has fewer species than the adjacent dipterocarp forest plot, it had the highest number of individuals representing various life forms, about twice the values for the two other plots. There were also several rare and endemic species of plants. At least two species, Terminalia surigaensis and Heterospathe califrons, both endemic to the Philippines were apparently highly restricted to ultramafic substrata and were not known in the two other plots or in other vegetation types in the logging concession. T. surigaensis has been found to accumulate up to 18.75 ppm of iron and 0.17 ppm of nickel, the highest for all the species tested. The soil in the ultramafic forest contained considerable amounts of heavy metals of chromium, nickel, cobalt, and iron, and had low concentrations of potassium, phosphorus, nitrogen and calcium. Leaf tissues of some of the dominant species accumulated concentrations of some of the heavy metals in the soil. The presence of heavy metals generally tends to inhibit other plants in tthe ultramafic areas. The environmental conditions of very high precipitation rate, light intensity, exposure, soil and wind could have influenced the structure and species identified for each plot. The diversity in three 1-hectare plots was found to be high in terms of species and family composition, and many species were endemic in the area. There was only a few distinct clustering of species related to the environmental conditions of soil, moisture regime, exposure, and importance value. Rare species whether endemic or not were present in small or localized populations in all the three plots studied. Species of the family Dipterocarpacea dominate in the dipterocarp plots and Guttiferae/Clusiaceae, Combretaceae, Myrtaceae, Palmae, and Cyatheaceae families in ultramafic forest. Very few species migrated from ultramafic areas and vice-versa