Forests of the ultramafic mount Giting-Giting, Sibuyan Island, the Philippines

Abstract

Mount Giting-Giting is an ultramafic mountain on Sibuyan Island, Romblon Province, Republic of the Philippines. It was studied along the Mayo's Peak Ridge which had rainforest from about 200m up to c. 1550m. The grassland and scrub vegetation from 1550m to the summit (2050m) of Giting-Giting was not explored. The mean annual rainfall at the base of the mountain was c.2100mm which included a dry season around February and March. There was a notable increase in wind speed with altitude and there was about one hurricane (called a cyclone in the Philippines) per year. Analyses showed that the soils are acid ( 4.4–5.5), and have relatively low concentrations of Olsen extractable phosphorus (0.22–2.07μg g−1), low exchangeable potassium (0.04–0.41mEq 100g−1), and relatively low (for ultramafic soils) Mg/Ca quotients (0.31–2.87) and exchangeable nickel (1.0-23.8μg g−1). Coupled plots (of 0.25 or 0.04ha) were studied at altitudes from 325m to 1540m. All trees ≥ 10cm dbh were enumerated and structural features of the trees and smaller plants were quantified. The forests were all of fairly small stature (maximum tree height 24m) but dense (up to 2180 trees ha−1 in Lower Montane Forest (LMF) and up to 880 ha−1 in Upper Montane Forest (UMF)). While different taxa could be readily recognized, specific determinations were often impossible from the mainly sterile specimens. The LMF was species rich with up to at least 111 species of tree (≥ 10cm dbh) per 0.25-ha plot. There was no family clearly dominant in the two lower plots at 325m and 385m (where the Dipterocarpaceae were probably reduced by logging). At 770m and 860m the Dipterocarpaceae accounted for 12.9% and 14.7% of the basal area respectively. There was a surprisingly high representation of the Sapotaceae (25.9% of the basal area) at 1240m. At 1540m, the Araucariaceae (Agathis sp.) dominated one plot (37.1% of the basal area) and the Myrtaceae the other (72.4%).