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E.S. Fcmando: Vegetation of the Philippine lslands 2
Panay and Mindoro and the northern section of Palawan and its adjacent small islands
constitute the Triassic and Jurassic continental fragments rifted off the margin of South China
during the Oligocene and colliding with the central Philippine lslands (Hamilton 1979; Holloway
1982; McCabe et al. 1982). Luzon, the largest island in the archipelago, is known to be
composed of accretions of terranes and basement complexes of diverse origins varying in age
from Late Jurassic to Early Miocene (Karig et al. 1986; Geary et al. 1988).
Audley-Charles (1981) has suggested that some Philippine volcanic islands may have been
above sea level for about 70-140 Ma or since the Cretaceous. The collision of the Gondwanic
AustraliaINew Guinea Plate with the Laurasian Indo-Malaysia in the region of Wallace's Line is
estimated to be in the mid-Miocene (about 15 Ma) (Audley-Charles 1981). This collision has
given rise to new islands through volcanic activity (Audley-Charles 1987). Some of the
resulting collision fragments could have been thrust north towards the Philippine area and
formed separate islands or accreted to existing larger islands. Wolfe (1983), for example, has
suggested that Catanduanes Island and the Surigao area of Mindanao may represent
fragments thrust towards the Philippine area from the southwest Pacific as the AustraliaINew
Guinea Plate rafted northwards from Gondwanaland.
There is recent evidence which indicate that fragments of the AustraliaINew Guinea Plate that
drifted north (what are now parts of Sumatra, Malay Peninsula, Thailand and Burma) much
earlier than the mid-Miocene collision at Wallace's Line, may have provided stepping stones of
dry land in Late Jurassic (1 60 Ma) (Audley-Charles 1987).
The Philippine archipelago is today linked by island arcs to its neighboring islands to the north
and south. The Babuyan-BatanesYAmi island arc connects to subtropical and warm
temperate Taiwan in the north. In the southwest are two island arcs that connect the
archipelago to Borneo, viz. the Mindoro-Calamian-Palawan-Balabac island arc and the Basilan-
Sulu-Tawi Tawi island arc. Another two island arcs link the archipelago to Sulawesi, Moluccas
and farther to New Guinea. One of these arcs, the Sangihe, runs directly south from the Davao
Peninsula on Mindanao and nearby Sarangani lslands to the Minahassa Peninsula in Sulawesi.
The second arc runs from Cape San Agustin on Mindanao and nearby Miangas Island then to
Talaud and towards the Sula lslands and Moluccas.
These island arcs have been postulated as migration routes for flowering plants in both
directions. The Taiwan-Luzon path in the north via the Babuyan-Batanes-YAmi island arc has
been recognized by Steenis (1965) as one of three tracks by which mountain plants have
entered Malesia. Lam (1945) also has earlier identified the island arcs south of Mindanao as
possible migration tracks between the Philippine archipelago and Sulawesi (Fig. 3). The
Palawan-Balabac island arc is closely allied with Borneo being geologically part of the Sunda
Shelf.
The vegetation of the Philippine lslands bear the influence of the climatic vicissitudes that
occurred in Malesia during the Tertiary and Quaternary (Whitmore 1981; Morley & Flenley
1987). These fluctuations, according to Morley & Flenley (1 987), have brought about changes
in sea levels, in the degree of seasonality in precipitation and in variations in temperature. The
lowering of sea levels provided more linkages with mainland Asia and Australia. Cooler
temperatures during the Late Quaternary allowed more area for montane plants to migrate.
The extensive seasonal climate in the Malesian Region during the Late Tertiary on the other
hand, has permitted migration of monsoon plants. There is, unfortunately, very little published
work available on fossil pollen or other palaeobotanical