Luzon rain forests

Luzon has never been connected to mainland Asia. At least 15 million years ago, friction between the Australian and Asian tectonic plates and volcanic activity created parts of the Luzon highlands, which over the next 10 million years morphed into their modern form.[2] This long period of isolation and complex internal geography is a primary cause for the great biodiversity and high degree of endemism found on the island of Luzon.[3]

During the Ice ages, sea levels were up to 120 meters lower than at present. Ice-age Greater Luzon connected Luzon with Catanduanes, Marinduque, the Polillo Islands, and several smaller neighboring islands. The ice-age land bridges allowed the animals and plants of these now-separate islands to mix, which made them part of the same ecoregion.[4]

In 2005, evidence for human occupation in northern Luzon since at least 25,000 years ago, was found in Callao Cave.[5] Evidence included chert flake tools, charred parenchymatous tissues, starch grains, grasses, and Moraceae phytolith.[5] The possibility of hunter-gatherers subsisting in Holocene tropical rain forests without support from agriculturalists was debated, based on the patchy and seasonal resources.[5] Wild forest animals are lean and lacking in calorie-rich fat.[5]

However, hunters and gatherers may have managed by strategizing and moving to correspond to the shifting food resources.[5] Information on skeletal morphology and diet is merely speculative as no human remains were recovered from this time period. This idea is supported by the Sierra Madre Agta of the recent ethnographic past who would plan their movements based on the seasons and available resources; they hunted in the montane forest during the rainy season and in the lowland forest during the summer.[5] Most of their food supply came from fishing, shellfish gathering, wild yams, nuts, and Caryota palms.[5]

In the same cave two years later, in 2007, the same scientist found a metatarsal bone dated to at least around 67,000 years ago, which is speculated to possibly be of Homo sapiens origin. The bone (Right MT3 – the small bone from the end of the middle toe of the right foot) has been identified as belonging to a species of the genus Homo. To tell if the bone belongs to an ancient anatomically modern human, a skull or mandible from the specimen is needed. This fossilized remain from Callao Cave is referred to as Callao Man.

During World War II, the Japanese invaded the Philippines, and a small band of the Communist Party of the Philippines (CPP) activists used the dense mountain jungles and vast swamps of the Luzon rainforest for protection.[6] The communist activists established a base of operations in the nearby Mt. Arayat and the Candaba Swamp.[6] These activists launched small yet annoying attacks against the Japanese.[6] On December 10, 1941, CPP leaders issued a manifesto vowing their support for the anti-Japanese efforts of the Commonwealth and the United States, and urging the peasants to support this united anti-Japanese front.[6] Resulting was the organization of the Hukbalahap, an acronym for the Hukbong Bayan Laban sa Hapon (the Anti-Japanese Army), in a small lowland forest clearing near Mt. Arayat on March 29, 1942 by the merging of the CPP with the remaining socialist and peasant organizations of Luzon.[6]

Mature forest in the crater floor of Mount Cagua

The Sierra Madre mountain range.

Above 1,000 metres (3,300 ft) elevation, the Luzon rain forests transition to the Luzon montane rain forests ecoregion. Montane rain forests are found in the Northern and Southern Sierra Madre, Mt. Sapacoy, Mt. Magnas, and Mt. Agnamala in the northern Cordillera Central highlands, and in the Zambales Mountains in the west. In the south of the island, enclaves of montane forest can be found on several volcanic and non-volcanic mountains that exceed meters elevation. The volcanic mountains include Mt. Makiling, Mt. Banahaw, Mt. Isarog, Mayon Volcano, and Bulusan Volcano.[2][7]

In some areas, annual rainfall can be about quadruple what the lowland rainforests receive (as high as 10,000 mm).[7] The Sierra Madres have very mild seasons, with a slight dry period between December and April. The Zambales Mountains and northern Central Cordillera highlands are more strongly seasonal with a longer dry period and slightly less rainfall generally.[7]

The dipterocarp trees of the lowlands are gradually replaced by oak and laurel forest species with increasing altitude.[7] The forests generally have less undergrowth and become shorter in stature as altitude increases.[7] With the decreasing temperature from increasing altitude, decomposition is slowed and results in a forest floor thick with humus.[7]

In the montane forests, epiphytes, vines, and moss-covered branches are very common.[7] The highest altitudes of montane forests are caller upper montane forest, or elfin forest, and are more extreme: trees are shorter in stature, and tree branches are so thick with moss and organic material that they can sustain aerial plants that are not typically epiphytes.[7] Many endemic animal species reside in the thick, matty soil of the upper montane forests.[7]

In fact, species richness is greatest along the highest elevations of the montane rainforests of Luzon.[3] Areas with the greatest levels of endemism are reported to be the Cordillera Central highlands, the Sierra Madre, the Zambales Mountains, and highlands on the Bicol Peninsula.[3]