- It is the largest remaining undisturbed rainforest in the country. It is also the most diverse and the richest in terms of biodiversity. Northern Sierra Madre Natural Park was established by virtue of Proclamation No. 978 on 10 March 1997. It covers an area of 359,486 hectars, including 287,861 hectars of land and 71,625 hectars of marine areas. Also known as the Palanan complex or Wilderness Area, the park is in eastern Isabela Province, in the northern half of the Sierra Madre mountain range. It is bounded by the Dikatayan River to the north, the Disabungan River to the south, the Cagayan Valley to the west and the Philippine Sea to the east.
The topography within the park ranges from relatively low hills with moderately steep slopes near the coast to higher mountains with very steep slopes, peaks and sharp ridges in the interior of the Sierra Madre Mountain Range. A series of peaks along the north to south range running through the park rise to well over 1,000 m, including Mt Cresta (1,672 m), Mt Divilacan (1,311 m), Mt Palanan (1,184 m) and Mt Dipalayag. The park is blessed with undisturbed lowland dipterocarp rainforest and it includes areas where undisturbed forest extends from the coast to the mountain peaks. There are some extensive areas of montane forest around the higher peaks in the park. The park�s coastline are other ecosystems, which are among the most diverse and productive in the world. These include estuaries, mangroves, beach forest, seagrass-beds, and coral reefs.
Almost all of the 6 species of Eagles found in the Philippines thrive here. These include the Philippine Eagle and the Rufous-bellied Eagle. There are large areas of lowland forest in the park, so it is very vital for threatened lowland forest inhabitants. Threatened or near-threatened amphibians and reptiles were also recorded in the area including the Philippine Sail-fin Water Lizard (Hydrosaurus pustulatus), Estuarine Crocodile (Crocodylus porosus) and Gray�s Monitor Lizard (Varanus olivaceus). At least 12 amphibians and 15 reptiles recorded from the area are Philippine endemics and other species not previously known from the park and from the Philippines. This area is also known to hold many endemic and rare species of plants.