Set aside as a "Reserve Forest" (RF) on 27 August 1881,[5] it was named after its dominant tree species, hollong or Dipterocarpus retusus.

[5] Although the sanctuary is currently completely surrounded by tea gardens and a few small villages, it used to connect to a large forest tract that ran to the state of Nagaland.

[5][6] As tea gardens began to emerge between 1880 and 1920, and villages were established during the 1960s to rehabilitate people from Majuli and adjoining areas who had lost their lands to floods, the forest became fragmented and the reserve became isolated from the foothills.

These plantations along with the natural vegetation subsequently created a forest stocked with a rich variety of flora and fauna (biodiversity).

It is the only sanctuary in India named after a primate due to its distinction for containing a dense hoolock gibbon populations.

[5][6] The tea gardens include Katonibari, Murmurai, Chenijan, Koliapani, Meleng, Kakojan, Dihavelleoguri, Dihingapar, Kothalguri, Dissoi and Hoolonguri.

[5] Nahor (Mesua ferrea) dominates the middle canopy with its spreading crown, casting fairly heavy shade over a wide area.

Other species that make up the middle canopy include bandordima (Prasoxylon excelsum), dhuna (Canarium resiniferum), bhomora (Terminalia belerica), ful gomari (Gmelina sp.)

bonbogri (Pterospermum lanceafolium), morhal (Vatica lanceafolia), selleng (Sapium baccatum), sassi (Aquilaria malaccensis), and otenga (Dillenia indica).

[5][6] The growing populations of tea garden workers also threatens the habitat since many people rely on the forest for firewood, traditional medicine and food.

Railway lines further divide the park, stranding a single group of gibbons in the smaller fragment.

Illegal logging and the encroachment by local people employed by the tea gardens degraded the habitat quality.