An almost unhindered human occupance for over three millenia of years and centuries of plough and pastoral culture has induced the natural vegetation in the region except in the northern part of Champaran and pockets of Tarai and some river banks. With a moderate rainfall and fertile soil the region is a natural habitat of a dense forest cover of Sal and other species like Shisham, jamun, mahua, ber etc. Even up to the 1840's a large area of the northern plains as well as sizable pockets in South Ganga Plain was densely forested. The remanents of once extensive sal forests are found in Sahasa and purnea districts. Elsewhere, the induced vegetation is seen in the form of a savannah with grasses and bushland dotted with trees of different sizes. The village wastelands or vacant places are covered by naturally-growing trees like peepal, banyan, tamarind, mahua, neem, babul, palmyra, date-palm, etc., while such grasses as bher, moonj, kans, jhau etc., are found in the diaras, tarai or otherlow lying areas, and the more omnipotent dub, motha, and other less pervasive types are found in the cultivated fields and mounds or field-boundaries and left-over lands. The planted fruit orchards and groves of trees consist of mango, jamun, gauva, mahua, jack-fruit, plums, lemon, etc., and also timber trees like shisham, eculyptus etc., while bamboo clumps grow around and within settlements. Also such thorny-bushy plants as the senhur, plum bushes tec., grow on waste or neglected lands, but the formaer is sometimes planted for fencing farms or orchards.

The Champaran forests are classified as (1) Siwalik sal, (2) Moist (Gangetic) high alluvial sal (3) Gangetic moist deciduous riverain forest.

There is some dofference in vegetal cover north of the Ganga on the one hand and to their south on the other. The groves and orchards, marshy vegetation etc., are more frequent, particularly in N. Bihar Plains. In the south, there is more open parkland vegetation and the frequency of groves decreases and isolated trees are common. Sabai is the most important grass of the region, particularly in the north.

In most of the districts like Saran, Muzaffarpur, Darbhanga and Saharsa in North Bihar plain, forest cover is particularly non-existent (below 0.01% of the total area of the district), while in most of the others it is negligible (below 1%) as Purnea (0.09%), Deroria (0.23%), and Patna (0.83%). Except in Champaran where the forest cover is much high (10.92%). In the districts south of the Ganga, the forests occur mostly outside the plains.