Coal forests were the vast swathes of swamps and riparian forests that covered much of the land on Earth's tropical regions during the late Carboniferous (Pennsylvanian) and Permian periods.^[1][2] As plant matter from these forests decayed, enormous deposits of peat accumulated, which later became buried and converted into coal over the subsequent eras.

Much of the carbon in the peat deposits produced by coal forests came from photosynthetic fixation of atmospheric carbon dioxide, which released the accompanying split-off oxygen into the atmosphere. This process may have greatly increased the atmospheric concentration of oxygen to possibly as high as about 35%, making the air more breathable by animals with inefficient respiratory systems, as indicated by the size of Meganeura compared to modern dragonflies.^[3]

Coal forests covered tropical Laurasia (Europe, eastern North America, northwesternmost Africa) and Cathaysia (mainly China). Climate change devastated these tropical rainforests during the Carboniferous period. The Carboniferous rainforest collapse was caused by a cooler drier climate that initially fragmented, then collapsed the rainforest ecosystem.^[2] During most of the rest of Carboniferous times, the coal forests were mainly restricted to refugia in North America (such as the Appalachian and Illinois coal basins) and central Europe.

At the very end of the Carboniferous, the coal forests underwent a resurgence, expanding mainly in eastern Asia, notably China; they never recovered fully in Laurasia. The Chinese coal forests continued to flourish well into Permian times. This resurgence of the coal forests in very late Carboniferous seems to have coincided with a lowering of global temperatures and a return of extensive polar ice in southern Gondwana, perhaps caused by lessening of the greenhouse effect as the massive coal deposition process extracted carbon dioxide from the atmosphere.

The coal forests seem to have been areas of flat, low-lying swampy areas with rivers flowing through from higher, drier land.^[4] When the rivers flooded, silt gradually built up into natural levees. Lakes formed as some areas subsided, while formerly wet areas became dry from silt buildups. When a forested area became dry enough to be set on fire by lightning, the resulting forest fire left charcoal, the fusain component of coal.

There seems to have been a rich and varied flora, with sets of species for each type of growing condition. The most varied flora seems to have been leafy vegetation, with many species of trees, bushes, creepers, etc. Thickets of Calamites seem to have favored the edges of lakes and waterways. Lycopsid genera specialized in various roles: Paralycopodites as a pioneer on newly silted lakes shallow enough for land vegetation to start; Diaphorodendron later when the ground had become peaty.

Other species specialized in re-settling land which had been briefly deforested by flooding: Synchysidendron and Lepidodendron in mineral-soil areas and Lepidophloios in peat areas. Cordaites may have favored drier areas of the swamp. In the later part of this period tree ferns tended to take over from lycopsid trees.

Some of the characteristic plants of the coal forests were:

• Sigillaria
• Lepidodendron
• Calamites
• pteridosperms

Animals inhabiting the coal forests were invertebrates (particularly insects), fish, labyrinthodont amphibians, and early reptiles. Amphibians were widespread, but once the coal forests fragmented, the new environment was better suited to reptiles, which became more diverse and even varied their diet in the rapidly changing environment.^[2]

These films and TV series are set partly in coal forests:

• Prehistoric Park Episode 5: ("The Bug House")
• Walking with Monsters Episode Two: "Reptile's Beginnings"