Forest types of the world

The cleanest global classification is the FAO Global Ecological Zones system, which carves the planet into climate-defined zones each forest must fit within^[3]. The Olson et al. ecoregions framework, developed at WWF and now openly republished as Ecoregions 2017, gives a much finer split — 846 distinct ecoregions globally — and is the dataset most conservation work uses^[1][2]. For a primer-level overview the six big buckets are enough.

Boreal (taiga)

The largest forested biome on Earth by area, ringing the Northern Hemisphere across Canada, Alaska, Scandinavia, and northern Russia. Cold winters, short growing season, dominated by a small number of conifer genera (spruce, pine, fir, larch) plus aspen and birch. Stand-replacing fire is part of the natural disturbance regime; old-growth boreal is therefore patchier than in temperate or tropical systems, with substantial unbroken landscapes still preserved as Intact Forest Landscapes mostly because they are remote, not because they are formally protected^[5]. The Russian boreal in particular holds the largest contiguous IFL on Earth.

Temperate rainforest

Cool, wet, coastal forest on the western edges of mid-latitude continents: the Pacific Northwest from northern California to Alaska, coastal British Columbia, southern Chile (the Valdivian forest), Tasmania, parts of New Zealand’s South Island, and small pockets in the British Isles and Norway. Dominated by long-lived conifers and southern beeches. Unusually high biomass per hectare, often the highest in the world. Most of the temperate rainforest old growth that survives is in this biome — including the Pacific Northwest stands described in that country snapshot and the BC stands in that one.

Temperate deciduous (broadleaf) forest

The forest most readers in eastern North America, central Europe, and east Asia know best. Maple, oak, beech, hickory, hornbeam, lime. Distinctly seasonal. Largely cleared for agriculture during the 18th and 19th centuries in Europe and eastern North America; the second-growth forest visible today in those regions has mostly regrown over the past 100 to 150 years on abandoned farmland. Old growth in this biome is now scarce — remnants like the Białowie‍ża Forest on the Poland–Belarus border, parts of the Joyce Kilmer Memorial Forest in North Carolina, and a handful of fragments in the Carpathians and Caucasus — but it does still exist, and the difference between it and surrounding second growth is striking on the ground.

Tropical moist (rainforest) and seasonally moist forest

The most species-rich biome on Earth and the one with the largest carbon density per hectare. Three big regions: Amazonia, the Congo Basin, and Insular Southeast Asia (Indonesia, Malaysia, New Guinea, the Philippines), plus smaller patches in Central America, the Atlantic Forest of Brazil, the Western Ghats of India, and Madagascar. The bulk of recent global deforestation, by area and by carbon, has been in this biome — the World Resources Institute publishes annual primary-loss numbers via Global Forest Watch and they have been catastrophic in the Amazon and Congo for the past decade^[6]. Old growth here is the world’s biggest remaining stock; its rate of loss is also the fastest.

Tropical dry forest and savanna woodland

A less-celebrated but very large biome: deciduous tropical forests and tree-rich savannas of seasonally arid latitudes. Cerrado in Brazil, miombo in southern Africa, the Central Indian highlands, parts of Mexico, and the Mata Seca region of north-eastern Brazil. Often overlooked in conservation finance because the trees are smaller and the carbon density is lower than rainforest, but biodiversity and endemism are high, and conversion rates are also high — the Cerrado is being cleared for soy at rates that exceed the Amazon’s by some measures.

Mediterranean forest, woodland, and shrubland

Forests of Mediterranean climate — hot dry summers, cool wet winters — including the Mediterranean basin proper, coastal California, central Chile, the Cape region of South Africa, and southwestern Australia. Dominated by sclerophyll oaks, pines, eucalypts, and a rich shrub layer. Heavily modified by millennia of human use; old growth is rare and mostly restricted to remote uplands. Fire is the central disturbance regime. The Cape Floristic Region of South Africa is among the most biodiverse small areas on Earth.

Mangrove forest

The coastal and estuarine biome of the tropics and subtropics, from the Sundarbans of Bangladesh and India to the western Pacific to the Caribbean to West Africa. Tree species adapted to salt water and tidal inundation: Rhizophora, Avicennia, Sonneratia, Bruguiera. Per-hectare carbon storage rivals or exceeds tropical rainforest, mostly held in waterlogged peat below the trees. Disproportionately important for fisheries (nursery habitat for a huge fraction of coastal-zone fish species) and for storm-surge protection. Coastal-development pressure has cleared roughly a third of historical mangrove area; current extent is mapped openly by the Global Mangrove Watch initiative^[4].

What this means for old growth

Two implications worth taking forward:

The rules differ by biome. “Old growth” in boreal forest is a fire-shaped patchwork of multi-aged stands — not the same picture as old-growth temperate rainforest with its emergent giants and continuous canopy. The five structural indicators in the foundational primer apply universally; the values they take vary by biome. A 200-cm DBH old-growth Sitka spruce on the BC coast and a 60-cm DBH old-growth black spruce in central Quebec are both old growth.

The remaining old growth is not evenly distributed. Most of the world’s remaining boreal IFLs are in Russia and Canada. Most of the remaining tropical primary forest is in Brazil, Indonesia, and the Democratic Republic of the Congo. Most of the remaining temperate rainforest old growth is in BC, Alaska, southern Chile, and Tasmania. Where you live in the world has a strong effect on which biome’s remaining old growth your individual action can most directly influence — which is a large part of what the country-shaded map is for.