The ferocious march of ice and cold chased warm and wet biomes toward the equator at breakneck speed. Most perished as the world grew cold. Mangrove forests backed into what barely tolerable corners of the world were left, waiting for icy death to take them. The frigid wave grew closer, its hands clutching at the forests' throats, but as it did, it weakened. The world found peace in its chilly new state. As it did so, its residents began to find stability, and against the odds, mangrove forests were there to see it heal.
Mangrove forests are perhaps the closest thing to a tropical biome the world has left, though they can also be subtropical, and doubtless would qualify as such on the old planet. They cling to precious life in only three locations warm enough to support them: The Amazonian Mangrove Forest, the East African Mangrove Forest, and the Dual Mangrove Forest. All are tightly bound to the equator, and all but the Dual Mangrove Forest are associated with a single continent, which instead is shared between Sundaland and Australia (see map). They are the last bastions of what is warm and wet on a desolate planet — from their point of view, anyway.
Losing them completely would be a horrible loss — even for a planet robbed of every last of its tropical biomes — for mangrove forests house some of the most productive ecosystems on Earth. The innumerable roots of mangrove trees and shrubs that typify them serve to hold silt in place, building up a soft soil and making them essential for maintaining the shoreline. They store much carbon in this sediment, trapping it beneath layers of dirt. They produce biomass from light and carbon so effectively that the ecosystem is constantly starved of nutrients, and this is despite all the nitrogen-fixing bacteria that live in their oxygen-poor soils and elsewhere. The loss of most of the world's mangrove forests actually helped the modern ones to live, for in dying, they stopped sequestering carbon, leaving more in the air to hold in heat.
Separated by impassable land and water, these relict biomes have gone their own ways, islands surrounded by hostile environments. There is some interchange though, and precisely in one direction: From the Dual Mangrove Forest to the East African Mangrove Forest. The East African Mangrove Forest was formed by the hasty retreat of the mangrove forests encircling the Indian Ocean, from Africa's east coast through Australia's west coast. The currents in the Indian Ocean flow counter-clockwise, providing precious warm water the East African Mangroves rely upon. However, this current also sometimes brings immigrants from through the Indonesian Strait. While difficult, a lucky population of animals may be disastrously ripped from the warmth of the Dual Mangroves, but be fortunate enough to survive the journey to East Africa. In this way, the two mangroves have very occasional transmission, but only one way: Resisting the current for the return trip is nigh impossible.
Despite this, all three mangrove forests still have much in common in the modern day. The equatorial sun is warm and the air moist and wet, ranging from quite hot and muggy in the summer to a welcome escape from the chill in the winter, even if frost is still possible. The coast is hard to define, covered in a tangle of flora with their characteristic mangrove roots. Some of these evolved not before the cold snap, but afterward, filling the gaps left open. Innumerable birds frequent the treetops, all kinds of mammals and reptiles stride through the mud, insects flit and crawl about, and fish seek protection from the open ocean in the roots. Land crabs clamber through the branches over the water; it is only because the mangroves survived that they, too, did. Beneath the waterline, rising up and falling down twice a day with the tides, a soft, silty mud houses burrowing animals whose constant action helps bring some of the nutrients buried back up to the surface. On occasion the oxygen levels in the entire water column will collapse, an inevitable disaster that the residents here have to avoid. Still, biodiversity is higher here than perhaps anywhere else.