Savanna

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

The savanna is often mistaken for a simpler cousin of the desert, a place that is merely hot and dry. It is neither. What actually draws the boundary of a savanna is not temperature at all, since mean monthly temperatures sit above 18C year round with only a modest swing between wet and dry seasons, but rainfall, and specifically the shape of rainfall over a year. A savanna gets real rain, 800 to 1600mm a year by some measures, enough to grow grass tall enough to hide a lion. What it does not get is rain spread evenly through the year. Almost all of it falls in a single wet season, and for months at a stretch, sometimes with under 60mm falling in the driest month, almost none falls at all.

That seasonal whiplash is what keeps a savanna from becoming a forest. Trees can survive the dry season, but not enough of them can establish fast enough during the brief wet season to close a canopy before the rain shuts off again, so the biome settles into an equilibrium of scattered trees standing over a nearly continuous grass layer, rather than tipping all the way to forest or all the way to bare ground. Fire is part of what holds that balance in place. Grass that cures out in the long dry season burns readily, and fire on savanna, including the African savanna, is common enough to count as an ordinary feature of the biome, one that keeps woody growth from closing back over the grass rather than a rare event.

The biome forms a band on both sides of the equator, sitting between the closed canopy tropical rainforest nearer the equator and the true desert farther away, on every continent that reaches those latitudes: East Africa's open plains, Brazil's Cerrado, northern Australia's tropical savanna. Two ecosystems already on this atlas, the Serengeti and the Cerrado, were classified under the temperate Grassland biome before this entry existed; both are correctly savanna, and the record now reads that way.

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

Two of this atlas's best known ecosystems, the Serengeti and Brazil's Cerrado, were filed under the Grassland biome from the founding wave onward, and by the ordinary meaning of the word, that was not unreasonable: both are open, sun struck expanses of grass stretching to the horizon. But ecology draws its categories from climate and structure, not appearance, and by that measure both places have always been something more specific than grassland. The Cerrado's own description already called it, without qualification, the most biodiverse savanna ecosystem on Earth, and carried the alias Brazilian Savanna. The Serengeti's own description already called it a savanna grassland ecosystem. The biome record simply had not caught up to what the entities themselves already said.

The distinction matters because temperate grassland and tropical savanna form under different climates. A savanna stays warm to hot year round and has a rainy season and a dry season, with most of its rain falling in just a few months; that long dry season is what keeps the grassland from closing over into forest. A temperate grassland, the prairie of the American Midwest or the steppe of Central Asia, is instead defined by real seasonal swings in both temperature and precipitation across the year, a different climate pattern that produces a superficially similar grass dominated landscape by a different route.

The Serengeti's own wildfire, nitrogen cycle and decomposition facts, and the Cerrado's own wildfire demonstration, already described exactly this savanna dynamic before either was reclassified: seasonal drought curing the grass into fuel, fire recycling nutrients and holding back tree cover, a new flush of growth on the next rain. Reclassifying both ecosystems under the newly minted Savanna biome does not change a single fact already on either page. It corrects the shelf they sit on to match what was already written on the label.