Question: In my world, I'd like to place a region of permanent forest fire, but I'm having trouble coming up with the minute details of the biome.

For example, my first idea was that it started when a lush, humid forest was exposed to a volcanic eruption, followed by a permanent but slow lava flow, lasting long enough that some plants have adapted and evolved into flora that relies on the constant fire and is itself so slow to burn it can outgrow whatever is lost to the fire.But how plausible is that, biologically speaking?

What is a better approach to creating a permanently burning forest?

Answer: 
Yes there is a better approach
==============================

Gas pockets feeding the fire...

[![https://en.wikipedia.org/wiki/Darvaza_gas_crater#/media/File:Darvasa_gas_crater_panorama.jpg](https://i.stack.imgur.com/rGdcw.jpg)](https://i.stack.imgur.com/rGdcw.jpg)

...or underground coal seam fires.

[![https://www.globalforestwatch.org/blog/fires/embers-under-the-earth-the-surprising-world-of-coal-seam-fires/](https://i.stack.imgur.com/wjT6B.jpg)](https://i.stack.imgur.com/wjT6B.jpg)

The volcano essentially did a naturally occurring version of [fracking](https://en.wikipedia.org/wiki/Hydraulic_fracturing), creating channels to the surface for gas, while land slides exposed coal seams, in some instances laying the coal bare on the surface.

These fires ebb and flow with the seasons, especially effective if they are not the four seasons we have on the temperate parts of the Earth (Winter, Spring, Summer, Fall) but instead Wet and Dry seasons like in the tropics.

The biology would have to...

1. Grow fast
2. Spread Effectively
3. Survive Scorching

Vines and brush — where the roots go deep — would work, the plants survive having the surface parts scorched off because when the water comes, the roots shoot and grow new surface parts and send vines in all directions.

[![https://tallcloverfarm.com/3547/brambles-gone-wild-how-to-remove-blackberries](https://i.stack.imgur.com/H56Gi.jpg)](https://i.stack.imgur.com/H56Gi.jpg)

*Blackberries are infamous for forming [brambles](https://en.wikipedia.org/wiki/Bramble) and spreading quickly*

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So, to summarize:

* The fires are fed by subsurface sources, like gas and coal, not by "new" biomass
* The biology does not so much "adapt" to the fires as it is just very good at spreading and "ignoring" the fires by hiding underground.
**Fire loops around the globe**

Your planet has its landmass as a band around the equator.
(or, alternatively, there's [almost] no open water, poles are covered in snow, leaving only the equatorial zone available to the biomass)

Somehow, there are no large rivers.
(plants still need to get moisture, and there's evaporation, which means there ought to be rain- or snowfall, which usually means runoff, but fires tend not cross large bodies of water... figure this out).

The equatorial zone is dry-ish, and e.g. typically covered by grasses.

The fire started (magically in only one direction), spread the entire width of the equatorial zone, and continues to burn, e.g. westwards.

By the time the fire has reached it's origin, that part has already sprouted new mature (and dry) grass, with seeds either preserved deep in the soil, or spread by wind, or spread by birds or even migratory animals.

Thus the fire continues to burn in a circle around your planet.

The long-lived plants died out, what remains are grasses and maybe shrubs.

The plants have therefore evolved to live in this cycle, and so have the insects and the animals.

P.S. some math

Fire boundary speed: 10% of wind speed (<https://link.springer.com/article/10.1007/s13595-019-0829-8>)

Typical wind speed: 10km/h
(<https://sciencing.com/wind-speeds-tropical-rainforest-23367.html>)

Thus, for a planet the size of the earth, the fire circumnavigates in ~4½ years, leaving enough time for plants to germinate, go through several life cycles and accumulate large amount of stale, dry biomass.