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: 
Quite a few plants - mostly trees - have evolved to [require fire to germinate](https://www.britannica.com/list/5-amazing-adaptations-of-pyrophytic-plants)! This seems like the seed of what you might need - combined with a geography that's appropriate.

Annual fires are common in several areas; but what makes them not permanent is the weather cycles. This could possibly change, though, and become permanent in a way - if your geography allows the fire to "travel".

Imagine a Pangea type world, a single large continent taking up 30% of the surface area of the globe, say. If that Pangaea is ringed by forest, and if the weather patterns comply, it's possible that you could have a *travelling fire*, which, say, is on the east from March to June, the north from June to September (northern summer), the west from September to December, and the south from December to March (southern summer). (West/East could easily switch, of course.)

The fire could literally travel in that way, ringing the globe every year. Then, cultures would possibly come up that would worship the "renewing fire", have rituals based on it, etc.

I'm not sure what you'd need for the weather to work out this way (you'd need the rainy season to not coincide with the fire, but to coincide with the *end* of the fire, for example), so that part might be a bit handwavy - but it seems like a possibility, if you're not going into meteological science fiction, at least!
**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.