just means they burn every year, there's less fuel load so the fire burns out before larger trees have time to burn.

There are several techniques which control traditional burning temperature. They're done at a time of year when the air and soil are only slightly damp, to produce the best temperature, and the conditions on the day are assessed, including the health of the soil. An experienced Elder can tell how hot a fire will burn just from holding a clump of dirt. That's because of the advanced science which these cultures have honed over thousands of years. The burning is also done in a mosaic pattern, a little piece at a time, so the geographical size is controlled and any changes in conditions can be responded to. And before the full burn begins, Elders will make some smoke to warn the animals that a burn is coming. Insects will climb to the treetops, because evolution has taught them it's safe up there.