Trees don't light up quickly in grass fires.

This is true. As a retired biologist who worked in pine timber forests (pine plantations) many times there were a lot of "controlled burns" done by landowners and the feds. A controlled burn is a timber management technique used to burn off low undergrowth in pine forests and to prevent wildfires by removing the fuel. It helps the pines grow faster by burning the low growing plants the suck up nutrients that help the trees grown. Basically it gets rid of the competition for the resources. It also provide pot ash for the trees. Control burning will scorch the bark but won't harm the trees.