Why would a forest ranger start a fire among pine tree?

Because pine cones release seeds when they explode. They explode because of the high temperatures the fire gives

Answer:

Fires are often set as controlled burns to create firebreaks as a safeguard against the spread of future fires. Firebreaks are area where the fuel (dead forest material and dead trees) have been removed and a new fire cannot cross it.

The lack of forest fires in managed forests can be detrimental as the dead wood provides many opportunities for extended forest fires.

Fires are also started to remove old trees from an area. This allows a diverse range of vegetation st grow in the burnt over area and support a more diverse forest ecology. This diverse ecology was prevented from happening by the shade and nutrient removal caused by the old pine trees. Studies show that new growth can be established in a burnt area within a few weeks and the area recovered (without the old trees) within two years.