Trees use shade to reduce the competition from ground cover plants and to reduce potential fuel for wildfires that might destroy them. Here is a small remnant of live oak forest whose canopy has shaded out most of the light needed to sustain much plant growth at or near ground level. The small spindly trees are mostly hackberry trees that are desperately trying to reach the sunlight. These hackberry saplings were planted by birds, whose droppings contained live hackberry seeds. There are few grasses or forbs growing in the understory. Keeping out these grasses and forbs limits potential wildfire fuel; therefore, wildfires had difficulty penetrating the forested areas. This battle for control of territory by trees, grasses and forbs has been waged for many millennia. Trees also defend against fires by growing a thick bark that resists fires.