What is "nurse tree"

A nurse tree is a larger, faster-growing tree that shelters a small, slower-growing tree or plant. The nurse tree can provide shade, shelter from wind, or protection from animals who would feed on the smaller plant.

For example, the Norway Spruce (Picea abies) and Larch (Larix) can function as a nurse for hardwoods. In the Sonoran desert, Palo Verde, Ironwood or mesquite trees serve as nurse trees for young saguaro cacti. As the Saguaro grows and becomes more acclimated to the desert sun, the older tree may die, leaving the saguaro alone. In fact, as the Saguaro grows larger it may compete for resources with its nurse tree, hastening its death. Consequently, young saguaros are often seen near trees, but old saguaros are not.