True moss belongs to the group Bryophyte, which doesn't flower or have a vascular system to suck nutrients out of the ground. Spanish moss, on the other hand, is an epiphyte. It absorbs all its nourishment right out of the air. So though it may seem to be taking over the oak trees in City Park, Spanish moss doesn't harm them at all. In fact, it actually repels insects, saving the trees from six-legged invaders.
Spanish moss is a study in contradictions.
Despite its hardiness, it is exceptionally sensitive to pollution. Because even dead limbs are kept intact by the plant's vascular system, Spanish moss can live to be centuries old. But because it relies on clean air from which to draw its nutrients, heavy pollution can be a real killer. Spanish moss is often used as an indicator of pollution levels in cities. The ancient trees that canopy St. Charles Avenue and spruce up City Park grow to massive size, typically 50 feet high and 150 feet wide.