Severe weather, in the form of tropical storms and hurricanes, also affects the structure of the Everglades. Between 1871 and 2003, 40 tropical cyclones struck the Everglades, usually every one to three years.[10][11] These storms alter the coastline, flush decaying vegetation from estuaries, strip weakened branches from trees, and disperse seeds, pollen, and plant material.[12] Hurricane Donna in 1960 affected 120 square miles (310 km2) of mangrove forests by depositing marl over the roots and depriving the trees of oxygen. It also eradicated orchids, bromeliads, and other epiphytes that once flourished in the mangroves; their reappearance may take a century or more. Donna also significantly spread buttonwood, saltwort, and glasswort, and epiphytes began to grow in new areas.[13] Although the lasting effects remain to be seen, Hurricane Andrew in 1992 also destroyed mangrove forests and snapped slash pines in half. However, regrowth occurred quickly, and sand deposited by the storm surge improved nesting conditions for crocodiles and sea turtles.[14]