Why are these places called "forests"?

Kelp are large brown algae that grow in cool, fairly shallow water close to the shore. Where many grow together, they form dense groups that rise from the seabed like trees, and so these places are called kelp forests. They are found mainly along the west coast of North America, in cold water that is rich in nutrients.

Kelp needs sunlight to make its food, so it rarely grows deeper than about 15 to 40 metres. Within that depth, it grows astonishingly fast: in the best conditions, giant kelp can grow up to about 45 centimetres in a single day.

A kelp forest is one of the richest places in the ocean. It gives food and shelter to thousands of kinds of fish, invertebrates and marine mammals, and few other ocean communities hold a greater variety of plants and animals. Seals, sea lions, whales and sea otters live in or visit kelp forests, and birds such as gulls, terns, herons and cormorants feed there. The thick blades of the kelp protect young animals from predators and from storms.

Kelp forests change with the seasons. Winter storms and El Niño events can tear the kelp and wash it away, leaving the forest ragged, but it grows back each spring.