General Information

Ironically, the area that is now Muir Woods National Monument was saved from logging because in the 19th century it was just too hard to get there. Located just 12 miles from the Golden Gate Bridge, this old-growth stand is now wonderfully accessible to anyone who wants to take a walk in the woods. If you’re looking for a backcountry experience, you’ll have to go elsewhere – around 1.5 million people visit the park each year. If you want a leisurely stroll in an ancient forest to see some of the tallest and oldest trees in the world, then Muir Woods is a great choice.
The 560-acre (226 hectare) park is 256 feet (78 metres) above sea level. It is surrounded by the 6300 acre (2400 hectare) Mt. Tamalpais State Park, and is part of the Golden Gate National Recreation Area, which stretches from the Presidio in San Francisco to 28 miles north at Point Reyes.

The park is cool, shaded and moist year-round, due to summer coastal fog and winter storms. Daytime temperatures average between 40 and 70 degrees F (5–21 Celsius), with an annual rainfall of about 40 inches (1 metre). Most of the precipitation occurs during the winter and spring months.
The Trees
The Latin name for the coast redwood is sequoia sempervirons, the ever-green sequoia, often translated as "ever-living." Redwoods are survivors from 250 million years ago and continue to be the dominant tree in the forests where they grow. Around 25 million years ago they could be found across the entire northern hemisphere, making them older than the Rockies or the Swiss Alps. The trees have changed very little since then. As the earth changed over this time the redwoods did not adapt well to meet the new challenges: they retreated over the years to remain only on the far west of the old continent.

They belong to the swamp-cypress family of conifers, or cone-bearing trees.
From over a dozen original species of redwoods, only three have survived to the present day. All of them live in mostly protected areas:

The Dawn Redwood lives in central China and grows up to 140ft (42 m) tall and 6 ft (2m) wide.

The Giant Sequoias, living in 75 groves in the Sierra Nevada Mountains of eastern California are the largest trees on earth in terms of volume, growing up to 270ft (82m) tall and 36ft (11m) wide. They live up to 3500 years. The largest is the General Sherman and lives in Sequoia National Park, just south of Yosemite.

The Coast Redwood grows only along a 475mile strip of northern California coastline, from Big Sur in the south to just over the Oregon border in the north.
They will not survive within reach of the salt spray from the ocean. The breezes dehydrate the trees, killing the foliage. They also do not survive too far from the ocean - they are not found further than 42 miles inland. The trees require a cooler climate and need the moisture from the coastal fog to survive the dry Californian summer. The highest elevation they will grow is 3200ft (1000m).
They grow best where the climate is warm and moist, the ideal temperature being around 65 degrees. Only in the far north of California is there enough rainfall to sustain the trees. Elsewhere, the trees rely on the California coastal fog for moisture during the summer months. In Muir Woods, the annual rainfall averages 40 inches (1 metre) – around 90% of it between October and March. The summer fog keeps the valley cool, reducing evaporation from the needles. Droplets also condense on needles and create ‘fog-drip’ which is the equivalent of around 10 inches (25 cm) of rainfall per year.
The soil in the redwood forest is rich and high in organic content. The trees prefer sandy soil, especially in areas with high alluvial deposits along riverbeds. They do not survive in soil with high mineral content.

The trees also grow far better on the north slopes. If you walk into Muir Woods from the entrance and follow a trail up to the right, the redwoods will very soon give way to a drier mixed evergreen forest. On the opposite (north) slope the redwoods continue for around half a mile.

The Coastal Redwoods are the tallest trees on Earth. The National Geographic Society measured the ‘Tall tree’ in Redwoods National Park in 1963 at a height of 367.8 feet (112 m). The tallest trees are taller than a football field is long, and taller than the Statue of Liberty or a 35-story building. The tallest trees grow in the low elevation slopes within a few miles of the coast. Here, they are protected from strong winds and are closer to water and moisture-laden fog. Under ideal conditions they will grow at 2-3 feet (60 – 90cm) per year. On average it takes a tree 200 years to reach full canopy size. They are also one of the widest trees on earth, reaching diameters at their base of over 30 feet, (9.3 meters) and one of the oldest trees in existence, the oldest on record having lived 2,200 years when it was cut down. The redwoods in Muir Woods are relatively young – none are over1000 years old. There are two explanations for this – one is that the environment is too dry and windy and the soil is too thin. The second theory is that the whole forest burned down around 1000 years ago.

The average redwood lives for 500 – 700 years. Most trees die from wind throw (falling over) – one of their few weaknesses is that their roots only penetrate 8 – 12 ft and spread 60 – 80 ft. They will intertwine with roots from other trees for strength. Others may die from excessive fire damage or a severe flood. The trees have no known diseases and do not suffer from insect damage. The color of the tree comes from tannin and related compounds in the heartwood and bark.