Old-Growth Nature at Muir Woods
Muir Woods is the only old-growth coastal redwood forest in the Bay Area and one of the last on the planet. Roughly 2 million acres of forest like this once covered a narrow strip along the coasts of California and Oregon. Today, 97% of that original range has been logged or altered. Most coast redwoods now grow in second- or third-growth forests or managed timber plantations. Muir Woods was spared that fate thanks to William Kent, who bought the canyon and donated it to the federal government in 1908. What remains is a rare, accessible example of a forest system shaped almost entirely by time.
What Defines an Old-Growth Forest?
Old-growth is hard to pin down by size or age alone. Those vary by species. Four characteristics, developed slowly over centuries, together define an old-growth forest:
1. Large Live Trees
Large live trees are a product of time and also the reason the rest of the forest works the way it does. The coast redwoods in Muir Woods are the tallest living things on Earth. The tallest specimen here reaches about 258 feet (roughly a six-foot person stacked 45 times). Further north, coast redwoods can reach 379 feet, 74 feet taller than the Statue of Liberty.
The average age of the coast redwoods in Muir Woods is 600 to 800 years, with the oldest at least 1,200 years. That is still young for the species. Coast redwoods can live up to 2,200 years. At that size and lifespan they anchor the carbon, nutrient, and water cycles of the entire forest.
2. Multi-Layered Structure
Old-growth forests commonly have three distinct vertical layers. Think of it as a house: the herbaceous layer is the carpet, the understory is the furniture, and the canopy is the roof. Each layer supports a different community of plants.
3. Dead Trees (Downed Logs & Snags)
The National Park Service once cleared all dead material from the forest to “keep it clean” and reduce fire threat. That practice ended once ecologists realized how central dead wood is to an old-growth system.
Fallen trees absorb rainwater and keep the soil moist. They shelter insects, amphibians, and small mammals. As they decompose, they release nutrients slowly and steadily. Most importantly, they serve as nurse logs: leaves and cones pile up on top, turn to soil, and new seedlings germinate in the rich humus. Trees that fall into Redwood Creek create falls and pools, slow flood surges, and add habitat for salmon.
Snags (dead trees that remain standing) are equally valuable. Insects feed on them and support pileated woodpeckers. Several bat species roost under loose bark and in hollows. Hawks, owls, and eagles perch on the tops. Bobcats find shelter in the larger cavities. Eventually a snag falls and becomes a downed log.
The saying at Muir Woods: only half a tree’s life is spent standing. The other half is spent on the ground.
4. Interdependent Communities
Every component of an old-growth forest depends on the others. Large live trees pull water and nutrients from downed logs. Redwood sorrel and California bay laurel need the layered canopy for protection and filtered light. In Redwood Creek, the redwoods stabilize the banks and keep gravels clean; the salmon that spawn in those gravels need cold, constant water that only the undisturbed canopy provides. Redwood Creek is one of the last California streams with its native coho stock, largely because the forest around it has never been broken.
“Among the scenes which are deeply impressed on my mind, none exceed in sublimity the primeval forests undefaced by the hand of humans… no one can stand in these solitudes unmoved.” (Charles Darwin)
Ferns, An Ancient Plant Family
The bright green fronds at knee height along almost every trail are ferns. Ferns and “fern allies” are vascular plants (they have xylem and phloem) but they do not produce fruit or seeds. They reproduce by spores.
Muir Woods is home to 13 species of ferns from six different fern families, plus horsetails and club mosses. Ferns have existed for more than 300 million years, predating dinosaurs, and grow on every continent except Antarctica. Common species here include western sword fern, lady fern, maidenhair fern, and gold back fern. See our Plants page for the full species list.
Mushrooms, The Hidden Network
Over 200 species of fungi live in the old-growth forest and surrounding hillsides. After the first serious winter rains, a slow walk produces caps the size of pinheads to the size of dinner plates, in brilliant reds, purples, and golden oranges.
What you see above ground is just the fruiting body. The real organism is the hyphal network threading through soil and dead wood, breaking down fallen logs and feeding nutrients back to living trees. Fungal hyphae form symbiotic connections with the rootlets of almost every plant in the forest: the fungus increases the tree’s ability to absorb water and minerals, and the tree supplies photosynthetic sugars in return. Neither side can function fully without the other. In other words, the mushroom you bend down to photograph is the visible tip of the underground network that keeps the entire forest alive.
Frequently Asked Questions
What is the difference between old-growth and second-growth forest?
Old-growth forests have developed over hundreds of years without major human disturbance and contain large live trees, multiple vertical layers, significant dead wood (downed logs and snags), and interdependent plant and animal communities. Second- or third-growth forests have been logged at least once and regrown. They lack the complexity and fallen woody debris that old-growth depends on.
How can a dead tree be important to the forest?
Dead trees, whether standing (snags) or fallen, shelter wildlife, hold moisture, release nutrients as they decompose, and act as nurse logs where new seedlings germinate. Fallen logs in Redwood Creek create pools and habitat for salmon. The saying at Muir Woods is that only half a tree’s life is spent standing.
Why are there so few flowers and fruits here?
Redwood tannins repel insects and the canopy’s deep shade limits the light reaching flowering plants. That limits pollinators and fruit production compared with open sunlit habitats.
Are the ferns really 300 million years old?
The fern lineage (the family of plants that reproduces by spores rather than seeds) dates back more than 300 million years. The individual ferns you see in Muir Woods are of course much younger, but the species are ancient.