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How Redwoods Grow and How They Burn

posted May 23, 2015, 4:57 PM by Joe Christy
We’re all familiar with the many basal crown sprouts that often ring the base of a healthy redwood and usually envelop a redwood stump. In a second- or third-growth forest they give rise to the fairy rings of mature trees surrounding a depression in the ground where a stump from a felled previous generation has rotted away. In some areas of our mountain these depressions can 25 feet across.
In April I visited the UC Landels Hill-Big Creek Reserve in Big Sur and had the opportunity to study a minimally altered, albeit drier, redwood ecosystem. I was struck by how different the trees looked and realized that almost all the redwoods that we see here in the Doon are, despite their size, quite young. In Big Sur there are very few straight, conical redwoods and those are confined to deep canyons protected from the brunt of winter storms. The typical redwood there has had the top blown out of it many times, resulting in a candelabra shaped or even flat crowns. The largest trees almost all have catfaces, the charred interior cavities that you can find in some of the neighbors of the Big Ben Tree in Fall Creek State Park, just east of the airport. How do catfaces get there?
The crown sprouts, as they grow into small trees, accumulate a lot of litter from the mother tree, creating a hump of duff that can bury the bases of the lowest limbs of the mother tree. One of the many fire adaptations of redwoods is their thick, fibrous bark, which, while it ignites quite readily, usually burns itself out as the low intensity fire it spawns quickly climbs the tree. The cone of litter caught in the crown sprouts however, burns slowly enough to burn through the bark of the young poles, igniting them and producing a much hotter fire, which can burn through the thicker bark of the mother tree and kill the growing cambium layer beneath. Given enough time, the cambium that survives under neighboring incompletely burned bark will grow across this scar.
The fire stimulates the growth of even more crown sprouts that begin accumulating a new hump of litter. If fire returns before the scar is healed, the catface will burn even deeper into heartwood of the tree. Most of the dead redwoods in Big Sur have broken off near the base of the trunk, which will show the remains of a large catface, deepened across the centuries.
Working on the Warrenella Shaded Fuel Break, I’ve had the chance to observe two remarkable secondary manifestations of this growing/burning cycle.
In areas of the Lockheed Fire scar where it burned though tanoak challenged redwood forest, the redwoods will all have charred bark 20 to 30 feet up the tree, but while there are plenty of charred tanoak logs on the ground, the surviving tanoaks trunks are uncharred since they had relatively little litter accumulated at their bases.
While removing a sickly stunted redwood to interrupt vertical fuel continuity, we discovered that though bark completely covered the trunk, the center of the tree had once been a catface. I could see how the new growth had curled in on itself until the two sides met, leaving an enclosed cavity which was full of decaying wood and even contained some branches growing inward, bleached from failing to reach sunlight.