Fire Cavities: Indicators of Past Fire Regimes in Coast Redwood
Basal cavities are ubiquitous in coast redwood. They are common in the humid coastal forests of Del Norte County and the dry forests of the southernmost extent of the species range. Tourists delight in them, bats take shelter in them and fire managers see them as an indicator of tree weakness and decline. Known locally as "goosepens", these basal hollows can be large enough to hold livestock, as they did for early Euro-American settlers. For centuries, large cavities also provided the occasional shelter for Native Americans. Because they often become so large, basal cavities fascinate us, as they add to the uniqueness of the coast redwood forest type.
How do these unusual features form? Are they a threat to the health and viability of coast redwood trees, or are they an inevitable quality of the species, given its evolutionary environment?
As shown at right, the key reason why these cavities exist is fire. While coast redwood likely evolved with fire, their ability to survive fires is decidely mixed.
We have all heard that coast redwood have adaptations to deal with fire--like the ability to sprout at their base and along their bole and branches and their often thick fire resistant bark and heartwood, but the highest flame lengths that occur during a coast redwood forest fire are inside the trees. Something about cavities make them appear to be the tree's Achilles' heal. By understanding the process of cavity formation and development, we gain insights not only into the reasons why coast redwood collapse, but an ultimate source of compositional diversity of the forest and the tradeoffs associated with fire management alternatives.
Cavities result from a complex interaction between repeated fires and decay associated with two fungi, Poria sequoiae and Poria albipellucida. Coast redwood heartwood is remarkably decay and fire resistant compared to other tree species, but these fungi thrive in sapwood behind newly formed fire scars.
Thirty years after sprouting from a logged stump, the redwood tree at left scarred across a quarter of its basal circumference. Now, three years after the fire, healing has partially closed over the scar, but not quickly enough to prevent formation of decay behind the scar and under the bark. Had this tree not been killed by a windstorm in 2007, the next fire may have burned out this decay and initiated a cavity. Concave features lend themselves to hotter fires because radiant heat is reflected off the opposing walls. If the scar surface would entirely heal over, the likelihood of cavity initiation is decreased. Given the healover rate, these small scars would have closed after a decade which was a common fire interval in this coast redwood forest.
Even when healing does close over scars prior to the next fire, the thin bark of the healover tissue readily succumbs, resulting in a line of fires along one section of the tree (right). Recurrent cycles of scarring, decay and healing associated with low intensity fires lead to clumped pockets of decay. These are easily excavated by hot fires, leading to a sudden increase in the basal cavity's volume. As cavity's burn, the high temperatures that result from radiant heat within the oven-like cavity can lead to the combustion of sound sapwood and heartwood. Without this radiant heat, exposed redwood heartwood is normally just scorched.
Coast redwood trees with cavities will collapse when cavities enlarge to where there is insufficient holding wood to support them. About 30 percent of old growth redwood trees that exist in the northern redwood region have some form of externally visible cavity, and nearly all redwood trees have some interior decay associated with past fires.
The risk of failure (i.e., mortality) depends on the type of cavity that the tree has, its internal decay attributes, and the intensity of the fire itself.
Classification of Redwood Basal Cavities
Basal cavities differ in terms of their height, width, depth and internal structure. Shallow cavities (Type 1) form from a single fire event, typically when there is a large amount of fuel at the tree's base. Radiant heat from adjacent tree (usually a sprout) can also lead to high, shallow scars. Deeper hollows normally reflect burn out of decay embedded within the tree, and they therefore provide indicators of repeated cycles of fire and decay lasting for centuries. Types 2-4 reflect the relative extent of burnout, leading to Type 5 trees that are perforated on two sides and are therefore highly vulnerable to collapse.
While this classification provides a rough indicator of the risk of an individual tree falling during a fire event, the extent of cavities within a stand provides an indicator of the past severity and/or frequency of fire. The Redwood Cavity Index (RCI) provides a plot-level measure of fire tied to tree size distributions. (Large trees are more likely to have cavities, given their age). The numerical cavity classes (shown above) are determined for 12 trees at a site over 1m, then a site score is obtained by taking the average. The percentage of trees over 1m dbh to have a cavity provides an alternative measure.