How to assess health of burned trees in Sonoma County

Many trees that graced our homes and hillsides have burned. The leafy green corridors that line many of our roads and trails and provided relaxation and beauty are now brown or blackened. Sometimes the only evidence there was a tree is a gray ghostly outline of ash on the soil.

Trees, the deep green domes of live oaks, white barked sycamores, fragrant bay trees, and soft boughed firs have been burned to various degrees by the fires that swept through our neighborhoods and landscapes. In our neighborhoods many trees are blackened sticks, and it is hard to picture the welcome shade they provided and soft green branches waving in the breeze.

Despite the gloomy appearance of most neighborhoods and hillsides, some oaks are already sprouting bright green leaves from burned twigs-seemingly miraculous in a brown and black scene. Grasses, ferns and bulbs are growing with the fall rains - bright chartreuse against blackened soil, reminding us that life will return.

Months after the fire, how are our trees faring and what can we expect in the future from them?

Trees have endured different degrees and duration of heat, and suffered with different degrees of severity. Some trees have leaves scorched brown, in others leaves have been burned off, and many have had leaves, twigs and all smaller branches burned and are charcoal black-all indicating different degrees and length of heat.

Trees respond differently to fire. Some have adaptations such as thick insulating bark, or smooth bark that doesn't allow flame to travel, others shed lower limbs as they grow lessening the ability of fire to travel up to the crown. Blue oaks have open leaf canopies, and California buckeyes defoliate in summer. Some trees like sycamores have thick or leathery leaves that tend to scorch, not burn. Others like maples have thin leaves with high moisture content, and shrivel when exposed to heat. A number of trees ,like our native oaks, have the ability to sprout from the base after a fire.

DEPENDS ON ENVIRONMENT

Fire behaves differently in different environments.

Grassland fires are generally very fast burning with lower heat severity due to a lack of hot burning fuels.

In these type of fires many trees may just be scorched.

In areas with an understory of shrubs such as chamise, Manzanita, coyote brush, and weeds such as Scotch broom - fires will be hotter.

Densely planted or growing trees, or those with large amounts of flammable bark and leaf litter such as eucalyptus create a hotter burn due to continuous fuels.

In urban settings, the houses themselves form the majority of burnable fuels and temperatures can be severe. The same trees that escaped with scorched leaves in more wild areas, in urban settings are often charred remnants of themselves.

ROOTS AND CAMBIUM

The main determination of survival for the above ground portion of a tree after a fire is whether the upper roots and cambium layer survived.

The cambium layer is the thin layer of growing tissue just under the bark that grows conducting cells and new wood.

If the cambium layer is dead, or more than 50 percent damaged, the tree will likely not survive or thrive in the future. The severity and duration of heat, and bark thickness determine whether a trees cambium layer survives a fire or not.

Assess your trees to determine extent of damage.

There may be both dead and alive portions of the trees. Check the viability of the cambium layer in the trunk and main branches by chipping a quarter sized piece of bark off in a few places.

If the cambium layer is white or pink and moist it is alive. Yellowish, or brown dry wood indicates dead wood.

Trees with just scorched leaves may be alive, though trees with very thin bark can be killed by low severity fires.

If twigs are dry and break easily, they are dead. If they are green and pliant, they are alive.

Cut back into twigs and smaller branches to see if there is any green and moist tissue.

Start with the bottom up and check several places. Brown or dry wood indicates that that portion is dead.

WHAT TO DO

Prune off dead branches to help tree heal at those sites and to prevent any personal injury from branches falling later.

Most conifers, except ponderosa pines will not recover from scorched crowns and do not resprout from burned branches.

Young Douglas fir, bays and many oaks can survive fire at ages as young as twelve years.

Deciduous broadleaf and other trees may resprout from branches, the base of the burned branch or at the soil level from latent buds.

If they do, they will likely need subsequent pruning to thin out the mass of new growth. You can wait and let them grow, then when the new shoots are about 2-feet long, you can select the strongest and best placed branches to grow for the future and cut the rest out.

Native oaks and redwoods can be fairly severely wounded by fire and still live many, many years. Big trunk wounds from long ago fires are a common sight. Severe burns can cause structural defects and tree failure. If over 40 percent of the cambium layer is compromised this can lead to tree failure in the future. The capacity of oaks to resprout from the base of tree, trunk and branches is strong.

How do you help trees recover? Fall rains are the best medicine for trees. Mulch with composted greenwaste or woodchips to boost soil organic matter levels. It helps develop soil structure so water and air can penetrate. Avoid parking under trees, it compacts the soil and damages roots. For trees that need summer water, begin before soil is completely dry in spring.

If you can, wait until spring or later to make decision about whether to remove a burned tree. See if new leaves appear. Oaks may take 3 to 6 years to sprout.

Consider engaging a professional arborist to evaluate your trees.

Information resource: bit.ly/2zndV97

Kate Frey's column appears every other week in Sonoma Home. Contact Kate at: katebfrey@gmail.com, freygardens.com, Twitter @katebfrey, Instagram @americangardenschool