Post-Fire Regeneration of Black Walnut

Oaks ‘n’ Folks – Volume 13, Issue 1 – February, 1998

California black walnut is found only in southern California and offers important habitat for many of California’s vertebrate and invertebrate species. Leaf production and flowering occur at approximately the same time in the spring, and mature fruit is abundant by late summer. By November or December, this tree begins to shed its leaves and mature fruit, supplying invertebrates and reptiles with shelter beneath the leaf litter; California ground squirrels and gray squirrels are provided food to carry them through winter; the foliage provides food for browsing herbivores; and the canopy and hollows in the trunks offer roost sites, shelter and nesting habitat for songbirds and raptors.

Many trees that are two to three decades old show signs of “heartrot.” A tree afflicted with this condition, caused by wood boring insects and fungi, has hollow spaces in its trunk. Tree loss can be caused by harsh winter storms, from the occurrence of heart rot or an infestation of mistletoe,or from a combination of these factors. After a tree dies, new sprouts commonly are seen within a month at the base of the desiccated trunk; this is why individuals of this species of tree can have multiple trunks of varying ages.

California black walnut is becoming increasingly scarce in the foothills of southern California. Recently, this endemic species was proclaimed by The Nature Conservancy Heritage Program as “rare,” and the walnut woodlands as “very threatened.” The remaining large stands in southern California are threatened by spreading urbanization, livestock overgrazing, and the proliferation of alien plant species.

Hopper Mountain National Wildlife Refuge (HMNWR), managed by the U.S. Fish and Wildlife Service, is located approximately six miles north of Fillmore,California. The 2,471 acre Refuge, which is closed to the public, protects habitat for the endangered California condor and functions as a base of operations for Refuge staff engaged in California condor reintroduction efforts. The Refuge contains more than 1000 acres of chaparral and coastal sage shrub, 900 acres of grasslands, 350 acres of oak and walnut woodland,and 169 acres of riparian habitat. Three acres of fresh water marsh also are present on the Refuge.

Within the oak and walnut woodland is one of the largest remaining stands of California black walnut in southern California. Recently, Refuge personnel have become increasingly concerned about the health of the walnut community because of the absence of seedlings and an infestation of mistletoe. Sage and introduced annual grasses choke the understory, presenting little opportunity for new growth and offering dense fuels for fires. Until 1997, fire was absent from the Refuge for more than 72 years.

On 5 August, 1997, a fire started in the grasslands north of the Refuge and within a few days, all 2,471 acres of the Refuge burned. Very little is known about the benefits or detrimental affects fire has on California black walnut or about vegetative regeneration by walnut. The fire presented Refuge personnel the opportunity to study the effects of the fire on this rare, endemic tree.

A few weeks after the fire swept over the Refuge, photo transects were established to document the regeneration of the Refuge’s woodlands, grasslands,and marsh. Within 36 days, walnut trees that had been subjected to light or moderately hot fire supported eight to 12 inches of new growth from stumps of burned trees; some showed evidence of new growth in their canopies. Areas that had been subjected to extremely hot fire required more time to regenerate.

In early December, graduate student Victoria Tenbrink from California Polytechnic and State University, Pomona, under the guidance of Professor Ronald Quinn, initiated a study to examine the post-fire regeneration of the walnut community at HMNWR. Tenbrink has used identification markers to identify numerous, individual sprouts at three sites to collect data on the phenology of the new trees, the trees’ first season response to fire damage, and the plant communities that regenerate in the woodland understory. Through her research she hopes to answer the following questions:

  1. How resistant are California black walnut trees to fire?
  2. What effects does fire have on these trees?
  3. Does fire control or encourage mistletoe?
  4. What native and non-native plant species return and flourish aftera fire?

Tenbrink also is monitoring the understory for the emergence and survival of new walnut seedlings. The study is scheduled to run from December, 1997 through June, 1998. The data will be used to enhance management strategies to protect the Refuge’s California black walnut community.

prepared and edited by Richard B. Standiford and Pamela Tinnin