Post-Fire Regeneration of Black Walnut
California black walnut is found only in southern California and offersimportant habitat for many of Califor-nia's vertebrate and invertebratespecies. Leaf production and flowering occur at approximately the same timein the spring, and mature fruit is abundant by late summer. By Novemberor December, this tree begins to shed its leaves and mature fruit, supplyinginvertebrates and reptiles with shelter beneath the leaf litter; Californiaground squirrels and gray squirrels are provided food to carry them throughwinter; the foliage provides food for browsing herbivores; and the canopyand hollows in the trunks offer roost sites, shelter and nesting habitatfor 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 insectsand fungi, has hollow spaces in its trunk. Tree loss can be caused by harshwinter 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 commonlyare seen within a month at the base of the desiccated trunk; this is whyindividuals of this species of tree can have multiple trunks of varyingages.
California black walnut is becoming increasingly scarce in the foothillsof southern California. Recently, this endemic species was proclaimed byThe Nature Conservancy Heritage Program as "rare," and the walnutwoodlands as "very threatened." The remaining large stands insouthern California are threatened by spreading urbanization, livestockovergrazing, 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 Fill-more,California. The 2,471 acre Refuge, which is closed to the public, protectshabitat for the endangered California condor and functions as a base ofoperations for Refuge staff engaged in California condor reintroductionefforts. The Refuge contains more than 1000 acres of chaparral and coastalsage shrub, 900 acres of grasslands, 350 acres of oak and walnut woodland,and 169 acres of riparian habitat. Three acres of fresh water marsh alsoare present on the Refuge.
Within the oak and walnut woodland is one of the largest remaining standsof California black walnut in southern California. Recently, Refuge personnelhave become increasingly concerned about the health of the walnut communitybecause of the absence of seedlings and an infestation of mistletoe. Sageand introduced annual grasses choke the understory, presenting little opportunityfor new growth and offering dense fuels for fires. Until 1997, fire wasabsent from the Refuge for more than 72 years.
On 5 August, 1997, a fire started in the grasslands north of the Refugeand within a few days, all 2,471 acres of the Refuge burned. Very littleis known about the benefits or detrimental affects fire has on Californiablack walnut or about vegetative regeneration by walnut. The fire presentedRefuge personnel the opportunity to study the effects of the fire on thisrare, endemic tree.
A few weeks after the fire swept over the Refuge, photo transects wereestablished to document the regeneration of the Refuge's woodlands, grasslands,and marsh. Within 36 days, walnut trees that had been subjected to lightor moderately hot fire supported eight to 12 inches of new growth from stumpsof burned trees; some showed evidence of new growth in their canopies. Areasthat had been subjected to extremely hot fire required more time to regenerate.
In early December, graduate student Victoria Tenbrink from CaliforniaPolytechnic and State University, Pomona, under the guidance of ProfessorRonald Quinn, initiated a study to examine the post-fire regeneration ofthe walnut community at HMNWR. Tenbrink has used identification markersto identify numerous, individual sprouts at three sites to collect dataon the phenology of the new trees, the trees' first season response to firedamage, 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 survivalof new walnut seedlings. The study is scheduled to run from December, 1997through June, 1998. The data will be used to enhance management strategiesto protect the Refuge's California black walnut community.
prepared and edited by Richard B. Standiford and Pamela Tinnin