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Forest officials seek answers to oak tree deaths
By Billie Jo Jannen
The Alpine Sun
ALPINE — Local U.S. Forest Service staff and county agriculture officials gathered in Alpine and Descanso earlier this month to explore the reasons for increasing oak tree deaths that have both residents and officials worried.
So far as officials can gather, the problem is not sudden oak death, such as that caused by a fungus-related epidemic in Northern California.
“We haven’t found a disease or cause,” said Supervising Plant Pathologist Pat Nolan of the San Diego County Department of Agriculture Weights and Measures. “It doesn’t appear to be a biological.”
Large die-offs of trees is not a new phenomenon in the area. Both officials and residents became very alarmed several years ago over the higher than usual numbers of pine tree deaths at higher altitudes.
As reports of local tree deaths accelerated, with tolls up to 15 and 20 percent in some places, CDF officials decided two years ago to do flyover mapping of dead trees in rural areas with an eye toward quantifying the problem and, perhaps, finding a common cause of death.
Both heat signature and spectrographic (light reflectivity) analysis were used to identify areas where tree deaths were highest. Combined with GIS (Geographic Information System) technology and global positioning, the resulting maps provide precise information about location and terrain that may be contributing factors in the trees' decline.
Reflectivity and heat signature should also offer other advantages over conventional photography or visual inspection. The devices can “see” what the eye cannot and should help identify trees that do not yet display overt symptoms of decline — a process that may be well underway before ill health becomes noticeable.
The Forest Area Safety Task Force, consisting of state, federal and local agencies, use this and other information to help decide which areas to focus efforts to protect area homes from wildfire, said District Ranger Tom Gillett of the U.S. Forest Service.
According to the FAST website, the pine tree die-offs have reached as much as 80 percent in some areas.
Last spring, oak deaths in lower altitudes began to accelerate sharply, creating a new source of worry. Primarily affected are Engelmann Oaks and Coast Oaks in the lower altitudes of Descanso, Pine Valley, and eastern Alpine, along with some Black Oaks at the higher altitudes.
The area of greatest impact is 75,000 acres of oak woodland stretching between Pine Valley and eastern Alpine. The epicenter of the area, where deaths are highly concentrated, is around Merigan Ranch, Gillett said. Some areas suffer as much as 50 percent mortality, he added.
“We’ve noticed an unusual amount of tree mortality since spring of 2005,” Gillett said. “The dead pines had stabilized after last year’s rain and then, suddenly, oaks started dying at an alarming rate.”
Even more alarming, the fatalities were not necessarily among the older trees, as might be expected normally, Gillett said. Many younger trees that should have been vigorous were among them.
Gillett said he, Nolan, entomologist Laura Merrill, and others made an initial field trip in late summer to begin gathering data. Flyovers such as those that previously focused on the pines were also made.
Once a tree shows symptoms, including browned-off leaves, bare branches, and dark spots on the trunk, it often appears to die almost overnight, Nolan said.
Nolan said that the normal threats, beetles and some pathogens, are present but would not ordinarily kill a healthy tree.
Drought damage still remains Nolan’s best guess as to the cause, though both she and Gillett are continuing to investigate to see if there is a new pathogen involved.
In a prior interview, Forester Tom Porter of the California Department of Forestry and Fire Protection, said he believes the problem goes back to the much longer drought of the 80s and 90s, which, he said, may well have damaged the tree roots and limited their growth in ways that are not immediately visible.
The relatively heavy rains that the county experienced for a couple of years in the late 90s promoted an abundance of rich growth above ground, giving an appearance of vigor, Porter said. But the aboveground growth of the trees' leafy canopies is faster than that of their roots — a survival mechanism that allows the leaves to photosynthesize nutrients to send down to roots, which would then have the energy to follow with growth of their own.
Porter said he thinks the roots didn't get the time they needed to grow before drought conditions returned:
"It may take five to ten years to get the roots back to a healthy level," Porter said. The result is that the roots can't collect enough moisture and soil-based nutrients to sustain the the now overabundant canopy, increasing the tree's stress beyond what it can cope with.
Water, which has been in woefully short supply in the 80s and 90s, is an essential ingredient for making tree sap and drought-stressed trees have a lot less of it than they need, leaving them far more vulnerable to infestation by oak bark beetles and ambrosia beetles.
"The trees make gooey sap," Nolan said. "The beetles get stuck in it."
Bark beetle lay their small, whitish eggs in sheltered holes and crevices in the bark. According to Dr. David Kellum, an economic entomologist with county agriculture. The larvae then tunnel around under the surface of the bark, destroying the phloem and cambium, which carry nutrients and water from the tree roots to to its branches.
The damage be seen by chipping away loose bark and looking underneath. The larvae's "galleries" look like little canals wandering around in the surface wood of the tree. Sometimes the entry holes made by the adult beetles to lay eggs are noticeable, along with small dots of oozing sap.
"Stressed trees give off a scent that lets beetles know they are susceptible," Kellum said, "It happens when they get out of balance."
What does FAST do?
The Forest Area Safety Taskforce was formed in the fall of 2002 to address the forest health concerns and threats to life and property due to the continuing die off of trees and brush in San Diego County.
Formed as a collaborative effort, FAST is made up of the Natural Resources Conservation Service (NRCS),California Department of Forestry and Fire Protection (CDF), County of San Diego, United States Forest Service (USFS), Resource Conservation District of Greater San Diego (RCD),San Diego Gas & Electric, Fire Safe Council of San Diego County, as well as community organizations and private citizens.
The priority of this joint venture is to remove dead, dying and diseased trees in and around mountaintops and communities at risk.
For further information, one may visit
Forest Area Safety
Taskforce.
Photo by
Joan Wynn/USFS: U.S. Forest Service Entymologist Laura Merriil, investigates a dead oak tree in Descanso, as Ranger Tom Gillett and USFS researcher Andi Koonce look on.
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