San Diego (special) -- There are special materials you can read to learn some secrets of Tu B'Shevat, but unless you know how to decipher their text, they may seem more mysterious than the Zohar or other books of kabbalah. 

These materials are the "autobiographies" written inside the trunks of trees by their patterns of rings. According to Jewish belief, on Tu B'Shevat, which comes this year on Wedneday, Feb. 11, all trees become one year older.

San Diego's Museum of Natural History in Balboa Park has on display two good examples of such autobiographies. One is a cross-section of a coastal redwood tree that was 654 years old when it fell in the 1950s; the other is a cross section of a fossil redwood that was 311 years old when it was buried in volcanic ash about 30 million years ago.

Judy Gibson, a curatorial assistant in the museum's botany department, knows the cross sections of these trees well. She recently spent the better part of a day simply trying to count the rings of the coastal redwood. Measuring 6 feet, 10 1/2 inches at its widest point, the cross-section of coastal redwood has some rings that are very wide, and some which are so close together that it takes a magnifying glass to tell where one ends and another begins. To accomplish her task, Gibson taped a piece of paper across the cross section, and

STATELY MEASUREMENTS--One of San Diego's most popular trees, located in Balboa park between Spanish Village and the Museum of Natural History, is this Moreton Bay Fig planted for the park's  opening in 1915.  It is fenced off to prevent root  injuries

marked the paper with a pencil at each place she counted a tenth ring. When she had finished, she had counted not 654 rings but 636. Did that mean that previous information was wrong? That it really wasn't 654 years old?

No, it meant that the slice was taken from a higher up portion of the tree, Gibson said.

"A tree is wider at its base than it is at its top," she explained. "As it gets older it gets higher, and the rings that are formed at the top of the tree don't include the ones that were formed when the tree was shorter.
"So I think what happened is that when this tree fell in Redwood State Park in Humboldt, California, they sliced a few slices off, and counted the rings of one of the slices, and it was probably nearer to the base of the tree than the one we got."

Well, 636 years or 654 --either way the tree had been standing long before Columbus discovered America in 1492, and long before Jews were expelled from Spain in the same year. 

In terms of Jewish history, the tree was standing about the time in 1300 when Mamluks were beginning to require Jews to wear yellow turbans and about the time that King Philip IV--under the influence of Pople Clement V--expelled 100,000 Jews from France.

The tree lived through many more Jewish troubles over the next seven centuries, and didn't die until a few years after the modern state of Israel was founded, at long last.

Gibson said the first thing one may discern is the pattern of knots or discolorations in a cross section of a tree. "There was a branch here," she said, pointing at a knot. "As the tree got bigger it engulfed the branch. That's what knots in wood are: branches that have been engulfed by growing (thickening) trees."

She said on some trees "you can find evidence of forest fires that may have burned one side or the other." While no such scars could be seen on this particular slab of coastal redwood, on others one can detect "a big scar that the tree has wrapped around and swallowed up."  Also, Gibson said, on many cross sections "you can see evidence of insects having bored into them." The next level of analysis is to compare and contrast the rings themselves, Gibson said. "Wide apart rings means that it is growing

CROSS SECTIONS--Kim Lande, public relations  director at the San Diego Museum of Natural  History, stands by a cross-section of a petrified  redwood that was buried in volcanic ash 30 million years ago, While Judy Gibson Stands by a coastal redwood cross-section of much more recent vintage.

very fast--perhaps because it is young, and there are good conditions." 
Narrow rings tell a far more ambiguous story.

Gibson explained that a tree "produces a certain amount of carbohydrates per year based on the sunlight and the amount of leaves it has on it." The mass is distributed over the entire trunk of the tree. Therefore the higher the tree grows, the less wide it is likely to become.

She explained that is a major difference between the Giant Sequoias, which get so thick that you can drive a car through an opening in their trunks, and the Coast Redwoods which are the tallest trees in the world, some reaching as high as 380 feet.

Gibson said some people mistakenly believe that a wide ring results from a "wet year" and a narrow ring from a "dry year." That is an oversimplification, she said. It's more helpful to think of a wide ring meaning a "good growing year" and a narrow ring as a "bad growing year."
Recently, she said, "I was hearing reports on the ice storm in the Northeast where all the branches were stripped off the trees. If they survive at all, they won't grow much even if it is a wet year, because they won't have the leaves to produce the material."

Another reason why a ring might be narrow, she said, could be if the tree were shaded by other trees, or if it were plagued by disease or insect infestation.

Pointing out the composition of the rings, Gibson said, "in a temperate climate the spring growth shows as this wide band of lighter color. The sap starts running, the sun starts shining and photosynthesis is going very fast. As the summer comes along things start slowing down, and as the fall comes and it gets much colder, you get the denser darker color."
The bark of a tree, Gibson instructed, "is its protective covering." One reasons for redwoods' longevity is that their bark is "relatively fireproof."
"The actual living part of the tree--the part that is growing and adding material--is only at the interface between the wood and the bark," she said. 

"A couple of years after the wood is formed, it fills in with resins. All the way from the center of the tree almost to the bark is dead and filled in- the substance we call wood. The only real living material on the tree is the vascular tissues that run up and down under the bark. That is called the 'cambium.'"

So far as Gibson could tell, the coastal redwood was healthy up to the time it died. What caused its demise then? "Maybe it just fell over, or it could be that it was hit by lightning," she said. 'It's a big mystery."
Less mysterious is what happened to the petrified tree that yielded the other cross section: it was buried in volcanic ash about 30 million years ago. Taken from the fossil beds on the desert slope of Oregon, the fossil slab has a pattern that suggests it came either from a giant redwood, like those found today in the Sequoia National Forest, or was part of a forerunner species.

Kim Lande, public relations director for the museum, said "nobody walks by this without touching it--it is one of the most popular exhibits." She held her hand at waist level and said, "when you have a kid this big, they just go 'wow!' when they see it. People are constantly asking questions about the trees. It is an amazing attraction."

Lande, a member of the Jewish community, said approximately 100,000 people visit the Natural History Museum each year, and trees--along with dinosaur bones, bugs and birds--are among the most commented upon attractions.

An explanatory plaque said that in order for a tree to become petrified, "conditions must exist to prevent decay. These are 1) rapid burial by sediments; 2) water percolates through the sediments to waterlog the wood, sealing it from free oxygen; 3) the water is rich in dissolved materials; 4) low temperatures 4 degrees centigrade or 39 degrees fahrenheit, or high temperatures 40 degrees centigrade or over 104 degrees fahrenheit."

The explanation on the plaque continued: "Chemical reactions occur between dissolved minerals and wood cellulose. The minerals crystallize on the wood's cell walls and eventually fill the cell cavities and the rock is petrified. The minerals that petrify this tree belong to a class of minerals called 'silicates.' ...Different colors and patterns are due to impurities such as iron, which is red; manganese, which is black, and nickel, which is green."

Gibson and Lande tell visitors that if they want to learn more about trees, there is another place, besides the Natural History Museum, where they can go to learn in Balboa Park.

Outside!

People who have lived in San Diego for many years may remember a giant redwood slice measuring 12 feet in diameter that sat on a walkway on the side of the museum. "I understand that it got infested by carpenter bees, who were making a nest of it, and eventually the city condemned it as a health hazard, and they had to get rid of it," Gibson said.

But between the museum and Balboa Park's Spanish Village, is a stately Moreton Bay Fig which was imported from Australia and planted in time for Balboa Park's opening in 1915. 

For generations, every child who visited Balboa Park used to climb on it, but now the tree is fenced off to protect its health. Its not children climbing on the tree that had the city worried; rather it was many feet scampering over its root system.

A sign on the fence tells this story about tree roots:

"It's a balancing act. A typical tree will have a root system at least the same size as its branching system. Large roots travel deep into the soil to anchor the tree. The small feeder roots that supply the tree with oxygen and nutrients often grow near the surface and are susceptible to injury. Soil compaction and root injuries will result in leaf and branch die back."