Science briefs: Farm advisor unravels oak mystery
Blue oaks on Kern County's foothill rangeland seemed to be vanishing, but livestock farm advisor Ralph Phillips suspects they were simply overlooked in a misunderstanding of oak tree biology.
A 1987 study revealed that, on average, an acre of Kern County rangeland had about 80 seedlings (less than 1 foot tall), 75 saplings (1 to 5 feet tall), but only 15 trees in the 5- to 10-foot-tall range. However, there were 85 trees per acre taller than 10 feet.
Perplexed by the small number of oaks 5 to 10 feet tall, Phillips undertook a study. In 1989, he marked 605 oak seedlings at three sites in Kern County to trace their survival, then recorded the exact height of each seedling (all under 1 foot tall). Phillips' study coincided with the 1986–1992 drought.
During the first 4 years, rodents killed 3% of the trees. The average height of the remaining seedlings actually fell three-quarters of an inch. The apex bud was present on virtually every seedling, indicating that grazing was not a factor.
“The oak is a unique tree,” Phillips said. “The seedlings would send up a green shoot in the spring. By the end of summer, the shoots dried up and the leaves fell off. The next spring a new green shoot appeared. Moisture needed for growth was limited, so each new shoot was smaller than the previous year's shoot.”
In 1990, Phillips began to research the age of his 605 trees. “We were very surprised to find that close to 20% of the seedlings (all under 1 foot tall) were over 10 years old,” he said. “One tree less than 6 inches tall was over 25 years old.”
Phillips thinks the small trees form a reservoir of oaks that grow tall only when conditions allow, such as when more water is available or a nearby mature oak dies.