RESEARCHERS LINK TREE RING RECORDS TO SAN FRANCISCO BAY’S SALINITY

FAYETTEVILLE, Ark. -Researchers at the University of Arkansas have examined tree rings from the little-known, old-growth blue oak forests of California and connected their history to that of the San Francisco Bay, demonstrating that recent salinity levels are higher than at any time in the past 400 years.

Geosciences professors David Stahle and Malcolm Cleaveland and research associate Matthew Therrell of the Tree Ring Laboratory at the University of Arkansas, together with scientists at the Scripps Institution of Oceanography in La Jolla, Calif., report their findings in the journal Eos, Transactions of the American Geophysical Union.

The scientists used the blue oak ring record and reports of salinity in the San Francisco Bay Estuary dating back to 1922 to reconstruct the bay’s historic salinity levels. The estuary serves as a breeding ground for marine life in the North Pacific Ocean. Prolonged, elevated salt content could potentially damage the ecosystem, but until now there has been little evidence of the bay’s historic salinity levels to compare with current levels.

The Tree Ring Lab researchers have changed that: The blue oak tree rings can be used to explain 81 percent of the variance in salinity at the Golden Gate Bridge from 1922 to 1952. And they provide a proxy for bay salinity that extends back 400 years.

"You go out to California and you see these gnarled trees on these brown, dry hills, and you wonder how they can be connected to one of the biggest estuaries in North America," said Therrell.

The answer lies in the trees’ and estuary’s need for water.

"The trees have seen the same rainfall that provides stream flow that then freshens the bay. They can provide us with a 400 to 500 year perspective," Stahle said.

The researchers used this data to reconstruct the estuary’s salinity for the past 400 years, based on the common precipitation influence on tree growth, runoff, and San Francisco Bay salinity. They found that the diversion of large amounts of water from streams and rivers leading into the bay, which increased dramatically after World War II, has caused salinity levels to soar past what would have been expected due to rainfall. In fact, the salinity has been higher in recent years than at any time in the past 400 years.

"This research clearly shows how we’ve altered that hydrological connection, between rainfall, runoff and estuary salinity levels," Therrell said.

The "old growth forests" that provided the insight into this historic record of estuarine salinity do not look like the giant columnar redwood and sequoia forests.

"These are often diminutive, twisted, knotty oaks, and they were not valuable for saw timber," Stahle said.

But these rainfall-stressed blue oak live for 400 to 500 years, and cover a vast area on the foothills of the Coast Range in Sierra Nevada. The great abundance and antiquity of blue oak in California has not been widely appreciated, and centuries-old trees continue to be cut for firewood, grazing land and development.

The blue oaks grow on steep slopes with thin soil, which may explain their rings’ sensitivity to rainfall, Cleaveland said.

"They’re having to rely on precipitation for growth," he said.

The researchers have established a network of 12 blue oak sites in California, taking harmless, soda-straw sized cores from an average of 30 trees at each site. Exact calendar dating is established by "crossdating," or cross-matching the growth rings of all trees back from the known date of collection for the outer ring on living trees.

They sample different age classes of trees, deadfalls and stumps to get a clear picture of the ring patterns.

"It’s a bridging process that takes us further into the past," Therrell said.

Wider rings indicate abundant water during the growing season, while small, narrow bands point to low precipitation, or even drought conditions, depending upon the actual width of the growth rings.

The researchers can correlate the more recent rings with climate data, then extrapolate back using mathematical equations to determine climate variability hundreds of years ago.

The blue oaks display some of the highest sensitivity to rainfall of any trees in the world, Stahle said.