Trees that have withstood the test of time can tell a great deal about the periods it has survived. Tree rings, in particular, can unveil hundreds of years of history. In a new research, scientists were able to decipher six centuries worth of marine history by looking at the rings of blue oak trees.

For the new study published in the journal Science on Sept. 19, Bryan Black of the University of Texas and colleagues analyzed tree ring data of long surviving blue oak trees to reconstruct 600 years of coastal upwelling along the California coast.

Upwelling occurs when typically cold and nutrient-rich water rises to the surface and replaces the original surface water, thereby maintaining cool water along the coast. Scientists track upwelling because it affects biological productivity. The phenomenon, for instance, encourages seaweed growth and supports phytoplankton blooms that are crucial for providing the base of food chains. Highly productive marine ecosystems are often found where upwelling is common.

Blue oak trees are sensitive to climate factors that are linked with upwelling. These trees, for instance, depend on precipitation, which also depends on the same atmospheric conditions associated with upwelling during winter.

By analyzing tree ring data of local blue oak trees, researchers found that of the 10 weakest upwelling years during the past 600 years, four occurred beginning 1950. Seven of the weakest winter upwelling years, on the other hand, were associated with the years after 1850.

"In the California Current, a 576-year reconstruction of climate variables associated with winter upwelling indicates that variability increased over the latter 20th century to levels equaled only twice during the past 600 years," the researchers wrote.

By comparing the tree ring data with other biological factors, the researchers also found that poor upwelling was associated with reduced biological productivity. Winters that were marked with extremely weak upwelling were found to have resulted in slower growth in fish and lower reproduction rate in seabirds, indicating that weak upwelling can negatively impact the productivity of marine ecosystems.  

"The annual growth data gleaned from fish, seabirds and blue oak trees are highly correlated and demonstrate a remarkable degree of connectivity across the coastal interface," said study researcher David Stahle from the University of Arkansas. "Taken together, the data not only provide a long historical context for interpreting modern variability in observational records, but may also inform decisions about managing coastal ecosystems."