According to climatologists, it's going to be wet this winter.

Experts on El Niño, the well-known global weather circulation pattern that often brings warm winters and heavy rain to the West Coast, recently released a chart showing the El Niño Southern Oscillation  (ENSO) strengthening across the Pacific. This is good news for those of us in SoCal hoping for rain.

Here's a recent satellite picture, from the NASA-affiliated Jet Propulsion Lab in Pasadena, showing the elevation of a Kelvin wave building along the equator as of two weeks ago. 

JPL's forecaster extraordinaire Bill Patzert, who earlier this year doubted that El Nino would significantly increase our chances of rain, now sounds convinced.

"In the American west, where we are struggling under serious drought
conditions, this late-fall charge by El Niño is a pleasant surprise,
upping the odds for much-needed rain and an above-normal winter
snowpack," he said.

But of all the experts who look to natural history to predict the weather, the most intriguing may be the research scientists specializing in oak trees. That's because in California two species of oaks bet their reproductive lives on being able to match a massive outpouring of acorn seed to a good rain year.

Of course, you don't have to be a scientist to look at oak trees, notice a heavy acorn fall, and wonder if the trees could sense an oncoming wet winter. People have been doing that for years, myself included.

But it's less easy to sample acorn production every fall in California at fourteen different sites around the state, publish the results for over a decade, and try to understand how and why millions upon millions of blue oaks around the state can agree that this is the year on which to produce a massive acorn crop. That's what scientists Walt Koenig and Jean Knops have done in the California Acorn Survey:

Discussing acorn production and "masting," in which some species of oak trees produce massive amounts of acorns in synchrony with fellow trees, responding to an unknown signal, Koenig wrote:

The results vary among species but clearly show considerable synchrony [trees deciding en masse to produce vast quantities of acorns on the same year]. Consider blue oaks, which are one of the most extensively distributed trees in California. They grow in foothill regions around the Central Valley over an area of some 20,000 square kilometers, about five percent of the state’s total land area… In other words, synchrony in acorn production extends to pretty much every blue oak, a population of 100 to 200 million individuals.

That's from Koenig's 2005 fascinating article in The American Scientist, The Mystery of Masting in Trees.

Masting is an extraodrinary phenomenon, known and studied for hundreds of years, but still mysterious to humans. Although Blue Oaks have been shown to synchronize their acorn production across California, and Valley Oaks also produce a widely-varied number of acorns, other species are less variable.

Some oaks surveyed by Koenig haven't produced well in decades; some always produce well.

Still, in The American Scientist piece, Koenig points to a remarkably good correlation of acorn production across the state by blue oaks. He assumes, as do most researchers, that the warm, wet winters typical of El Nino drive the production of acorns the next fall.

He doesn't assume that oak trees anticipate an El Nino. He believes a past El Nino tend to make for favorable conditions — plenty of moisure in the ground, and warm weather in the spring — to produce a good crop the next year. 

In the words of Victoria Sork, a tree scientist at UCLA who studies pollen dispersal patterns, and finds Koenig's work persuasive: 

If there is a lot of rain this year, it could affect next year's crop.
 However, one of two things could happen.  If there is too much rain
during pollination (March and April) then fertilization will be poor.
 Or, the rain will be good for the trees and they will mature a large
portion of the fertilzed flowers, resulting in a good crop.

But acorn-watchers such as myself like to look at acorns on the ground, and speculate about the winter to come.

Is that wrong?

Surely such a correlation could be true, even if we can't yet understand it.

And since last winter was not good for rain at all, the trees cannot be responding to last year's rain to produce a good crop this year (although they could be responding to a warm, dry spring).

I've been asking around, and my neighbors report this year has been very good for acorn production. A friend who lives in the heavily wooded Camp Bartlett tells me they're "everywhere you look."

(Cross-posted at the Ojai Post)