Early season rainfall pH measurements in Big Sur

Here is some news for those of you interested in following the story of acid rain in Big Sur. The first rains have arrived here in Big Sur in a big way, 8.45″ of rain fell between Oct. 12 and 14. The pH readings on the first 3 inches of rain averaged 5.41, the highest value (lowest acidity) yet recorded in over three years of measurements.

Another small storm brought some rain today (Oct. 19) which totaled just 0.14″, with a pH value of 4.79. This value is more in line with readings from the previous three years and represents a significant drop (increase in acidity) compared to the heavy rains last week.

What do you suppose is going on?

On the origins of fire scars in California redwoods

Anyone who has spent time with the redwoods has no doubt seen and even ventured inside the giant trees with fire-scarred trunks. Certain trees are so severely scarred that you wonder how they are even able to stand. Some trees contain cavernous fire-carved rooms in their base with multiple entrances and even window-like openings. I’ve marveled at these trees and looked carefully at the orientations and shapes of their scars, and in doing so have found some odd things.

Having worked on fire lines in Alaska, Colorado, and California I’ve seen how fire scars are formed. A large quantity of fuel piled at the base of the tree is usually required to ignite a fire hot enough to penetrate the bark and scorch the cambium.

A characteristic burn pattern is seen on slopes where the vast majority of fire scars occur on the uphill face of the trunk (more than 90% in places)[1]. This is due to a couple of factors. First, hot air currents tend to drive ground fires upslope, especially during the day when fires burn hottest. (Keep in mind we’re talking about large ground fires, not large canopy fires which would more likely kill the tree.) The wind-driven fires tend to burn relatively quickly around the lower parts of the trunk, but eddy effects allow the fire to linger on the uphill side. Second, fallen leaves and branches tend to move downhill and accumulate on the uphill side of the trunk. Together these factors seem to account fairly well for the uphill side tendency of fire scars.

So imagine my confusion . . . 

Sudden Oak Death

In recent years a whole lot of attention has been paid to Sudden Oak Death, a stem canker disease (Phytophthora ramorum) that has sickened countless oaks in California. I, too, believe that Sudden Oak Death is a particularly aggressive pathogen that requires our careful observation and study. To be sure, I often inspect the sick coast live oaks and tan oaks here in Big Sur and at least half have bleeding stem cankers.

Sudden Oak Death casaulty in Big Sur

Still, there are many other problems I am seeing in these forests that cannot be attributed to Sudden Oak Death. Pines are becoming infected with pitch pine canker, sycamores are sick with anthracnose, bays are toppling from a root collar fungus, and redwoods are losing their tops. It seems that something larger is going on . . .

Acid rain in Big Sur

“Having been in my progress so often misled by taking for granted the results of others, I have determined to write as little as possible but what I can attest by my own experience.” – John Dalton, Independent Scientist (1766-1844)

UPDATE (May 2, 2009): In the past 24 hours it has rained 0.93″ here in Big Sur with a pH of 4.56. I have amended the seasonal data accordingly.

UPDATE 2 (May 5, 2009): An additional 0.08″ was recorded, but the sample was too small to obtain an accurate pH reading. The seasonal data have been amended.

Anyone who thinks acid rain does not occur in pristine, unpolluted environments had better think again. Or better yet, go get some litmus paper and check it out for yourself.

Scientists studying Sudden Oak Death have dismissed acid rain as a relevant factor in oak mortality, pointing out that areas of severe decline are near the coast, upwind of the major sources of pollution, thus rainfall could not be acidic. Here along the coast of California storms blow in from the ocean where there are few sources of manmade pollutants. Rainfall pH, then, is not expected to be any more acidic than about 5.6, which is the theoretical pH of unpolluted rainwater in equilibrium with atmospheric CO2.

Acid precipitation in southeast Alaska

However, I have learned from past research that rainfall in pristine environments can sometimes be quite acidic[1]. So for the past three years I have been recording rainfall at my home in Big Sur using a Stratus RG202 rain gauge. Readings of rainfall pH have been made with a high-precision Beckman (Model Φ250) pH meter using a standard two-point (4.00 & 7.00) calibration. The rain station is located 1.2 miles from the coast at 36° 16′N; 121° 49′W, and 922’ (281 m) elevation.

Here are the results: