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Treebeard's Stumper Answer
18 October 2002

Water Stress

We've had recent autumn weather swings from 105° baking heat to cold drizzle this week. But make no mistake, it's been an extremely dry year since last Christmas, and the ground is still parched. Much of the country has had record drought (and wildfire) this year. Now look at the hills. Native oaks and chaparral are still green, and many plants are even ripening seeds and fruit. We survive with dams and aqueducts. How can our native plants survive for nine months with almost no rain when our garden plants wilt every hot afternoon even with watering?

I use a coffee can for my rain gauge, and I use the local Santa Ynez River swimming holes as my ground water guage. Here are two photos of our favorite DMS Hike Club swimming hole two years ago (left) and this year (right), both in September. Use the bumpy ledge on that triangular rock to get your bearings. We managed to find another spring-fed swimming hole for Hike Club this year.

The swimming holes at our river dropped as much this year as they did during the 6+ year extended drought a decade ago. The river dried out so quickly this year that the already full Lake Cachuma reservoir is still in good shape. No one is talking publicly about a water problem except the Forest Service fire crews.

It's been another remarkable year. We had good rain until Christmas, but then the tap went dry. By my reckoning, this is the fourth driest spring ever documented. Not a record, but still serious drought. This rainfall data is from my SB Rainfall Data page, updated with this year's rainfall data from the National Weather Service. The NWS index page is not up to date, but you can access monthly data in the form "<>" where you type mm = month (01-12) and yy = year (99-02). See my Rain or Shine (5 Jan 2001) stumper and my Santa Barbara Weather (etc.) page for more local weather data and links.


Like the Lorax, I'll speak for the trees in this stumper. If the river dropped that much in a single dry year, then the trees and shrubs with their roots in the ground must also be in bad shape. Despite water stress, the native plants are mostly green, and some are even producing seeds and fruit. How do they do it?

Camp under a tree, and you'll learn that fog drip can provide moisture without rain. Trees also store much water in their wood. That's why you can't burn fresh-cut oak in your fireplace unless you season it by letting it dry out for months. A cord of green oak logs can hold enough water to fill six 55 gallon drums! Wood holds water in tiny xylem tubes that connect roots to canopy. Water evaporating from the leaves pulls more water up from the roots. Native plants are adapted to reduce that loss, but even they will die without enough water. I hope it rains soon.


Leo Tolstoy began Anna Karenina with "Happy families are all alike; every unhappy family is unhappy in its own way." Ursula K. Le Guin countered in The Dispossessed with "The sunlights differ, but there is only one darkness." Both come from Plato's idea that there is one good and many ways to fail. But like Popeye, the weather is what it is. The only "normal weather" is the statistical average between drought and deluge. Every weather year is different in its own way. This is a very dry year for us in central California and across the country. Native trees and shrubs are in trouble. The few drops of rain yesterday didn't help much.

Green wood can hold a lot of water. I've had green oak spit at me when I split it with an axe! That claim that a cord of oak contains six 55-gallon drums of water comes from University of Georgia Warnell School of Forest Resources. Is it really possible?

One source on the Web gives these statistics for Coast Live Oak (Quercus agrifolia):

Species Cord Weight (pounds)
Cord Weight (pounds)
Coast Live Oak 3766 - 4840 6120 - 7870
average weight: 4303 6995

A cord of firewood is a stack 4 ft x 4 ft x 8 ft = 128 ft3. But real cords of stacked firewood depend on how close the wood is stacked and vary from 70 - 90 ft3. Caveat emptor. I'll use the averages.

Water content of wood is measured in a way that is not intuitive. It's not the percent water content of green (wet) wood. A sample of green wood is weighed, then put in an oven set at approximately 190 degrees F° and allowed to dry for 24 hours. The sample is then reweighed, and the difference tells how much water was lost while drying. The moisture content is then found with this formula. Note that moisture content can be more than 100% with this method.

                      weight of water      wet weight - dry weight
  moisture content = ------------------ =  -----------------------
                     dry weight of wood           dry weight

For our local Coast Live Oak, this comes to:

                     wet weight - dry weight    6995 lbs - 4303 lbs
  moisture content = ----------------------- =  ------------------- = 0.63 x 100 = 63%
                          dry weight                  4303 lbs

That cord of oak firewood lost 6995 - 4303 = 2692 pounds of water while drying, more than a ton.

Water weighs about 8.3 pounds per gallon. So six 55-gallon drums hold:

               gallon       pounds
  6 drums x 55 ------ x 8.3 ------ = 2739 pounds of water
                drum        gallon
Six drums of water in a cord of green wood is reasonable. That's a lot of water! You feel the weight when you stack green wood or carry it up the stairs or overload your pickup truck!

It sounds like another stumper that trees have to use water to get water, but it's true. We humans are good at recycling water from our intestines to use again. Diarrhea happens when we can't do this, and serious dehydration is possiblle. (Don't think about this too much!)

Plants are different. Water has a one-way ride through a tree. Water transport is powered in part by osmosis and capillary action, but mostly by transpiration which is evaporation from special stomata pores in the leaves. It's remarkable that this passive process can carry water hundreds of feet into the air.

A molecule of water evaporates from a stomata pore on a leaf and pulls the water molecule behind it up a notch through the xylem tubes in the wood that form a continuous column of cellulose microfibrils from root to canopy. That molecule pulls another and so on down to the roots where osmosis takes over. Water is sticky. It adhers to surfaces like wood and other water. That's why ponds have surface tension that can support water striders. That's how towels and sand castles work, and how water can creep up a thin glass tube by capillary action. It has to do with the bipolar "Mickey Mouse" shape of a water molecule. The hydrogen "ears" are on one side with a more positive charge, and the oxygen side has a more negative charge. Opposites attract, so water molecules tend to line up and attract each other in thin films within a tree's xylem microtubules. It works, but trees have to lose water by transpiration to pull up new water. This is no help in a dry year, and water stress results when the water column is broken.

Our native California trees have several adaptations to reduce this water loss from evaporation from the leaves and survive in this arid climate. The deep problem is that any reduction in transpiration is paid for by a loss in new water transport. Maybe this doesn't matter in the summer when the soil is dry and the wood is already saturated? Our native trees and shrubs are the survivors of millennia of cycles of drought and deluge. Here are some factors that I think allow our natives to survive our winter-wet / summer-dry climate:

Obviously there's lots more to talk about!

Most of our local forest looks surprising good for such a dry year, but I am seeing some signs of extreme water stress in our trees. The photo (right) shows brown leaves on California bay (Umbellularia californica) and madrone (Arbutus menziesii) trees near my house on San Marcos Pass. They appear to be dying though the other trees around them look ok. I'm worried that water stress can weaken a tree against disease and insect attack. I'm watching these particular trees for symptoms of the Sudden Oak Death (SOD) disease that is devastating California forests north of us. As far as I know, SOD has not been found in Santa Barbara County. But it is a big problem in Big Sur (Monterey County) not too far north, and it's spreading.

As I write this, we've had a cloudy week and even some sprinkles. But the ground is still bone dry, and we're especially at risk on San Marcos Pass because of winter snow damage in past years. Wildfire season is not over until we have two inches of honest rain. That's nearly as much rain as we had all this year!

Here are some Web links for more research:

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