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 "<http://www.nwsla.noaa.gov/climate/sba.f6_mmyy.html>" 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.

Month:	Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Total
2001-02	0.61	3.98	2.12	1.50	0.47	0.45	0.13	0.10	0.05	0.03	0.08	0.24	9.76
Average	0.50	1.70	2.94	3.83	4.20	3.05	1.28	0.30	0.09	0.03	0.06	0.25	18.25

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.

Notes:

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) DRY	Cord Weight (pounds) GREEN
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:

• Every weather year is different and rarely "normal".
• All plant species are the survivors of millenia of swings between drought and deluge.
• All plants find a microclimate that suits them, at least so far.
• All plants alter the microclimate to trap humidity and make it available to other plants and critters.
• All plants can store water in their wood, roots, and leaves during the rainy season for use in dry times.
• All plants can control transpiration water loss by closing their stomata leaf pores or having fewer of them.
• Some plants have narrow needle-like leaves or leaf segments to reduce water loss. (chamise, red shanks, coast sagebrush)
• Some plants have juicy succulent leaves and stems to hold more water. (prickly pear, ice plant, giant coreopsis)
• Some plants have hairy or hoary leaves to reduce water loss. (white sage, yerba santa, buckwheat, brickellbush)
• Some plants fold or turn their leaves to avoid direct sunlight. (coast live oak, laurel sumac)
• Some plants have thick sclerophylus leaves with a waxy cuticle to reduce water loss. (coast live oak, ceanothus)
• Some plants have a woody lignotuber root-burl that can hold water and survive a wildfire. (chamise, scrub oak)
• Some plants collect fog drip and direct it to their roots. (redwood, eucalyptus)
• Some plants can maybe absorb fog moisture directly through their leaves. (redwood?)
• Some plants are summer deciduous even in a wet year. (gooseberry, giant coreopsis, buckeye)
• Some plants are winter deciduous or semi-deciduous, but they can drop their leaves early in a dry year. (poison oak, elderberry, foothill ash, milkwort)
• Some plants wilt by day, but they come back at night. (tomatoes)
• Some plants look dead from water stress, but they come back when it rains.
• Some plants survive a dry year, but they don't produce seeds or fruit.
• Some plants survive a dry year, and they do manage to produce seeds or fruit.
• Some plants (especially introduced weeds) can produce some seeds in a drought year even when the natives fail.
• Some plants produce seeds that won't sprout for years until conditions are right. (fire, etc)
• Some plants will die from water stress.

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:

• There has been drought (and wildfire) across the country this summer. We couldn't BBQ at the Live Oak Music Festival last summer. There's info at the National Drought Mitigation Center home page and the NOAA's Drought Information Center. The NOAA US Drought Monitor still looks bad. Is this dry year caused by global warming?

• How trees move water to the top is a classic stumper discussed at Scientific American: Ask the Experts here and here. Also see USA Today. There's a more technical info at Wood and Moisture Relationships (pdf) from Oregon State University; a forestry class lecture on Water Relations; a good explanation of water transport from the The Lake Charles Bonsai Society; and this paper on Water Shortage in Oak Seedlings (pdf). There are instructions for measuring the moisture content of wood here. Is it possible to imitate nature and build a totally passive microtubule pump to lift water from a deep well?

• There's good information on seasoning firewood at Firewood - How Does it Stack Up?. There's more info here, here, and here. My stumper on Burning Bogs and Exploding Pancakes (10 Nov 2000) explains why it's dangerous to burn green wood in your fireplace or wood stove.

• There's some discussion of the importance of fog drip here. Interesting research by U.C. Berkeley plant ecologist Todd Dawson suggests that "Redwood canopies can directly absorb fog" for as much as much as 40% of their water intake. These trees create their own fog drip and alter the microclimate below. There are newspaper accounts of Dawson's work here and here. This is interesting research. I've camped under the drippy eucalyptus forest at Morro Bay State Park, so this is not surprising.

• Water transport in trees is entirely passive, so it doesn't stop when trees are turned to lumber. This creates problems for wood furniture and floors. There's discussion at Wood Floors Online, Water & Wood: Warping Nightmares, and Wood’s Advantage.

• Sudden Oak Death (SOD) is an emerging problem in central California. We're still off the map here in Santa Barbara, but we have many of the same susceptible species near my home on the north slope of San Marcos Pass. We live in a beautiful forest of trees that are usually found further north, including tanoak (Lithocarpus densiflorus), California bay (Umbellularia californica), madrone (Arbutus menziesii), and big-leaf maple (Acer macrophyllum), along with surprising shrubs like huckleberry (Vaccinium ovatum) and thimbleberry (Rubus parviflorus). Sudden Oak Death first appeared in the 1990s, apparently caused by the native fungus Phytophthora ramorum. I'm worried, and I'm watching the dying bay and madrone trees that I photographed (above) by our mailbox. The California Oak Mortality Task Force has info and reports. There's a GIS map of the spreading infection. There's more info from U.C. Berkeley, U.C. Davis, U.C. Ag and Natural Resources, and CalFlora. A Google search returns thousands of links.

• I have many local weather stumpers including Frosty Leaves (25 Jan 2002), Mediterranean Climates (19 Oct 2001), Rain or Shine (5 Jan 2001), Storm Damage (26 March 1999), Cold Winter Sun (6 Dec 1996), and The Big Freeze (18 Oct 1996). My Late Bloomers (12 Oct 2001) stumper has info on late blooming native plants. My Wet Sand, Dry Sand (14 Apr 2000) stumper has more info on the stickiness of water. My stumper on Frosty Leaves (25 Jan 2002) has info on gutation, which is another kind of water transport in plants. There's lots of local weather info and links at my Rain or Shine (5 Jan 2001) stumper and my Santa Barbara Weather (etc.) page. My SB Rainfall Data page has 90+ years of local rainfall statistics.

Back to Stumper

Last modified .