Saturday, April 5, 2014

Water logged

As a unilateral gesture of good will to a neighbor, I'm cutting down one of my climate mitigation assets. My sycamore tree shades my house in the morning and would shade my neighbor's house during our blistering afternoons, especially in autumn. But in return for these services, the tree drops leaves, quote generously I should add. Despite it's benefits, I figure we all must live together and cooperate, so "timber!"

Another part of the mitigation benefit is indirect. My tree absorbs atmospheric CO2. I've been asked if plants get all their carbon from atmospheric CO2 as opposed to getting some from the soil. I believe that the atmosphere is their only source, as I've described here: Rubisco Enzyme. In this mitigation, I'm still doing well, for I keep a small stream of new trees ready to replace any that may die in other parts of my yard. My remaining trees shade mostly my home's western exposure which I've recently learned is the most effective cooling benefit you can get from trees. Lake Elsinore's Climate Action Plan has this to say: 
Researchers have found that planting deciduous trees or vines to the west is typically most effective for cooling a building, especially if they shade windows and part of the building’s roof (USEPA, 2009).
But back to my tree and the reason for this post: While cutting, I removed a wedge and was impressed with how wet it was to the touch. I've known that much of a live tree's weight is in water. Having a freshly removed, wet slice made a good sample to weigh. The slice would quickly lose it's water, so I could get a before and after measurement. Here is my slice. You can see the wet wood but also that the thinner portion has started to dry out, hence, the color difference.

Also from this photo, you can infer a significant change in water availability. Four growth rings near the middle are thicker than  the 11 outer rings, suggesting that 11 years ago the tree had a change in water availability, and therefore, a change in it's growth rate. When I moved into my home about 14 years ago I inherited a water logged yard, front and back. Within three years, the yard was replaced with low-water, native plant landscaping. And the tree was still happy.

My slice has been on a scale for 7 days. Initially, it weighed 152 grams. Today it weighs 82 grams. So, 70 grams was water, and 82 grams is wood and any remaining water. It should be noted that the water content will vary with humidity. I'm not surprised that the tree was nearly half water by weight. I am surprised that the wood was saturated to the middle of the tree. Wood tissue's primary function is the transport of water, but I hear that this transport is suction via evapotranspiration through the leaves. My water logged tree had no leaves because I started cut off the limbs in January. My guess is that the evapotranspiration saturated the tree and the osmotic suction of the wood cells held the water in place. And, the approximately 20 years represented in this slice s not enough time for the water conduits to become clogged.

Though wood's primary function is to transport water, it also provides structure, allowing trees to grow taller to compete for sunlight. When crafting with wood, the cabinet maker knows to allow for wood's first calling. Wood workers allow for normal contraction and expansion due to humidity changes. This is why classic furniture will have panels set into a frame rather than be a solid slab. (Think of the design of a panel door. The modern panel door build form metal or composite material often imitates the traditional wood door by included panels purely for aesthetics.)

Many of us know that a board left on the ground and in the sun will warp. The warp is caused by the differential drying. The ground side stays cool and wet, the top side dries out faster. As the drying wood cells on one side shrink, the board warps. This is a flaw in wood as a building material, a feature that has be compensated for. However, I have evidence that this same quality to warp can be a benefit to a plant.

Two years ago I was given a limb of a cactus. I tossed it into my yard and left it, much like discarding a scrap of wood in the yard. After a few months, it warped. 

I believe this warp is from uneven exposure to the summer sun. I dropped the cactus in July and by August, the differential moisture changes curved the limb so that's two ends were upright, allowing the cactus to better compete against other ground plants for sunlight. The cactus is now rooted in at it's middle. And the cactus is happy.


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