Melissa (Lissy) Enright, a graduate student in ECOSS at NAU, shared her research on the hydraulics (water movement) of giant redwood trees with the high school students in the iCREATE class at NAU.
Lissy shared some of her background with the students including working on an oceanographic vessel with the Sea Education Association, working for the National Park Service in Hawaii, and for American Conservation Experience as an AmeriCorps Member in both Flagstaff and Alaska! She also worked with Honko, an NGO (non-governmental organization) working in partnership with the coastal communities of Madagascar on mangrove conservation. She showed the students the website for the Student Conservation Association where you can search through their list of opportunities, including those for students under 18 years old!
Lissy's experience helped her to get a "real job" working with the US Forest Service in Alaska doing a Forest Inventory and Assessment. She also worked in Northern California where she now has her research area studying water stress at the top of Redwood trees.
Lissy illustrated how the stems of the tree branches can get embolisms (air bubbles) from high water stress. She climbs the trees using a jumar system and then brings the branches home to Flagstaff to study the hydraulic conductivity of the tree.
Lissy asked the students why the pine needles and branches might be ore water stressed at the top of the trees than down below. Even though the base of the redwood is in an often moist environment, the tops of the trees are more exposed to sun; but the primary reason is that water is pulled all the way up the xylem (water tubes) of the tree from the ground to 250-400 feet! So the top of the tree is more like a dry climate.
Lissy took the students into the laboratory where she measures the hydraulic conductivity of the branches. She had them assist her with several experimental procedures she uses to determine levels of water stress in the redwood trees.
She told us: We measure hydraulic conductivity on the hydraulic line, a system of tubes that connects a stem segment to an upstream reservoir of solution (water with a tiny bit of potassium chloride)suspended a meter above the sample, and a downstream balance. The water flowing through the sample is measured as it gets to the balance. We use this measurement to infer about the degree of embolism, or air bubbles blocking the flow of water, present in the xylem cells. If the conductivity is very low, for example, we presume that there are a lot of embolisms, and the tree the sample came from was subject to a lot of water stress.
In the image above, a student measures pressure for another experiment. Lissy explains: When a piece of a plant is clipped off, the water inside sucks back away from the cut surface. This is because there is tension in the water in plants. If the plant is more water stressed, the tension is greater. We can measure this in units of pressure with a pressure chamber. We first cut a sample, then insert it into the chamber with the cut end extending out of the hole in the top. When we turn up the pressure in the chamber, the water will be forced back to the cut surface. We measure the pressure at which the water reaches the surface, and know that that number is equal to the tension that existed in the water in the plant before we cut it. If it takes a lot of pressure, then the plant was very water stressed.
Thank you, Lissy, for the wonderful presentation and the engaging hands-on research in your lab!