|Graduate student Karri Meador records the growth of young aspen trees at Crystal Creek in Yellowstone's Lamar Valley. Note the absence of medium-height trees. (photo Jean Arthur)||
Aspen and willows tell Yellowstone talesBy Jean Arthur
Long before sunrays warm the Indian paintbrush and sage, Montana State University graduate students Karri Meador and Levia Jones hike among Yellowstone National Park's willows and aspens with a pressure bomb in a suitcase.
They assist Duncan Patten of the MSU-based Big Sky Institute in determining what environmental factors affect aspen and willow communities—considered barometers of the health of the diverse ecology of Yellowstone's northern winter range. Understandably, water sources and soil nutrients play a role in the woody plants' life cycles.
"We use a pressure bomb to determine how hard the plant must work to move water—the amount of tension the water column is under in a plant stem," explains Meador, a graduate student in land resources and environmental sciences from Powell, Wyo. "Essentially this tells us the plant water potential or plant water stress."
Jones loads the pressure bomb chamber, a mechanism that increases the pressure on the plant sample in the chamber until a drop of water appears on the cut surface.
"The water column in a plant is always under pressure so that when I cut a plant sample, the water column is sucked back into the plant," says Jones, also a graduate student in land resources and environmental sciences from Alaska.
They crank up the pressure in the chamber using pressurized nitrogen and watch the exposed cut surface.
"When the water first appears at the cut surface, we record the pressure reading," says Meador. "This value is equal to the tension of the water column at that time of day. The lower the pressure reading, the lower moisture stress the plant is under and vice versa."
She notes that the reading represents the plant's interaction with the available water. Every three weeks, they return to four study plots: Eagle Creek near Gardiner, Crystal Creek inside Yellowstone, and the Crandall and Sunlight Basin areas, outside Cooke City in the Shoshone National Forest. They take predawn readings to show the moisture tension prior to evaporation when the plant is under minimal stress. Another reading at midday shows the plant's water demands as it actively transpires and pulls water from the soil.
In addition to moisture, the researchers examine willow and aspen community stature and resiliency to browsing by ungulates such as elk.
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