The Only Good Use for a 20-Gauge is Shooting Trees - a fieldtrip reflection

Graduate and undergraduate students spread out across a hillslope as the morning twilight begins to light the valley – armed with shotguns, a drone, and pole pruners. Sounds like the opening of a story told around a campfire. But this is another day in the life of WyCEHG scientists who are going out to measure water stress of trees in the Happy Jack area near Laramie.

Daniel Beverly, an affable PhD Student and his equally friendly field partner a Bernese mountain dog called Aldo are leading the day’s fieldwork. He measures tree stress along a hill’s gradient. Just off trail from the parking lot, the slope is covered with instruments humming away during field season. Wires run across streams, solar panels provide energy to boxes containing multiple wires and curiously shaped objects. While this is a relatively heavily used trail – instrumentation is tucked away. Hummingbirds dart across the slope that pops with midsummer color of yarrow, daisies, yellow paintbrush, sage, and flax flowers.

Calm prevails until the stroke of the hour, every third hour (unless you are a large friendly dog who seems calm throughout the day), when field techs and graduate students jump into action. Coffee and reading are set aside, sample bags are stuffed in pockets, and people begin hiking to the top of the hillslope to collect leaf matter from the sample trees as they work their way down to the valley. Collection methods vary from shooting branches with a 20-gauge shotgun to sawing branches with arborist tools. Volunteers are eager to remove branches using the shotgun, and some have quite good aim. Once a usable sample is removed from each tree, it is bagged and brought down to the field station for processing. The first measurement was recorded at 5:00 am, and this cycle will continue beyond the setting of the sun. This measurement strategy allows the researchers to observe diurnal changes in sap flow.  

Tree sap is the fluid transported in xylem cells of a tree which creates pressure within the tree. In addition to water, sap contains minerals and nutrients. Those presence and quantity of nutrients can determine the health of the tree, but for today’s purposes the sap content in the top third of the tree is what will reveal stress. Drought stress can be seen by applying pressure to a sample of leaf and stem material in a pressure chamber. Once the pressure on the sample becomes great enough to force liquid out of the stem, a quantity is recorded. This occurs for each sample throughout the day. The higher the pressure required to release liquid, the more stressed the tree. On this overcast cool day, Beverly does not predict high pressure readings. Aldo seems quite content to be in the field under these conditions, though it seems the researchers are hoping for a more dramatic temperature flux. 

The team will come back throughout the field season to establish a picture of seasonal stress and the vulnerability of this forest. With a changing climate, researchers will have tools to respond to the forest’s needs and better prioritize species protection. If you find  yourself on the east side of the headquarters trail at Happy Jack, wave hello to the WyCEHG scientists who may be just off the trail collecting data and helping us better understand our local forests.