Another Year Among the World’s Largest Trees
What hidden forces decide where the mighty sequoias take root in Yosemite’s Mariposa Grove? Our Virginia Tech team returned to the Mariposa Grove in Yosemite National Park, driven by this question. This time, our goal was to gather data to better understand the factors influencing the distribution of sequoia groves. In other words, why do sequoias grow where they do? Armed with new ideas, objectives, and hypotheses, we set out to push the capabilities of our nodal STRYDE system, which we use to enhance our understanding of the structure of the subsurface. We deployed nearly 2,000 nodes across six seismic refraction lines, with one line stretching up to an impressive 700 meters (~2,200 feet or almost ½ a mile). The locations for these lines were carefully chosen based on two key factors that we believe significantly influence where sequoia trees prefer to grow.
Image: a seismic refraction line with ~15 nodes.
The first factor is the topographic wetness index (TWI), a measurement derived from digital elevation models (DEMs) that combines slope and flow path information to identify areas with higher water accumulation or "wetness." In other words, TWI tells us where we are more likely to see water on the surface. The second factor is bedrock geology. Our study area spans three distinct geologic units: El Capitan Granite, Bass Lake Tonalite, and an unnamed metavolcanic unit. We hypothesize that sequoias may favor areas with higher wetness and more easily weathered bedrock, such as the metavolcanic unit. To strategically place our seismic lines, we created a diagram similar to a Punnett square, covering different combinations of TWI and bedrock geology (see below). This approach allowed us to select locations that intersect one or more of these combinations (Shaded boxes), providing a comprehensive data set for our study.
We are excited to share this data with you at the upcoming American Geophysical Union (AGU) Conference this December. Our initial findings are already revealing promising differences in bedrock depths across various geologic units, suggesting that the metavolcanic unit may offer greater water storage potential. As we continue to work on this data, we hope even more exciting discoveries will emerge.
Image: The field team taking a break at Olmstead Point
Aside from the interesting scientific discussions and planning meetings we have before and after the field season, many things we encounter in the field we are unable to plan for. Like many scientists, our fieldwork ambitions often exceed the capacity of our field crew, and this project in the Mariposa Grove was no exception. We faced several challenges, including large numbers of tourists, dense white thorn bushes, and hazardous terrain covered with fallen logs and branches. These obstacles required creative problem-solving. For example, we used walkie-talkies and stationed team members near trails to alert the data collection crew when tourists were approaching, minimizing interference in our data set. To deal with the thorny bushes, some of us double-gloved and used duct tape for extra protection. We also quickly learned the safest ways to fall when navigating over branches and logs to avoid injury. With many illnesses and injuries striking our team we persevered to collect the best data we could. However, the field site offered rewards that far outweighed the difficulties. The grove's stunning scenery and cool, shady climate made for a pleasant working environment. Between tasks, our crew could often be found resting beneath the majestic canopies of the giant sequoias. After long days in the field, some of us would explore secret waterfalls, relax on the beaches of Bass Lake, or take in the breathtaking sights that Yosemite has to offer. If you ever have the chance to experience the majesties of the Mariposa Grove, I encourage you to!
Image: A relaxing moment in the field