Jean Allen combines remote sensing data and ground measurements to study water stress among Sedgwick’s iconic oaks
September 2023
Nikki Evans
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We spent some time with Jean Allen, PhD student in Geography at the University of California, Santa Barbara, to learn about oak trees during drought, the power of combining field measures with remote sensing data, choosing a career in the environmental sciences, and a bird that sings ABBA. Read on to learn more!
| 1. Tell us a little about your research at Sedgwick. What are you hoping to learn in this project specifically and what broader implications does it have? The oak tree population in the Santa Ynez Valley is currently highly stressed due to increasingly severe and frequent droughts. As a result, recent years have seen elevated tree mortality rates and reduced succession of new adult trees. My research focuses on using a combination of field observations and remote sensing to better understand water stress and mortality risk in these oak savannas. I’m especially interested in the blue oak because Sedgwick Reserve is located in one of the hottest and driest regions it can naturally survive in, so even slight increases in temperature or reductions in precipitation can pose a serious threat to the tree population. Monitoring how these particular trees respond to water stress can help us understand tree mortality risk, identify high-risk groups of trees, and gain a clearer picture of the complicated relationship between climate and ecosystem health. 2. Your research involves measuring leaf area index (LAI). What is LAI and what can we learn from it? Leaf area index, or LAI, is basically a way to measure the leafy biomass loading on a tree. It’s a unitless measurement, and kind of a weird one: imagine standing underneath a tree with a laser pointer and pointing it directly up at the sky, and counting the number of leaves that the laser passed through before exiting the tree canopy. That measurement, averaged across the entire tree canopy, would be the LAI. There are a lot of different ways to measure LAI, but my lab group uses light interception, where we measure the radiant intensity of photosynthetically active wavelengths of light above and below the tree canopy to determine how much light was intercepted by the leaves. A tree with high LAI will have a flush, leafy canopy that absorbs a lot of light, resulting in less light passing through the canopy, whereas a scraggly tree with sparse leaves will absorb less light and have a brighter understory. Our instruments can measure this light attenuation to calculate LAI. LAI is useful in my research in two main ways. One is as a direct measurement of water stress: since water-stressed trees grow fewer and smaller leaves, they often have lower LAIs, whereas healthy trees often have higher LAIs. So on its own, LAI can provide insight into which trees are stressed and at greater risk of mortality. The second way is as a tool for interpreting remotely sensed data. LAI has an outsized impact on many common remote sensing indices and derived metrics. In order to tease out subtle variations in tree health from remotely sensed imagery, we have to account for that strong (and potentially confounding) LAI signal. 3. How does your work connect with NASA’s Surface Biology and Geology High-Frequency Time Series (SHIFT) campaign and remote sensing in general? I loved being part of the SHIFT campaign! My main research goal is to leverage remote sensing time series, like the series of hyperspectral imaging flights conducted as part of SHIFT, to scale up field measurements of water status and tree mortality risk from a small batch of in situ measurements to a landscape scale. Fieldwork is incredibly valuable, but unless you have a whole army of people to do it with, you’re never going to be able to measure and monitor the water status of every tree on a given landscape. However, we can use those field measurements to calibrate and train models that can turn airborne and satellite imagery into ecosystem-level insight. Within my research, I aim to use my own field measurements of LAI in combination with additional data collected by my colleagues at UCSB in the Landscape Ecophysiology and Function (LEAF) Lab to use the SHIFT imagery dataset to map tree water stress and mortality risk. 4. What do you enjoy about doing fieldwork? What is something cool you’ve experienced out in the field? My favorite part about doing fieldwork at Sedgwick in particular is how many different projects are going on at the reserve. Sometimes, while walking between the trees I collect data at, I’ll take a slightly different route than usual and stumble across something new; some instrument I’ve never seen before stuck to a tree, or a stake or monitoring station stuck in the ground. Often I don’t find out what it was that I had found until later, while talking to a colleague or meeting someone new out in the field. One time I was singing ABBA while walking between study sites, just for fun, only to realize that I was right next to a microphone that was monitoring bird calls. I hope that their system didn’t interpret “Dancing Queen” as some new, unrecognized bird. 5. How did you get interested in geography? Like a lot of people, my journey to geography was pretty winding and tumultuous. I really wasn’t a born geographer, or a born ecologist for that matter. I was really a math kid up until I started college. I went to math camp and even participated on my high school’s math team. But, like a lot of people, I was also really intimidated and horrified by the threat that climate change poses to our natural world and to people everywhere. I got into geography as a way to take my love of numbers and algorithms and apply that to solve important problems, and I’ve found a real home for myself in the world of environmental remote sensing. I’m very lucky to have some incredible friends and colleagues to work with and we’re able to bring our different experiences and knowledge to the table to do some really interesting science. 6. Do you have any advice for people hoping to launch a career in geography or related sciences? I think that the biggest piece of advice that I have is probably something that everyone has heard a thousand times before; follow your passion! I think the idea of pivoting or launching a career can be really intimidating, and sometimes it can feel like you have to have everything perfectly planned out. But I like to say that everything you do begets more of itself. If you focus on pursuing what you love and doing work that you feel proud of, you’ll end up on a good path. |
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