Thesis Christ: Trees Salty About Water Loss
“Turns out, pollution is bad. Shocker. There’s actual measurable, bad effects of pollution in trees. Yoinks.”
Over the course of a year, a tree on the side of a busy road can lose 3,000 more gallons of water due to pollution than its average water loss. Biology Senior Erica Bull is studying the specifics of this mechanism on giant sequoias (Sequoiadendron giganteum) in the Portland area. She has two field sites: a “dirty” site along SE McLoughlin Blvd., and a “clean” site on Reed’s campus away from any roads.
“I found that the foliage at the dirty site had a cuticular conductance that was 90 percent higher than [that] at Reed. This site isn’t like a landfill or right outside of a factory or anything like that. It’s just a roadside location that’s a high-traffic road, and these trees are each losing 3,000 gallons of water more than they should be losing,” Bull said.
Cuticular conductance measures how much water is lost through the waxy layer of a leaf’s surface. This results from plant water loss to a dryer atmosphere, particularly during periods of drought and hot weather, when plants rely on their cuticles to retain such moisture. Bull hypothesizes that this vast water loss is caused by hygroscopic pollution. Hygroscopic pollution is like when your sleeves get wet while washing dishes, and the water travels further up your sleeves. If you took organic chemistry at Reed, you did a lab that used thin layer chromatography (TLC), which works similarly to hygroscopic pollution. In TLC, the silica pulls polar substances up the coated paper, similar to the process of hygroscopic pollutants wicking water away from leaves and into the atmosphere.
Bull explained just how crucial this research is, as climate change is rapidly impacting forest mortality rates, particularly in or near urban areas. “[The layer formed by hygroscopic pollution] pulls the water straight out of the leaf, and the plants really need that water so that they don’t die,” Bull said. “[Burkhardt’s theory] really was proposed to explain the forest mortality that we’ve been seeing in the modern day. We’re living in the era of anthropogenic climate change, and we’re seeing a lot of forest mortality in response to that. We’re seeing a lot of forest mortality due to changing temperatures, due to changing pollution, and people are trying to pin down exactly why that’s happening.”
Bull is currently delving deeper into this mechanism, exploring the components of this hygroscopic layer. She found a statistically significant increase in the presence of calcium chloride, a common road salt also found in cement, and ammonium in the dirty site samples. Bull is hoping to finish collecting data by the end of spring break so that she can spend the last quarter writing her thesis.
Her advice to those who have yet to thesis? “My advice would be to breathe, first of all. I would say try to nail down your methods as soon as possible... I think everybody’s thesis process is really individual, and not everything is widely applicable, but don’t be afraid to look stupid in front of your advisor,” Bull said. “Talk with your advisor about it, because they just want to help you” Bull’s own thesis advisor, Professor Aaron Ramirez, has provided tremendous guidance throughout this process.
Bull hopes that her thesis shines some light on the mechanism behind rising tree deaths. She argues that an understanding of the processes behind these rapid changes in climate, pollution, and tree health is crucial for successful planting and survival of trees in cities.
“There’s been a lot of studies about how having trees in the city positively impacts human health and air quality and that sort of thing. [But] we haven’t looked a lot into the other side of that, where the trees are being negatively impacted by us,” she said. “There’s [an] effect on the plants for sucking up our nasty pollutants, and we can’t pretend that that’s not significant.”
Bull will present her thesis work at the Ecological Society of America Conference, a week-long environmental science meeting in Kentucky this August. She also hopes to publish her results in an academic journal after she finishes her thesis, planning to write her manuscript with Ramirez over the summer after graduation.