Vast Array Of Sensors Will Watch The Forest Breathe

By Jen Schripsema

In an ancient parable, three blind men come across a large, strange animal. To identify it, each man feels a different part of the animal's body. After returning home, they are surprised by their various impressions: a wall, a tree, and a snake. While each man truthfully and accurately described what he felt, only a man who could see the whole animal could accurately identify it: an elephant.

Unfortunately, most research on forest ecology has followed this parable's poor example: discrete studies undertaken independently by numerous researchers whose findings are never fully integrated.

Now, researchers at Oregon State University are trying to change that. Creating a comprehensive picture of forest ecology is the difficult task of the Forest Ecophysiology and Ecohydrology Lab. "We need to bring together the ecosystem scientists, the atmospheric experts, the engineers and soil scientists, and try to put all the pieces back together to really understand how the whole system works,” says director Barbara Bond.

OSU researchers are undertaking this difficult task in difficult terrain. Most forest ecology is studied on flat land because it is easier and less costly than working in the steep terrain that characterizes most forests. But it seems that the processes that characterize forests on flat land don't always apply to hillier terrain. "On flat land, we usually think of vertical exchanges of things: carbon dioxide comes in from above, water transpires and it goes straight up. It is what we call one-dimensional exchange: up and down and up and down. In mountain systems, things don't go up and down anymore; water moves sideways, air moves sideways. It creates an entirely different connectivity than we find on flat land and we don't know how to characterize that very well yet,” says Bond.

To understand these forest processes, researchers with the lab are measuring a myriad of soil, air, and plant properties in a watershed of the H.J. Andrews Experimental Forest. Mountain ecosystems are characterized by patchiness and high variability; because of these factors and the large number of parameters being measured, researchers need large amounts of data to make firm conclusions. To aid in data collection, they are using an array of sensors wired to electronic dataloggers that can measure and record data independently.

Last year, the OSU lab received a $1.1-million engineering grant from the National Science Foundation to develop a new way of providing power to these sensor arrays. The proposed technology will use radio waves broadcast from a central hub to provide power passively to sensors in the area. Development of this technology will probably take two or three years. Once in place, researchers will be able to use many more sensors over a much larger area, furthering the goal of developing a comprehensive picture of the forest ecosystem. "It's a very ambitious project. It's a whole quantum leap in how we might use sensor arrays,” says Bond.

Although the sensors now in use allow researchers to spend more time analyzing data and less time hiking up steep slopes to gather samples by hand, they are not perfect. The Ecophysiology and Ecohydrology Lab powers its sensors by stringing wire across the landscape, which can be dangerous. "Animals chew through wires; it's just a mess,” says Bond. Using wires also limits the size of the area sensors can monitor.

However, the current generation of wireless technology requires batteries at both the receiving and transmitting ends. Batteries contain dangerous acids and metals, things "you don't want to be distributing across a landscape, especially a wild landscape, because they can sit there and decompose over time and cause environmental problems,” says Bond. And, they eventually will run out of power.

Even with these limitations, the current sensor array has yielded significant results. For one, the findings have shed light on just how airflow in hilly terrain differs from that in flat land. Especially on cloudless nights, cold air rushes down slopes like a river of air. At the site where Bond and colleagues wre working, this river of cold air is fast, well-developed, and over 100 ft in height. Researchers are still unsure how these drainages interact to form "airsheds” and what implications they have for mountainous forest ecology.

Interestingly, researchers may be able to use these airsheds to estimate plant stress. This potential application would allow researchers to see stress caused by climate change before plants show visible symptoms. The ecosystem processes involved are complicated and still not fully understood. Briefly, many people are familiar with the simplified equation for photosynthesis: carbon dioxide + water + sunlight = glucose + oxygen.

Carbon dioxide is present in the atmosphere in two stable isotopic forms, C12 and C13. During photosynthesis, plants show a preference for C12 carbon dioxide. However, when plants are stressed by drought, disease, or climate change, they will fix through photosynthesis essentially any carbon they can get their leaves on. Bond likens it to the adage "beggars can't be choosers.” Researchers have understood these plant-level processes for approximately 20 years.

OSU researchers are just beginning to understand what happens to the carbon after it gets fixed through photosynthesis. They have found that the amount of time it takes for that carbon to return to the atmosphere is surprisingly short. "Turns out it just takes a few days for much of the carbon to go from the leaf level of fixation, down to the soil, be used by bacteria, and then the bacteria release it as respiration. That respiration comes back out of the soil and carries with it that C12/C13 signal that occurred from the photosynthetic discrimination a few days ago,” says Bond.

Therefore, by analyzing the isotopic ratio of carbon dioxide respired by the ecosystem and carried by airsheds through the forest, researchers can get a sense of how discriminatory plants are being about the carbon dioxide they fix. Presumably, this measurement will indicate plant stress levels. Bond cautions that this overview is simplified and still not fully understood.

With respect to climate change, this research also has implications for carbon sequestration. This idea is to offset the amount of carbon dioxide being released into the atmosphere through burning fossil fuels by storing carbon in "sinks.” The large biomass and longevity of trees make forests the principal sinks in terrestrial ecosystems. "Almost all the research that is going on right now on carbon sequestration is on relatively uniform flat areas because that's what we know how to measure,” says Bond.

However, this and other recent research has called into question how applicable the carbon sequestration principles developed from studies in flat terrain will be in complex mountain ecosystems. The speed with which carbon is shuttled between plants, soils, and the atmosphere suggests that much of the carbon plants fix winds up in the atmosphere only a few days later and isn't stored long-term in plant mass. Therefore, increasing the photosynthetic rate of plants may not lead to carbon sequestration. The link between those two processes doesn't appear to be as tight as researchers once thought.

Sensor arrays to be used in the near future will help shed light on all of these complex interactions. The work joins other efforts across the country investigating the future of sensing technology. For example, the University of California's Center for Embedded Networked Sensing in Los Angeles, a National Science Foundation Science and Technology Center, studies the diverse applications of sensing technology, including monitoring seismic activity and aquatic microbial communities.

Jen Schripsema earned a B.A. in biology from Colorado College. She is currently pursuing a master's degree in technical communication at the University of Washington.

Image at Top:

Several sensors are mounted on this 37-meter tall tower to measure vertical profiles of carbon dioxide, humidity, temperature, wind direction, and wind speed. Photo: Tom Pypker