This story first appeared in the SERC quarterly Newsletter spring 2006

 

Dr. Jess Parker;
Investigating the Shape
of Forest Function

As the first tentative leaves of early April succumb to the lengthening of days with their annual verdant explosion of life, the forests at SERC announce the indisputable arrival of spring. The energy in the woods is palpable beyond the twittering of young birds and the dancing of dappled sunlight. It is the energy of an enormous giant stretching and yawning itself awake in the early hours of the morning.

Like large breathing organisms, forests are complex and elegant living systems. They house an amazing diversity of plants and animals, providing food, shelter and security. They interact with the atmosphere, effectively "inhaling" carbon dioxide and "exhaling" oxygen. Forests reflect sunlight and heat into the spaces above them, while shading and cooling the spaces below them during the day. At night, the forest canopy insulates the space below it, trapping heat like a protective blanket. The trees of a forest exchange nutrients and carbon with the soil, and channel rainwater through the delicate weave of leaves that form their canopies.

Understanding how these complex and intricate systems function is critical to understanding some of the most important questions facing humanity today - questions about the role of forests in providing refuge for the world's biodiversity and their capacity to serve as sinks for the excess carbon dioxide human activity is pumping into the atmosphere.

Building the Tool Box:

SERC researcher Jess Parker is trying to understand forests in all their complexity. Much of his work is focused on the relationship between the physical shape of a forest and the way it functions. "I want to know how the structure of the forest impacts the balance of energy and material exchange with the atmosphere, how it affects the microclimates inside the forest and the diversity of habitats," Parker said.

"How many kinds of places are there in the forest?" he wonders, explaining that some forests contain lots of open spaces between the branches while others are denser. Parker wonders how that structure impacts their function. For example, open spaces like the holes in Swiss cheese may provide better places for bats to hunt than denser areas. But accurately assessing the organization of the forest and how this influences functions has been an evolving process that has only recently become possible for forest ecologists.

"When I first started, we climbed individual trees," Parker said, "but we could only measure close to the trunk. We couldn't see the open spaces, the air spaces between trees."

Their measurements were essentially two-dimensional. "But forests are fundamentally three-dimensional," he said. "Every little bit is different and we need away to account for these different bits and the way they function based on what we can measure."

To do that, forest ecologists like Parker have had to get inside forests in ways no one had done before. For the past 15 years, he and his colleagues have been literally building their ladder higher and deeper into the spaces within the forest. The development of three new tools in particular has provided them with the important rungs they've needed to really get at the nature of how forests work.

Canopy Cranes:

When Smithsonian Tropical Research Institute scientist Alan Smith first used a construction crane to access the forest canopy in Panama, Parker knew right away that a new door had opened. "It became clear that the way we looked at forests before, just walking around the forest floor, was really a small part of the picture," he said. That was in 1990. Now canopy cranes are used to access all levels of forests throughout the world.

Eddy Flux Monitoring Networks:

Forest canopies play an important role in the uptake of carbon dioxide from the atmosphere and the release of water vapor. Advances in technology have provided researchers with affordable and reliable tools to make very rapid real-time measurements of the rates of these exchanges. With these tools mounted on the research tower at SERC, Parker is able to create a detailed and accurate picture of what's happening throughout the forest as a whole. With eddy flux monitoring stations now established around the world, forest ecologists have developed a network for creating a global picture of the exchange of heat, CO2 and water vapor between forests and the atmosphere.

Remote Sensing:

Only a few decades ago, remote sensing meant simply aerial photography. For forest ecologists, that meant pictures of the top of the canopy. Although useful, such photos were limited in terms of the information they provided. Today, remote sensing tools are penetrating surfaces with radar and lidar technology. With the advent of lidar (light detection and ranging) technology ecologists were able to visualize the interior structures within a forest.

But Parker was interested in seeing more detail than what aerial lidar could achieve. The limitations of scale and cost of aerial lidar inspired him to develop a portable lidar system. With his new tool, Parker can walk through the forest scanning the spaces from below to create a three-dimensional picture with details to the scale of one meter. His tool has been described as a sort of CAT-scan for an organism hundreds of feet tall by many acres wide. Now he can get inside the structure of the forest and answer questions he's been curious about for a long time.

The Meaning of Shape

Years of looking for relationships between shape and function have led Parker to an understanding of a few remarkably simple concepts that belie the complexity of the science behind them. He's using those concepts to push the limits of what we know about forests.

"By measuring the shape of the canopy and height of the forest, we can estimate a great deal about its function," he said. A canopy's shape is determined in part by the type of forest it is, but also by its age and stage of development. All the trees in a young forest tend to be the same height because they are similar ages and are all growing rapidly in competition with one another. An older forest will have gaps where trees have fallen and younger ones have started to fill in. This mixture of young and old trees creates a bumpier canopy surface. Parker has introduced the term rugosity to describe the bumpiness of a canopy surface.

The rugosity of a canopy surface and the height of the trees tell Parker the forest's stage of development, which leads him to conclusions about how rapidly it's growing and how much biomass it contains. From this knowledge, Parker can calculate the net exchange of carbon dioxide between the forest and the atmosphere. He hopes to soon be able to calculate growth rate and make inferences about a forest's complexity with these same measurements. Parker is testing the limits of what these simple measurements can be used for.

Rugosity-Shaping a Bigger Question

Rugosity reveals more to Parker than the age of the forest, however. When he and his colleagues compared measurements of heat reflected from forests with lidar-based measurements of rugosity and forest service maps of old-growth and young forests, he found that higher rugosity, old-growth forests reflected 10 percent less of the sun's heat energy. And if it's not reflecting that energy, it must be absorbing it. What his measurements don't tell him is where that energy is going? What is that old forest doing with that extra 10 percent?

Parker would like to find out. "The energy you keep is the basis of the food web," he said. "The more energy absorbed, the bigger energy base for the system in an old-growth forest." He suggests that the extra energy is like getting a 10 percent raise in salary. "What would you be able to do with the extra resources? If you had more salary, you could pay debts or ward against crises, maybe fend off stressors and pressures." Parker said he doesn't know if that's how old forests are using the extra energy they absorb. But now that he knows they're doing it, he wants to know why.

As May matures into June in the SERC forest, the energy of spring awakening will calm into the steady pace of humid mid-Atlantic summer. Leaves will thicken and turn dark green and shadow will fall on the forest floor. Jess Parker will be there, walking among them, looking for the extra energy among the old growth, and trying to understand the meaning of spaces.


 

For more information, or to reach Dr. Parker, please contact SERC science writer Kristen Minogue.