Mark Rudnicki, assistant professor in the Department of Natural Resources Management and Engineering, is conducting research at the Southern Maine Massabesic Experimental Forest on the nature of wind flow and tree sway.

“I started looking at tree sway during my dissertation research,” Rudnicki says. “And of course, as is typical of most of these investigations, more questions came up than were answered.”
Wind affects the growth and structure of plants. So why are trees in a forest taller and more slender than a single tree out in the open? Trees in the open experience much larger wind forces than trees in a forest, as most of their wind energy is dissipated by colliding with neighbors. Trees exposed to more wind force are thicker and shorter, which gives them the strength required to hold up under such wind forces.

To measure wind sway, a sensor is attached on the main trunk of each tree in a group of 35 trees. Rudnicki relies on PhD student Vincent Webb to attach the sensors near the top of each tree.

“He’s an experienced arborist,” says Rudnicki.
The sensor measures tilt at a rate of ten times per second, whenever wind causes the tree to sway. From recording these measurements Rudnicki is able to calculate the distance of tree sway simultaneously for all trees in the group.

“We want to understand how trees collide with each other,” Rudnicki explains. “By knowing the sway dynamics, we begin to understand the intensity of collisions when the wind blows.” This load force, or drag on the tree crown, produces energy — energy that must be absorbed by the base of the tree trunk or dissipated through collision with other trees.

“That collision effect is a group-level phenomenon,” Rudnicki says. “We want to learn how they interact and adapt to wind load as a group to enhance stability and how it controls the structure of the entire canopy.”

Rudnicki hypothesizes that how the trees move or sway is not random but is part of a finely tuned organization. Understanding this process could increase productivity and reduce losses for commercial forestry. Where loggers once clear cut forests, professional foresters now employ strategies of selective tree cutting. However, foresters know that remaining trees can be uprooted or broken by wind (“windthrow”) after thinning, and they would benefit from silvicultural prescriptions that are adapted to minimize these windthrow losses.

When a few trees are cut out of a canopy, the entire organization of that canopy changes. “If we understand how the trees hold each other up, we might be able to adjust our thinning process to work with this phenomenon,” Rudnicki notes.

Another aspect of tree physiology concerns acid rain, a particular issue for the New England states. Calcium is an essential nutrient for trees and plays an integral part in how a tree reacts to stressors such as frost, heat, insect attack, and wind force. “We know that acid rain neutralizes the calcium in soil,” Rudnicki says. “This calcium depletion could be causing our forests to slowly decline.”

Rudnicki is also interested in the relationship between global warming climate changes and the subsequent increase in wind events. Webb is designing a theoretical model for tree sway and collision that could one day help anticipate how climate changes and severe weather patterns affect our forests.

“The work that Mark is doing is bringing to light some exciting implications for forest sustainability in a changing climate,” says Professor Emeritus David Miller. “Wind storms are the major periodic disaster that destroys existing tall forests in the Northeast. One of the predicted climate changes is increased storm intensity and more frequent storms. Dr. Rudnicki’s work is likely to lead to forest management practices which can mitigate this effect.”

According to Miller, Rudnicki’s research will present numerous real-world applications such as air pollution control, carbon cycling, movement of airborne bio-terrorist materials, and the growth and development of canopy biota and wildlife.
“Rudnicki is one of only two or three researchers in the world who are expert in the physics and measurement of forest canopy motion,” Miller points out. “His work is five to ten years ahead of the mainstream research community.”

“Mark is working at the forefront of his field by supplementing his tree movement measurements with state-of-the-art micrometeorological measurements to gain a complete understanding of winds both in and above the forest,” says April Hiscox, postdoctoral fellow in the Department of Natural Resources Management and Engineering. “His interdisciplinary approach to his research is an innovative way to advance the science in forestry, ecology, and
meteorology.”