Complex canopies help forests recover from disturbances

Extreme weather and natural disturbances often capture headlines for their dramatic impact on forests, but it’s the more common, moderate-severity disturbances – like smaller fires, ice storms, and pest outbreaks – that may play a bigger role in shaping ecosystems than previously recognized. 

“Since they’re more common, they’re probably playing a larger role in the ecosystem than we might have appreciated before,” said senior author Brady Hardiman, an associate professor of forestry and natural resources and environmental and ecological engineering at Purdue University. 

“At any given time, a huge fraction of the forested landscape is undergoing or regrowing from a moderate-severity disturbance, which took out some of the trees but not all of them. The forest is not regrowing from scratch.”

Subtle effects of forest disturbances 

In an effort to delve deeper into this topic, a team of researchers from Purdue, used lidar data to examine changes in forest canopy structure resulting from these disturbances. The findings, published in the Journal of Ecology, are based on data from the National Ecological Observatory Network (NEON) across several U.S. states. 

The team discovered that lidar, a technology that measures distance with laser light, can reveal subtle effects of disturbances on forests. Study lead author Dennis Heejoon Choi, a postdoctoral scientist at Purdue, notes the capability of lidar data to detect these subtle signals.

This research is significant because it helps clarify the role of moderate-severity disturbances in forest ecosystems, which has been less understood compared to catastrophic events. 

Complex forest canopies

Forest canopies, which include the upper layer of trees, play a crucial role in biodiversity, nutrient cycling, and sheltering various organisms. Complex canopies, which are not uniform and include a variety of tree sizes and species, are particularly good at absorbing light and supporting ecosystem functions.

Elizabeth LaRue, assistant professor at the University of Texas at El Paso and co-author of the study, compares complex canopies to a block of Swiss cheese, with its holes and variations, as opposed to the uniformity of a cheddar block or a Christmas tree farm. This complexity, the researchers found, may help forests better withstand and recover from disturbances.

“You can measure things that might be equivalent to a block of cheese. A block of Swiss cheese would be more complex than, say, a block of cheddar. Some of the metrics we use essentially measure how many holes you have in your block of forest,” she explained.

Supporting forest growth and recovery 

The study also explores the idea that managing forests to enhance canopy complexity could improve their resilience to various disturbances, supporting continuous growth and recovery. This approach requires a significant amount of data, which the researchers obtained from NEON’s comprehensive and repeated observations across the continent.

Handling the NEON data posed challenges, especially in maintaining consistency across different lidar technologies over time. The research team, supported by Purdue’s Rosen Center for Advanced Computing, worked to normalize data despite these challenges, aiming to provide comparative metrics that reflect changes in forest canopies accurately.

Broader implications 

This work contributes to the goals of the Institute for Digital Forestry, which aims to develop tools and methods for more detailed and frequent measurement of individual trees and global forest health. 

By integrating forest ecology with computer science and engineering, the institute seeks to provide innovative research in the field of forestry, asking new questions and finding new answers about how forests grow, change, and respond to the world around them.

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