Complex tree canopies are found to help forests recover from moderate disturbances

Extreme events destroy entire forests, dramatically uprooting complex ecosystems and local communities. Researchers have become familiar with such attention-grabbing phenomena over the years. However, they know less about more common moderate-to-severe disturbances, such as relatively small fires, snowstorms, and outbreaks of insects or pathogens.

„Because they are so common, they play a larger role in the ecosystem than we previously appreciated,” said Brady Hardiman, associate professor of forestry and natural resources and environmental and ecological engineering in Purdue University's College of Agriculture.

„At any given time, a large portion of the forested landscape is undergoing moderate-to-severe disturbance or regrowth, which has removed some trees, but not all of them. The forest is not growing new.”

An article published in Journal of Ecology Purdue University researchers and their co-authors identified how moderate-to-severe disturbances cause different types of changes in forest canopy structure. Hardiman and his colleagues based their findings on lidar (light detection and ranging) data collected at five sites of the National Science Foundation's National Environmental Monitoring Network in New Hampshire, Massachusetts, Virginia and Tennessee.

„The most interesting finding of this study is that multitemporal lidar data can detect subtle signals of disturbances,” said the paper's lead author, Purdue postdoctoral scientist Dennis Heejun Choi.

Neon began collecting data ten years ago. Repeated observations of NEON collecting species at specific forest sites are relatively rare, especially at the continental scale.

„Neon is a large-scale modeling effort,” said co-author Elizabeth LaRue, an assistant professor of biological sciences at the University of Texas at El Paso. „It's great that we now have the data to do something like this over time.”

Structural dimensions of forest canopy include height, openness, density and complexity. Previous research by Hardiman and others has documented that structurally complex canopies absorb more light and that their complexity is linked to essential ecosystem functions. These include nutrient cycling, providing shelter and nutrients to organisms, and promoting biodiversity.

Comparing an old-growth forest to a Christmas tree farm provides a simple contrast to the complexity, LaRue, a Purdue Ph.D. Alumni. Trees planted in the same row on a farm are all approximately the same age and height. An old-growth forest, meanwhile, shows more variation with trees of different sizes, ages, species and shapes.

„You can measure things that are equivalent to a block of cheese,” he said. „Swiss cheese is much more complex than a block of cheddar. Some of the metrics we use measure how many holes are in your forest.”

Researchers examined differences between episodic and pulsatile disturbances, discrete events, compared to long-term events. The co-authors looked for patterns of changes in canopy structure following disturbances. They found that forests with more complex canopy structures are better able to withstand disturbances and recover.

„Canopy structure is something we can modify through management actions,” Hardiman said. „Managing forests to improve structural complexity can make our forests more resilient to various disturbances in ways that allow them to continue to grow following those disturbances.”

Crunching the NEON data was a computationally intensive process that required resources from Purdue's Rosen Center for Advanced Computing. Includes the process of accounting for changes in lidar technology and the different sensor configurations used over the years. New lidar systems with strong beams produce dense point clouds — 3D data sets that represent the shape of forest canopies.

„We tried to equalize the point density from year to year to make comparable measurements,” Choi said. The challenge, he noted, was to balance the improved measurement capabilities of the new sensors with stability and relative demand over time.