The Auburn University Southern Forest Nursery Management Cooperative has discovered the cause of loblolly and longleaf pine seedling toppling during strong wind events, and has developed a solution to this issue.
Over the past several years, there have been many reports of container-grown seedling toppling during hurricanes and other strong wind events. A research team led by Scott Enebak, a School of Forestry and Wildlife Sciences professor and the director of the Southern Forest Nursery Management Cooperative, has discovered that the reason behind the toppling is that the traditional seedling containers do not allow for suitable root architecture to be established.
In order to examine different root architectures, Enebak’s team constructed a clear plastic box in which they could grow multiple seedling types. Strings were run through holes in the plastic every square inch so that the roots of the seedling would stay in place. Once the seedlings developed in the box, they were placed in sandboxes and were left to grow for three months.
“From the sandboxes we could see the architecture of the roots coming from the container and that’s when we realized some containers have very, very poor root architecture, so if you were to take that seedling out and put it in the field, the roots would never become established and would never be able to stabilize the tree,” Enebak said.
Poor root architecture is characterized by a lack of horizontal root growth at the top of the root ball. The goal of the research is to develop a container that results in lateral and tap root growth, which will lead to better root establishment and minimize seedling toppling.
What the clear box experiment showed was that the quality of root architecture was dependent upon the container used for seedling development. “We basically found that some containers are good at making good root architecture and some are very poor,” Enebak said.
Seedling containers vary in size, shape and number of root pruning holes, and what Enebak’s team found was that plastic containers with many root pruning holes provided the best root architecture out of all the samples. “If you want to avoid toppling, then you want to stay away from hard plastic containers with few root pruning holes,” Enebak said.
Enebak said the reason root pruning holes provide better root architecture is because the holes stop roots from growing downward and circling in the bottom of the container, and instead allow roots to grow in all directions.
“When seedlings only have roots at the bottom of the root ball it will not keep the tree in place,” said Ryan Nadel, a SFWS research fellow who specializes in root morphology for the Southern Forest Nursery Management Cooperative.
In response to these findings, container manufacturers are beginning to move away from traditional container types and are now producing containers with many root pruning holes.
“The severity of toppling in young pine stands ranges from five percent to as much as 80 percent. The stands with high levels of toppling have to be replanted, which is a costly endeavor both in time and money,” Enebak said. “As the new container systems become available and seedlings grown in those new systems are outplanted, we should see an overall decrease in both severity and incidence in toppling.”
Enebak said once all seedlings are grown in the new-style containers reforestation efforts will be more successful, tree mortality will decrease and there will be a greater economic return to landowners and seedling producers.
Funding for this project was provided by the Alabama Agricultural Experiment Station and the National Science Foundation’s Center for Advanced Forestry Systems.