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Shrubs can help or hinder a forest's recovery after wildfire

Research from the University of California, Davis, is shedding light on when and where to plant tree seedlings to help restore forests after high-severity wildfires, and it has a lot to do with shrubs.

In hotter, drier areas where natural regeneration is weaker, well-timed tree planting can boost recovery by up to 200%, but the outcome also depends on competition with shrubs, concludes a in the journal Forest Ecology and Management.

"Generally, where there are more shrubs, the climate and soil are more hospitable for plant growth," said lead author and assistant professional researcher Derek Young. "But what that also means is there's more competition for trees."

In areas where a lot of shrubs are present, it's best to plant seedlings within a year of a wildfire to avoid competition from these woody plants. In areas with fewer shrubs, planting three years after a fire would be more effective because some of these woody plants would have grown back but not so many to consume available nutrients and water.

"Some vegetation in those really harsh sites might actually facilitate tree establishment by providing shade," Young said.

Data-based findings

Understanding how to foster recovery is critical to restoration efforts as climate change intensifies the frequency and severity of forest fires. Land managers also must use data to help direct limited resources, said Andrew Latimer, senior author on the paper and a professor in the Department of Plant Sciences.

"We're aiming to help optimize tree planting by targeting it to where it's really needed," Latimer said. "Doing this matters because we're facing a reforestation backlog—limited capacity to replant and a lot of severely burned area."

Looking at past events

Researchers surveyed areas in the Sierra Nevada that were representative of a mix of climates and management strategies in California and had been replanted with conifer tree seedlings one to three years after intense wildfires. In each of the five 400-square-meter circular plots, which included replanted and nonplanted areas, the team counted seedlings, shrub cover and other environmental details.

This allowed researchers to gauge how replanting affects the composition of forests and map out the best strategies across wide swaths of land that would be challenging to survey on foot.

"I think the real benefit is being able to make those predictions across a huge landscape," Young said. "Now we have quantified the effects of certain environmental variables that allow us to make those maps."

Latimer is experimenting to see how removing shrubs two years after a fire affects tree regeneration. Young will soon use aerial imagery and drones at wildfire sites to determine how management actions affected forests in the 40 years since the trees burned.

Quinn Sorenson, who was in the Department of Plant Sciences at the time of the analysis, also contributed to the research.