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Plant age drives mortality, reproductive success and population dynamics

Recovery of big sagebrush populations after fire is inhibited by the loss of adult plants and the limited ability of new seedlings to survive or reproduce — a limitation with negative population consequences that last for years to decades after post-fire seeding restoration efforts, according to a recently published study by the U.S. Geological Survey

“The recovery of big sagebrush habitat is one of the largest, if not the largest, ecosystem restoration challenges in the U.S. right now. Hundreds of thousands of acres burn each year, and millions of dollars are invested in trying to restore big sagebrush in these areas,” said USGS scientist and lead author of the study, Robert Shriver.  “This study could lead to new and more cost-effective strategies for land managers trying to restore big sagebrush populations after fire.”

Big sagebrush is one of the most iconic plants of the American West, and more than 300 species of conservation concern, like the sage-grouse and pygmy rabbit, rely on big sagebrush ecosystems. Wildlife like mule deer, elk, pronghorn, sage sparrows and sagebrush voles also use sagebrush for food and habitat.

Although big sagebrush is found throughout the Intermountain West — from Montana south to Arizona — human activity, nonnative plant species invasions and wildfire have resulted in widespread loss of big sagebrush. To complicate matters, active attempts to restore big sagebrush after fires have had mixed results, even at locations that previously supported healthy populations.

To better understand why it is so difficult for big sagebrush to recover, USGS scientists studied 531 burned sites in the Great Basin. Following fire, these sites were reseeded with big sagebrush by the Bureau of Land Management. Using data collected on the re-established big sagebrush individuals, combined with statistical and mathematical modeling, USGS scientists were able to reconstruct the rates of survival, growth and reproduction of big sagebrush at each of the 531 sites since they were seeded.

“One of the biggest challenges we face in trying to understand what controls big sagebrush recovery after fire is that there isn’t a lot of long-term monitoring data available. This was a great opportunity to partner with the Bureau of Land Management to return to areas seeded decades ago and assess what drove post-fire dynamics of big sagebrush,” said Shriver.

The USGS scientists found that even though many sites originally supported healthy, mature big sagebrush populations before they burned, and those sites are still capable of doing so, big sagebrush populations may not necessarily recover after seeding. This is because newly established big sagebrush populations are mostly composed of small, young plants that have low survival and reproduction rates, resulting in rapid population decline and the potential for local extinction.

“What we find is that rather than recovering, most populations decline for years or decades after seeding, leading some to completely die out at individual sites. A big driver of this is the biology of big sagebrush itself; without some larger plants to anchor the population by surviving and reproducing, recovery becomes really difficult,” said Shriver.

For example, smaller plants (6 inches or shorter) had only an 8% chance of surviving from year to year whereas plants greater than 30 inches tall had nearly a 100% chance of surviving from year to year. Likewise, older, larger plants are much more reproductively successful than younger, smaller plants.

However, there may be strategies that could work to re-establish big sagebrush.

“This work suggests some good options for improving restoration going forward,” said Shriver. “First, at high elevations, restoring populations through seeding is often successful because plants grow faster and reproduce more than at lower elevations. But at low elevation sites where it is warmer and drier, planting nursery-raised seedlings of sagebrush may be a more effective strategy than seeding to increase survival and reproduction and speed up recovery.” This approach would be similar to wildlife conservation programs that rear animals to a certain age before releasing them into nature: applying this general strategy to big sagebrush may help it survive those early years when it is most vulnerable.

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