Rain dries up; silverswords see hardship
In the eyes of a typical observer, the Haleakala silversword appears to be thriving in the harsh climate atop the Valley Isle’s historic crater.
However, scientists are seeing a disturbing trend for the silvery plant, due to a steady decrease of rainfall atop Maui’s highest volcano.
In a paper published in the scientific journal Global Change Biology, a team of researchers from the University of Hawaii at Manoa, the University of Arizona and the U.S. Geological Survey found a rapid decrease in rain over the past 20 years, compared with the prior 60. Researchers found that rainfall had declined 20 inches from 1990 to 2010, causing many silverswords to die before reaching adulthood.
“Fifteen years ago, it was pretty rare for these plants to die in the young seedling stage,” said Lloyd Loope, a retired scientist with the USGS and contributor to the paper. In a process known as “bloom and doom,” the striking rosette plant grows 20 to 90 years, disperses its seed, and dies shortly after.
“But now in its severe state, the silverswords aren’t reproducing and only growing to be small- to medium-sized plants.”
Silverswords can be found between Haleakala’s 7,000- to 10,000-foot elevation, and they are experiencing hardships because of the trade wind inversion becoming more frequent, Loope said. In this weather pattern, clouds are incapable of reaching higher altitudes, thus causing less rainfall.
USGS Director Marcia McNutt said she was surprised with the federally protected plant’s decline in numbers because of its history as a hardy plant.
“The silversword is an amazing story of selective biological adaptation of this distant cousin of the daisy to the high winds and sometimes freezing temperatures on the high slopes and thin soils of Haleakala volcano,” she said. “Despite the successful efforts of the National Park Service to protect this very special plant from local disturbance from humans and introduced species, we now fear that these actions alone may be insufficient to secure this plant’s future. No part of our planet is immune from the impacts of climate change.”
The paper was originally created as a result of Loope’s research, dating back to 1982, where he documented the remarkable recovery of the silversword after it reached critical lows in the early 20th century. His research included a study that sought to create a representative sample for trends in the general population.
Nearly every year, Loope and biologists Forest and Kim Starr monitored around 400 plants and followed their growth cycle until 2010. Scientists saw the sample population reach a peak of 500 in 1990, but it experienced a sharp decline two years later.
By 2010, there were fewer than 100 plants.
Loope and the Starrs’ findings intrigued Paul Krushelnycky, a biologist with the University of Hawaii at Manoa and senior author for the paper.
“When you’re hiking the crater, it’s not really obvious that level of mortality is occurring,” said Krushelnycky, referring to the abundance of silverswords at the crater. “So we were asking ourselves, ‘Is that trend really representative of the larger population?’ . . . I don’t think those plots are too far off.”
Although researchers widely agree that the number of silverswords is decreasing, they could not give a definitive figure for the population of plants, with estimates ranging from 50,000 to 100,000. Krushelnycky said it has been difficult to reach an accurate estimate because of technical limitations and the crater’s grueling terrain.
“It’s exhausting,” he said.
The Starrs have done much of the legwork in gathering population estimates, which involves using binoculars and counting the number of plants in an area.
“There’s a fair amount of hiking and camping,” Krushelnycky said, adding that researchers use geographic positioning technology to map each plant, “but some of the populations are on cliffs and areas we can’t walk to.”
Researchers said recent improvements to remote sensoring and aerial photos have helped with silversword monitoring protocol, and researchers hope to have accurate estimates in the near future.
Krushelnycky said he hopes to learn more about the threatened plant with “seedling drought tolerance experiments.”
In the first round of preliminary results, he found that seedlings performed differently based on varying levels of water. He also found that plants that grew more slowly and had larger roots lived longer than plants that grew quickly and had long leaves.
The findings have caught the eye of Haleakala National Park, which draws 1 million to 2 million visitors a year, with many driving up to the crater to observe the rare and ancient plant.
Krushelnycky said he will verify his conclusions after a second round of results later this year, and then use the data to help silverswords thrive atop Haleakala.
“If we can understand variations in drought problem areas, hopefully it can point to some management actions and we can place seeds in other places,” he said.
* Chris Sugidono can be reached at firstname.lastname@example.org.