Warming climate could be silversword’s toughest challenge
The Haleakala silversword is one of Maui’s most spectacular native plants. Known as ahinahina to Hawaiians and Argyroxiphium sandwicense subsp. Macrocephalum to botanists, it is the quintessential plant of Haleakala Crater and grows nowhere else on Earth. This much-loved species has survived many threats over the last century, but a warming climate may prove its toughest challenge yet.
In the late 1800s, silverswords were so plentiful that explorer and naturalist Isabella Bird wrote of finding “not . . . one or two, but thousands of silverswords, their cold, frosted silver gleam making the hillside look like winter or moonlight.” In the decades that followed, feral goats munched silverswords as they roamed the crater and two-legged souvenir hunters yanked the unusual plants up by their roots. Anecdotal accounts from the 1920s and ’30s indicate that the silversword population was in dramatic decline.
After Haleakala National Park fenced the summit and removed the last of the goats, there was good reason to believe that silverswords would recover. Visitors learned to take photos rather than live plants as souvenirs. In 1982, biologists began documenting the recovery of this threatened species that had become part of the park’s allure.
They counted silverswords at various sites throughout the crater nearly every year, but were surprised by the results.
“Data from plots show a really obvious trend when you look at them over the last 30 years,” explains Paul Krushelnycky, a University of Hawaii researcher studying the silverswords. From the early ’90s or so, you get a steady decline.”
He investigated potential causes of the decline, first looking at invasive species. “Ants were a concern – they could be impacting the pollinators,” Krushelnycky said. “But it doesn’t look like that’s happening right now.”
While Krushelnycky said he fears the invasive Argentine ant could impact silverswords in the future as the insect’s population expands, ants are not causing a problem, at least for now.
When Krushelnycky compared silversword population data to climate data, specifically rainfall during the drier summer months, he saw a clear pattern: as summer rainfall declined, so did the silverswords.
Today, the silversword population is only about 25 to 30 percent of what it once was. “As summers got drier, drought stress was an obvious part of the picture,” he said.
Scientists predict this trend will continue. Rising temperatures in Hawaii affect the inversion layer, causing the ring of clouds that surrounds Haleakala in the afternoon to become shallower. This cloud layer drifts through the crater, providing water via condensation for the silverswords, which have hair on their leaves specially adapted to collect this moisture. As the height of the inversion layer continues to drop – as it’s predicted to with rising temperatures – clouds drift into the crater floor less frequently.
Consequently, less moisture is available to the silverwords living at the crater floor. “What we are seeing is consistent with basic predictions of climate change – that plants at lower elevations will have to move up,” Krushelnycky said.
The effects of climate change are frequently talked about. Rising sea levels threaten coastal ecosystems and even zoning regulations for coastal buildings. But the rare plants and animals found in the alpine ecosystem in Hawaii are extremely vulnerable. These species are uniquely adapted to harsh climates, and sudden changes in their environment will leave them vulnerable.
Krushelnycky is looking for genetic variations among plants to see if some populations are more drought tolerant. At the same time, he’s looking to see if elevation factors into silversword survival. His findings will likely influence future decisions about where to collect seeds and where to plant keiki – results that may bring hope for the plant synonymous with Haleakala.
To learn more about the impacts of climate change on the silversword, go to www.usgs.gov/newsroom/article.asp?ID=3490#.U4jzcnK-2G4
For more about Hawaii’s changing climate, see www.soest.hawaii.edu/coasts/publications/ClimateBrief_low.pdf.
* Lissa Fox Strohecker is the public relations and education specialist for the Maui Invasive Species Committee. She holds a biological sciences degree from Montana State University. Kia’i Moku, “Guarding the Island,” is prepared by the Maui Invasive Species Committee to provide information on protecting the island from invasive plants and animals that can threaten the island’s environment, economy and quality of life.