The Haleakalā silversword is one of Maui’s most spectacular native plants. Known as ‘āhinahina to Hawaiians and Argyroxiphium sandwicense subsp. Macrocephalum to botanists, it is the quintessential plant of Haleakalā 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 hill-side look like winter or moonlight.” In the decades that followed, feral goats munched silverswords as they roamed the crater and 2-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 Haleakalā 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 Hawai‘i researcher currently 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,” says Krushelnycky. “But it doesn’t look like that’s happening right now.” While Krushelnycky fears the invasive Argentine ant could impact silverswords in the future as the insect’s population expands, ants are not currently causing a problem.
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 throughout the crater is only about 40-50 percent of what it once was. “As summers got drier,” says Krushelnycky, “drought stress was an obvious part of the picture.”
According to scientists’ predictions, this trend will continue. Rising temperatures in Hawai‘i affect the inversion layer, possibly causing the ring of clouds that surrounds Haleakalā in the afternoon to become shallower. This cloud layer currently drifts through the crater, providing water via condensation for the silverswords—which have hairs 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. Another potential cause of the drying in the crater is an overall increase in the frequency occurrences of the trade-wind inversion. “What we are seeing is consistent with basic predictions of climate change—that plants at lower elevations will have to move up,” says Krushelnycky. But for species in an alpine ecosystem on an isolated island in the Pacific, there’s only so far they can go.
The effects of climate change are frequently talked about in the news in Hawai‘i. 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 Hawai‘i are extremely vulnerable. These species are uniquely adapted to harsh climates, and sudden changes in their environment will leave them vulnerable.
Krushelnycky is currently 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 spell hope for the plant synonymous with Haleakalā.
Learn more about the impacts of climate change on the silversword: http://www.usgs.gov/newsroom/article.asp?ID=3490#.U4jzcnK-2G4
Learn more about Hawai‘i ’s changing climate: www.soest.hawaii.edu/coasts/publications/ClimateBrief_low.pdf.
*The original article, as printed in the Maui News, overstated the decline in silversword population indicating the remaining population is only 25-30% of what it was when monitoring began. While this is correct in certain monitoring plots, the population throughout the crater is 40-50% of what is once was. This correction is reflected above.
By Lissa Fox Strohecker. Originally published in the Maui News, May 11th, 2014 as part of the Kia‘i Moku Column from the Maui Invasive Species Committee