Maui’s rare silversword plants on Haleakala are dying from increased warmth and drought

It’s hard enough to survive as a Maui ahinahina, or silversword plant, in the harsh conditions of the stark, alpine desert of Haleakala Crater — the only place on earth where this federally threatened subspecies exists — without getting stomped on, torn up by the roots or run over by a truck or car.

Although removing or damaging a silversword is a federal crime, off-trail hikers carelessly step on the delicate plants and cause erosion of their habitat, poachers steal them and, although motor vehicles are forbidden in the crater, several ahinahina were killed or damaged on a late September night by a vehicle that left the paved road at the 10,023-foot summit, leaving crushed plants and tire marks in the cindery ground.

The hit-and-run culprits remain at large, but the carnage they caused is yet one more loss in the epic devastation of Argyroxiphium sandwicense subsp. macrocephalum, whose numbers plummeted about 60% from 1991 to 2013, leaving about 40,000 plants remaining in the wild at last count, according to Paul Krushelnycky, a researcher at the Department of Plant and Environmental Sciences in the College of Tropical Agriculture and Human Resources at the University of Hawaii at Manoa.

Contributors to this general decline are also human, but on the global and collective scale: “Another culprit that’s been harming the Haleakala ahinahina is increasingly warm and dry conditions at the top of the mountain, which we think is related to man-made climate change,” said Krushelnycky, who has been studying Haleakala silverswords in the field since 1993. (Separate and unique subspecies of silversword grow in the West Maui Mountains and high up on Mauna Kea and Mauna Loa.)

But, Krushel­nycky added, he couldn’t be 100% certain the decline was completely a result of global climate change, because it wasn’t yet clear “what effect (the climbing CO2 level) has on local rainfall patterns.”

That said, “a big part of the decline is due to changes in rainfall, and as it gets warmer, it’s going to make the water-stress these plants are experiencing that much more intense.”

1-time bloom

In 2019, Maui led the other Hawaiian islands in a year of record high temperatures and heat waves. In July, Kahului tied its all-time high of 97 degrees since record keeping began in 1954, and record highs continued to be matched or surpassed through October, the month when Krushel­nycky conducts his annual monitoring of wild silverswords.

He and his colleagues count and measure the plants in four plots that lie between 9,500 and 7,050 feet, the highest and lowest elevations of the silversword’s natural range.

At 7 a.m. one late-October morning, the thin mountain air was chilly as Krushelnycky and Molly O’Grady, an entomological research technician with the UH Research Corp., started their descent into the crater on the Keoneheehee, or Sliding Sands, trail from the summit.

At the trailhead they pointed out scarred earth and tire tracks from the September incursion.

Walking down the trail, whose rough surface looked and felt like a new black-sand beach, one had a transfixing panorama of a sort of alpine painted desert, its slopes streaked white, red, brown, black, green and gray, with monumental cinder cones rising hundreds of feet from the vast crater floor, which stretches 7.5 miles long by 2.5 miles wide.

“I always feel like I’m on Mars here,” O’Grady said with a smile.

Here and there, to either side of the path, clusters of ahinahina, which means “gray” in Hawaiian, blazed silver in the morning light, their long, spiky leaves radiating from round cores.

It was past blooming season, which starts in May and continues through September, and this hadn’t been a big flowering year, Krushelnycky said, but several past-peak ahinahina still sported a few bright purple flowers amid dried blossoms on single, central stalks 3 feet tall. Silverswords, he said, tend to grow in clusters, as new plants sprout from seeds dropped by the blossoming stalk to the ground around the parent plant.

At intervals, dead plants, known as “bloomers,” lay crumpled on the ground, dried up and withered. “Ahinahina blooms just once in a lifetime, and right afterward it dies,” Krushelnycky said.

Many lived 40 to 90 years or more before blossoming, the scientist added, although fewer seedlings nowadays were living long enough to bloom.

He waited for some hikers who were blazing a shortcut between traverses of the switchback trail, advising them to stay on the trail to avoid damaging the ecosystem; everywhere one looked, improvised paths had cut red scars through the vegetation.

“You’re almost stepping on a silversword behind you,” he warned one man, who quickly lifted his back foot.

A measure of survival

At 8,500 feet Krushelnycky and O’Grady left the trail, their boots crunching over the cindery ground.

This elevation marked “the threshold between where wild populations are pretty much consistently declining and where they’re OK,” Krushelnycky said.

The sloping ground was studded with the silver plants. Staked to the ground beside each one lay a 2-inch, numbered metal tag. Some plants were smaller than their tags.

Using a folding wooden ruler, Krushelnycky bent down to measure each plant across its widest diameter of live silver-green tissue. “When they’re under stress, there’s some shrinkage of diameter,” he said.

He called out the plant’s tag number (9) and size (35 centimeters) to O’Grady, who entered the data in a logbook.

Whether a plant had grown, shrunk or died, “it’s not the year-to-year changes that we focus on so much as looking at long-term trends,” Krushelnycky said.

Coming up at the edge of a dead bloomer was a new seedling, 1 centimeter in diameter, that hadn’t been there last year.

Another seedling had died since last year. Krushelnycky pulled its tag.

One big, dead plant probably hadn’t had a chance to flower, he said, as it had no remnant of a stalk.

There were approximately 278 plants when he started tagging this plot in 2012, and he had tagged 839 plants over the years. At last count there were 433 living plants. Many had seeded but many had died, he said.

Asked whether he got depressed by the attrition, “I try not to,” Krushelnycky replied.

Death at lower elevations

As the sun rose higher, the day grew warm, tempered by a high-desert breeze, and more colors emerged in the landscape. Bracken and grasses spilled down cliffs between bare rock buttes, forming steep alpine meadows.

“Do we have a great job or what, spending all day out of doors up here?” O’Grady asked, pausing to stretch and look around.

The wind grew stronger, whistling and moaning, and clouds filled in the blank blue sky between the cinder cones. The largest cone, which resembled the Sphinx, was 600-foot-high Puu o Maui.

Puu o Maui had once sustained an estimated 20,000 silverswords, according to past censuses. This dropped to under 5,000 in the 2013 census, Krushelnycky said.

He said plants growing at lower altitudes in the silversword’s range are dying in the hotter, sunnier conditions brought on by climate change, but higher-altitude populations are doing better — for now. Studies he published in 2013 and 2016 indicate “a much darker outlook for plants in the lower portion of the range and a more stable pattern for plants in the upper portion.”

Unfortunately, he added, “about 90% of the plants are in the lower portion,” where higher air temperatures and low rainfall were associated with lower survival and growth.

Over the past 20 to 22 years he found, there has also been higher incidence of tradewind inversion (TWI), which caps the cloud layer below the summit of Haleakala, leaving it clear and exposing the silversword habitat to more sun.

In addition, “the higher incidence of TWI is likely one of the causes of the lower rainfall at the top of the mountain, i.e. the silversword habitat,” Krushelnycky said.

In the higher-elevation plots where the plants are accustomed to a drier environment, higher air temperatures and higher TWI incidence tended to be correlated with higher survival and growth, the 2016 study showed.

Nowhere to rise

Silversword population studies began in 1982 under the auspices of the late Lloyd Loope, a U.S. Geological Survey scientist who set up 11 demography plots in the crater. It was Loope, Krushelnycky said, who inspired him to study silverswords, although his specialty had been entomology.

Krushelnycky also expressed gratitude for the work of other longtime collaborators on Hale­akala silversword demographic studies, all of whom monitor their own plots and share findings and analyses: Lucas Fortini of USGS, Don Drake at the University of Hawaii, and Forest and Kim Starr of Maui.

In November, after he and O’Grady had finished counting his four plots, Krushelnycky shared the results in a follow-up phone call. The number of plants had decreased by about 5% to 10% from last year, “which is a pretty typical pattern for most recent years,” he said.

However, while he wouldn’t know for sure until the next census of all the wild silverswords in the crater, which is scheduled for 2023, “I have a feeling that over the last five to six years, the rate of decline has been somewhat slower (than from 1991 to 2013),” he said.

He was thinking this because “since we established these newer plots, especially at those higher elevations, the number of plants has increased because there was a big seedling recruitment event and a bunch germinated.”

While most silversword seedlings end up dying off, his two higher plots still had a net gain while the two lower plots were at a net loss, he said, “but the rate of loss seems like it’s a little bit slower over the last five to six years than it was over past 20 years. Maybe that’s because it’s been a little bit wetter over that more recent period.”

In other words, while the population is still declining, it may be declining more slowly.

This was cold comfort, however, because the ahinahina’s range is finite. As warming trends continue, “the silverswords can’t go any higher on the mountain because they’re literally at the very top. They can’t shift their entire population uphill. They can only retract from the bottom,” Krushelnycky said.

As an entomologist, he was also tracking endemic species of insects that depend on ahinahina to survive, including several fruit fly and predatory wasp species, a moth, a planthopper and a long-horned beetle.

Could ahinahina evolve to withstand hotter, drier weather before they go extinct? In a paper published last month in the journal Eco Monographs, Krushelnycky and Fortini found “some differences between lower and higher elevation plants, traits (in higher elevation plants) that would promote drought resistance.”

But those high-elevation plants didn’t have higher survival rates when outplanted, so the genetic differences didn’t seem to confer a large survival advantage, “at least under the conditions we observed,” Krushelnycky said.

While governments worldwide speak of adaptation to climate change, the Haleakala silversword is a reminder that, sometimes, adaptation just can’t happen fast enough.

>> READ MORE: How humans have helped silverswords survive