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Study offers clues to restoring threatened silverswords

Silverswords grow in Haleakala Crater. A new study indicates that silverswords that grow in wetter areas have a more difficult time handling drought conditions.— Photo courtesy of Paul Krushelnycky

At Haleakala National Park, the only place in the world where the rare and endangered silverswords grow, numbers have declined about 60 percent in the past few decades.

Researchers may have found a way to inspire a comeback.

During a three-year study by University of Hawaii at Manoa College of Tropical Agriculture and Human Resources and the Botany Department, researchers found ways to restore the population by focusing on starting new plants in a greenhouse or nursery.

“Probably the most important outcome of the study was that we think we now understand why plants at lower elevations in the crater have been dying disproportionately,” said lead investigator Paul Krushelnycky, an entomologist in the Department of Plant and Environmental Protection Sciences. “This has helped guide strategies for future management and has led to follow-on research that should indicate the most suitable regions of the mountain for silversword restoration work.”

The study, “Clinal variation in drought resistance shapes past population declines and future management of a threatened plant,” was published in Ecological Monographs of the Ecological Society of America.

Silversword populations have dropped in recent decades due to reduced rainfall and warmer temperatures. Krushelnycky said Tuesday that researchers have been working closely with Haleakala National Park to collaborate on restoration strategies.

If climate conditions continue to swing toward the extremes, he said the pattern of dwindling populations could continue.

“One surprising result was that populations in wetter areas have suffered more as conditions get drier, which is essentially the opposite to what one would normally predict,” he said. “This appears to happen because plants growing in wetter areas develop less drought resistance, and so experience higher mortality during harsh dry seasons.”

Understanding which geographic areas and climates on the mountain are best for the silversword is more important than trying to discover more resistant genotypes, he added.

The experiment included two greenhouses located at high and low elevations, and planting a group of silverswords into three garden plots near the bottom, middle and summit of the Haleakala silversword range, tracing growth and survival over three years.

Researchers found that lower elevation plants are less drought resistant than higher elevation plants, and high-moisture plants were dying more often than low-moisture plants.

The study suggests that more restrictive watering methods may increase the silverswords’ chances of survival in the first couple of years after out-planting.

No matter where the seed or stem came from, silverswords were more likely to survive at higher elevations.

“This appears to happen because plants growing in drier conditions develop more roots relative to leaves, which helps them when they encounter dry conditions later,” he said. “Not surprisingly, we also found that larger plants do better than smaller ones, so growing them to a larger size before outplanting will also be helpful.”

Argyroxiphium sandwicense, subspecies macrocephalum, or ‘ahinahina in Hawaiian are delicate plants that have evolved to survive in the harsh climate of Haleakala, the National Park website says. The silvery hairs, fleshy leaves and low-growing rosette allow the unique plant to survive in hot, dry climates like the desert cinder cone slopes of the Haleakala Crater, the website says. Silverswords flower once, sending up an eye-catching flowering stalk, and then die soon afterward, scattering drying seeds to the wind.

* Dakota Grossman can be reached at dgrossman@mauinews.com.

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