Visitors to the peak of Maui’s giant volcano in Hawaii may catch another stunning sight: giant cylindrical plants with swordlike silver leaves jutting out from their bases. These silverswords, which can grow taller than the average human, have been rapidly disappearing over the past 30 years. Now, ecologists think they know why.
Maui’s silverswords (Argyroxiphium sandwicense subsp. macrocephalum) had been declining for centuries, victims of feral goats and tourists eager to uproot living souvenirs. Even before the silversword was declared a federally threatened species in 1992, conservationists had fenced the barren slopes of their habitat, rid the area of goats, and planted silversword seeds. The efforts seemed to be working until the 1990s, after which the Maui species declined by 60%. Plants that sit farther down the volcano have suffered the most, even though they live in wetter conditions.
In 2016, Paul Krushelnycky, an ecologist at the University of Hawaii in Honolulu, noted that this recent drop coincided with more frequent changes in the trade winds, east-to-west winds that flow up the volcano. Increasingly, the trade winds’ cool, moist air has been trapped midslope by warmer air, creating hotter, drier conditions for the uphill plants. Such inversions have always been common, Krushelnycky notes, but now they are even more frequent, likely because of climate change.
To understand why silverswords at lower elevations are the most vulnerable, Krushelnycky and his colleagues grew seeds taken from plants living at low, medium, and high elevations in greenhouses and small outdoor plots. They watered some of them regularly and some of them irregularly, to compare growth under different conditions. They also measured how many plants survived. The researchers thought that plants from the highest elevations—where conditions were driest—would do better in the artificial “drought” scenarios, thanks to their presumed adaptations to dry living.
Seeds planted at lower elevations and subjected initially to moist conditions were the least likely to survive later droughts, regardless of which plants the seeds came from, the team reports in Ecological Monographs. “This suggests that traits acquired in response to the wetter growing conditions, not genetic differences, made them less drought resistant,” Krushelnycky says.
Many researchers worry that increasing fluctuations in climate conditions can have a detrimental effect on ecosystems, says Bruce Baldwin, a botanist at the University of California, Berkeley, who was not involved with the work. But that “can be difficult to demonstrate convincingly,” he says. The new findings, he adds, show that silverswords’ early adaptations to their environments “can [later] work against them.”
The work also suggests a way forward. Rather than transplanting silverswords from high elevations—where conditions are harsher—to lower ones, Krushelnycky says it may be more productive to transplant them to places with more consistent moisture, like lower grade slopes. Toward that end, he and his colleagues are planting silverswords in a wide variety of habitats to see which are best.