Compared to some plants, like ferns or mosses, the well-named Haleakalā silversword, or ʻāhinahina (Argyroxiphium sandwicense subsp. macrocephalum), appears forbidding and invincible with its metallic-colored, pointed leaves braving desert-like conditions in its mountaintop home. You wouldn’t think that something like a shift in cloud patterns could cause so much trouble to this charismatic Hawaiian plant. But, you would need to think again. 

A new study, published in Climate Change Responses, points to how a shift in atmospheric circulation patterns may make a difference in the survival of silversword populations. Paul Krushelnycky, ecologist at University of Hawai‘i-Mānoa, led the research to study the effects of changes in temperature, precipitation, and solar radiation on populations of silverswords over 80 years of data records. 

The team found that Haleakalā silversword numbers were increasing in the middle twentieth century, likely due to restrictions on gathering and reductions in browsing animals, until 1990 when numbers began to falter and dive in the lower population. This decline coincided with lower rainfall in the area that has been tied to increased occurrences of the trade wind inversion. 

The trade wind inversion (TWI) forms when rising air is impeded by dry descending air originating near the equator. This caps the vertical formation of clouds, resulting in the familiar cloud layer that forms on the upper slopes of Hawai‘i’s higher mountains. The TWI occurs most days, and on Haleakalā it usually holds the cloud layer just below where the silverswords reside, typically keeping them in sunny and dry conditions.

When this TWI pattern is disrupted, clouds, fog, and rainfall reach the higher altitudes. Fog and clouds then bring water to the plants and provide shade, which slows evaporation. It is easy to understand that a change in that occasional water source could seriously disrupt how the silverswords survive their harsh environment. Scientists discovered that this disruption of the TWI has now been happening less often, creating drier conditions above the inversion layer. 

It turns out that the occasional break in the TWI pattern may be crucial to the silverswords, especially in the lower range where the habitat is wetter. The decline of the population was most affected there as opposed to the drier, higher reaches. “Case studies like these are important, because they show how complex and subtle changes in climate can have strong effects on plants and animals,” commented Krushelnycky.

The bad news is that climate projections anticipate that the TWI may occur more often and even shift down the mountain, farther from silversword habitat. Additionally, some climate projections predict lower rainfall in both the wet and dry seasons on the upper reaches of Haleakalā. There is some good news, though, brought to us by the history of these unique plants. "On the bright side, the long term data set shows that the silversword population can rebound and grow quite quickly, as it did in the middle of the 20th century when it was protected from threats and rainfall was higher,” says Krushelnycky. “But, this also implies that the future health of this species will probably depend on a return to wetter conditions."

To read the full article, “Change in trade wind inversion frequency implicated in the decline of an alpine plant,” please visit this site. This work was supported by the Department of Interior Pacific Islands Climate Science Center, which is managed by the USGS National Climate Change and Wildlife Science Center. The center is one of eight that provides scientific information to help natural resource managers respond effectively to climate change. This study was also supported by the National Park Service, the Pacific Islands Climate Change Cooperative, and the Hauʻoli Mau Loa Foundation.

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