The rare and threatened Haleakala silversword plant on Maui is experiencing its lowest numbers in 45 years, as dry weather trends have reduced rainfall at its mountain habitat.

The number of silverswords in an adjusted count at Haleakala was 30,709 in 2013 — down from the census count of 73,951 in 1991. The plants had been experiencing a rebound since 1971, when 49,750 were counted in the first full census.

The new study, conducted by University of Hawaii researchers, was published in the journal Climate Change Responses in January.

According to the study, there seems to be a strong parallel between a lack of rainfall and the silverswords’ decline in numbers. There have been fewer days of wet weather at higher elevations in recent years. Through a weather phenomenon called “tradewind inversion,” dry air originating near the equator is preventing clouds from rising and providing rain on the upper flanks of the dormant volcano, scientists said.

The Haleakala silversword had its greatest resurgence in recorded numbers in 1991, after efforts were made by Halea- kala National Park officials to build fences to keep out grazing wild animals and to prevent people from collecting the plant.

The Haleakala silversword grows from an elevation of 7,050 feet to 10,000 feet and has a life span of 20 to 90 years.

Tradewind inversion has had a profound influence on mountain vegetation because it limits the vertical development of clouds. In Hawaii, the shift to more tradewind inversion days is associated with average rainfall declines of 6 percent in dry seasons and 31 percent in wet seasons, the scientists said.

In the study, scientists said tradewind inversions have occurred more frequently at Haleakala and other mountains in Hawaii since the 1990s.

A 2012 UH study found that tradewind days had dropped to 210 per year from 291 in 1985.

As shown in their recovery by 1991, the silverswords have the capacity to multiply provided there are protections and wetter weather, said Paul Krushelnycky, a UH researcher at the College of Tropical Agriculture and Human Resources.

“One of the things we did learn is the population did seem to rebound pretty quickly when conditions were favorable,” he said.

Scientists noted that rain in 2015 was heavier than in recent years, which has prompted many silversword seedlings to sprout. “That was great. We’re excited about that,” Krushelnycky said.

But he also recalled that there was heavy rain in 2004 but many seedlings died during years of subsequent dry weather.

“It seems they need to make it through a few years to deal with the stressful conditions,” Krushelnycky said.