Unlike most other flowering plants, the male and female reproductive organs of the edible valerian (Valeriana edulis) are housed in separate plants. They can’t self-fertilize the way hermaphroditic plants do. Now, researchers studying the sex lives of this alpine herb in the Rocky Mountains reveal that a warmer, drier climate ups the ratio in favor of males at higher elevations. And that’s likely because male and female plants have different water needs, according to findings published in Science last week.

Based on a 2011 survey of the valerian plant (also called tobacco root) across an elevation gradient in the central Colorado mountains, the frequency of males decreases with elevation. At the lowest elevations, which tend to be hotter and drier scrublands, males make up 50 percent of the flowering individuals. At the highest elevations, with wetter and cooler tundra settings, males represent just 22.7 percent.

To see how climate change impacts the plants, a team led by William Petry of ETH Zürich examined sex ratio data on 15 valerian plant populations across its 1,800-meter (5,900-foot) elevation range from between 1978 and 2011. During those decades, precipitation and soil moisture in the area decreased as temperature increased 0.21°C (0.38 °F) each decade.

They found that as the climate became warmer and drier, the percentage of males at higher, up-slope locations increased, altering pollen availability. Male and female valerian plants respond to climate change differently – likely because of the higher costs of reproduction in females – and their varying water-use efficiencies could be used to predict their population sex ratios.

"Understanding the responses of both sexes is important, because each sex must find mates of the opposite sex to reproduce," Petry explained in a statement. "No past work has connected ecological differences between males and females to their responses to climate change and the subsequent consequences for populations." Other plants with separate male and female individuals (called dioecious plants) include asparagus, aspen, gingko, holly, papaya, pistachio, spinach, and willow.