Male and female plants get together in the heat

Foto: Will Petry


Gender differences make a big difference – especially when certain plants start to adapt to a warmer climate. A North American species of valerian may survive by changing the distribution of male and female plants to improve conditions for reproduction. This is ten times faster than simply spreading into cooler areas, which is what most other plants do.


By Peter F. Gammelby

It is well known that many species of plant respond to the warmer climate by spreading slowly towards cooler habitats – either by moving farther up mountain sides (if there are any in the vicinity), or by seeking out cooler latitudes.

However, the operative word here is ‘slowly’. This is because the plants on the outskirts of the current territory of the species in question have to mature before they can produce enough offspring to colonise a new area.

A team of researchers has now discovered that, for the past 33 years, one particular species of Valerian – Valeriana edulis – has been dealing with the warmer climate in a completely different and much quicker way: it alters the gender distribution within its existing populations

The plant grows in the Rocky Mountains, so it has every opportunity to spread upwards.

The study was conducted by a team of researchers from several American universities and Aarhus University, and the findings have recently been published in the recognised journal Science.

Uphill for the males

In order for a species to be able to alter its gender distribution, it has to have a gender distribution to start with – and Valeriana edulis does. It is dioecious, which means that just like the vast majority of animal species, each individual plant is of just one gender, either male or female. If the female plant is to form seeds, its flowers have to receive pollen from a male plant’s flowers.

Normally, the female Valeriana edulis plants thrive a little higher up the mountain than the male plants, so the higher you climb above sea level, the fewer male plants are to be found in the population. At a height of 2,000 metres, the gender distribution is thus fifty-fifty, but at the cooler and moister conditions at 3,800 metres, there are only around 20 per cent males.

However, as the mean temperature has risen all the way up the mountainside, it appears that the male plants are increasingly ‘moving in’ with the females on the ‘upper storeys’.

The extra pollen they bring has boosted pollination at the higher altitudes: the female plants in the formerly predominantly female areas are now forming seeds in almost all their flowers, whereas fewer than half the flowers were pollinated previously.

Sons at breakneck pace

The male plants are not physically crawling up the mountain, of course. What the researchers have observed is the populations in the higher reaches of the mountains are producing more ‘sons’ than they did before.

The gender distribution is moving up such that the level at which there are equally many males and females is now half a kilometre higher up than it was 33 years ago, when the plant population in the area was last studied.

In other words: the most productive section of the population is moving up at the breakneck pace (for mountain plants) of 17.5 metres per year, and can easily keep pace with the changes in the climate.

Over the past 33 years, the mean temperature here has risen by 0.63°C, precipitation has fallen by 6.3 mm, soil humidity has dropped 4.5 per cent, and the snow now starts to melt almost ten days earlier.

This, in turn, has resulted in the valerian plants starting to flower earlier.

Thirsty females

The reason why the female plants normally live at higher altitudes than the males is that more water is needed to form seeds than to form pollen, as Amy M. Iler, a Cofund Junior Fellow at the Aarhus Institute of Advanced Studies, Aarhus University, explains.

“We don’t know exactly why the male plants don’t occur at higher abundances with the females in the wettest populations to begin with. You would think they could cope with the extra volume of water, and you’d think a more even gender balance would be an advantage for the population given that it translates into more efficient propagation,” she says.

Amy M. Iler doing field work in Rocky Mountains. She's a field biologist and currently a Cofund Junior Fellow at Aarhus University. Foto: Paul CaraDonna

First study of its kind

The most important aspect of the current project, however, is that it marks the first time that anyone has examined how climate change affects the sex ratio of a population.

“Up until now, researchers have focused on how the geographic ranges of various populations are moving. We have now shown that males and females react differently to climate change, that these gender-specific reactions are of significance to the viability of the populations, and that the speed at which this particular species’ properties are reacting to climate change is remarkably high. Changes in the gender distribution have ecological consequences and affect dispersal, population growth and interaction with insects and herbivores,” says Amy Iler, who continues:

“The significance of gender distribution probably also applies to other dioecious plants, but this will require additional studies.”

A minority among plants

The vast majority of plants on Earth have both male and female flowers (monoecious), twin-gender flowers (hermaphrodites), or reproduce asexually (apomixis).

That is not to say that dioecious plants are extremely rare, however.

Even though Valeriana edulis is dioecious, it is not the same species as Valeriana dioica which is to be found in Danish marshes and meadows, and which (as the name suggests) is also dioecious.

Other common dioecious plants in Denmark include asparagus, holly, spinach, willow, aspen, ash, sea buckthorn and hops.