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Sex on Fire and Vachellia caven

In Central Argentina grows a tree which can produce flowers with different sexual characteristics? Why? Is there an advantage to changing your sex life if you're a plant?

What sex should you be if you want to maximise your reproductive success?

The answer depends on what you are and where you are. A study by Marquez and colleagues looks at the case of Vachellia caven. V. caven (or Roman Cassie or Espino or Arimo and in older texts Acacia caven) can be found in South America, in Argentina, Bolivia or Chile. It grows to a height of four or five metres, with sharp thorns and yellow flowers. It has a few uses. The flowers feed bees for honey. The seed pods tan hides. The wood burns for fuel. The flowers are used to make perfume, and you can use the thorns to stab anyone who complains about your perfume.

Vachellia caven
Image: Vachellia caven Penarc / Wikipedia

The tree is part of the Leguminosae. Like it looks, that’s the legume family making it a relative of beans or peas. As a result, like beans or peas it’s able to work with bacteria to fix Nitrogen into the soil. That’s an important skill, because V. caven often grows on sites with a high burn frequency, and repeated fires can deplete resources. If resources are scarce then a plant that can change how it allocates resources can gain a competitive advantage over its rivals.

For V. caven that choice might be in the flowers it produces. Marquez and colleagues say the plant is andromonoecious. This means it produces two kinds of flower. One is your typical flower, a hermaphroditic flower with male and female sex organs, so it produces pollen, but also has ovules that can be pollinated. The other is a purely male flower. This only produces pollen. Marquez’s team refer back to earlier work that shows that these pollen-only flowers are cheaper to make. They also note that if your flowers rely on pollinators, then a wildfire that has trashed their habitat will disrupt the pollinators. So, how does this affect sex? If a V. caven plant is in a regularly firestruck zone, will it respond by making more male flowers? It’s cheaper and might make better use of the reduced number of pollinators. Will the loss of resources also mean that reproductive success is reduced?

Marquez’s team tested this idea by going to the Chaco Serrano in Central Argentina, by Córdoba. At the site, they selected plants at three regularly burned sites and compared them to plants at three unburned sites. They did this by measuring the basal diameter of the trees, pollination, fruit set and the sex ratio of the flowers.

What the team found was that the number of male flowers did not increase with fire frequency. Instead, it was the broader trees that favoured hermaphroditic flowers. The thinner trees tended to conserve their resources and grow more male flowers. Fruit set was also similar at both burned and unburned sites, though Marquez’s team did find the more male plants had higher fruit set at burned sites.

The variation in male:hermaphrodite ratio within populations gives V. caven some ability to react to repeated fires. In their paper, Marquez and co-authors say: “…[W]e show here for the first time that the positive relationship between maleness and female fitness can be modified by environmental factors. But, why did this relationship only occur in burned sites? One possible explanation is that plants growing under high fire frequency regimes experience lower competitive pressure for abiotic resources and thus increased outcrossing rates would better and readily translate into increased fruit set.” Effectively, when fire clears the landscape of other plants, the V. caventrees are able to concentrate on reproduction more and fight for resources less.

So, for V. caven there is no single sexual solution. It’s sexual diversity, and the ability to fix nitrogen to help replace lost nutrients, that makes the tree a survivor.