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A global study reveals a surprise: The driest places have the highest diversity of plant traits
Ecological common sense holds that plant diversity is higher in rainy areas such as rainforests than in drylands. However, a new global study on plant trait diversity overturns this general principle.
What is plant trait diversity?
Trait diversity refers to differences in the physical, chemical, and physiological properties of organisms that affect how they perform in their natural environment. For plants, a trait might be the rate of photosynthesis, the size or shape of the plant, or its chemical composition, which can be related to the function of the plant (from Total, 2024).
What is dry land?
Drylands are tropical and temperate areas with an aridity index of less than 0.65. Drylands cover 45% of Earth’s land surface and are home to one-third of the world’s population. Drylands include sub-humid, semi-arid, arid and hyperarid ecosystems, such as Mediterranean landscapes, grasslands, savannahs and deserts.
So far, plant traits have been investigated mainly in agricultural areas or woodlands with adequate rainfall.
So far, plant traits have been investigated mainly in agricultural areas or woodlands with adequate rainfall.
Although drylands cover around 45% of the land area, occur at all latitudes and on all continents, and are projected to expand due to droughts caused by climate change, only 10% of dryland plants have been identified and classified in the global plant trait database established over the past 20 years.
To better understand how dryland ecosystems function, especially in the context of climate change and intensified land use, more than 120 scientists from 25 countries have formed an impressive collaborative network.
To better understand how dryland ecosystems function, especially in the context of climate change and intensified land use, more than 120 scientists from 25 countries have formed an impressive collaborative network.
They conducted a global survey of 300 different dryland plant species across six continents using standardized protocols. They measured more than 130,000 individual plant traits across plant species worldwide.
They expect that under conditions of higher drought, plant trait diversity will be lower; only plants with traits that allow them to tolerate extreme water shortages and heat stress will emerge.
It turns out that below a certain threshold — less than 15.75 inches (40 centimeters) of rainfall per year — global trait diversity increases dramatically.
It turns out that below a certain threshold — less than 15.75 inches (40 centimeters) of rainfall per year — global trait diversity increases dramatically.
What’s more, in the driest places, trait diversity was twice as high as in wetter areas. Contrary to the belief that deserts lack diversity, the drier the environment, the higher the trait diversity, the researchers found.
In fact, when an area is dry enough that plant cover and soil fertility have “collapsed” and there is bare ground between plants, as in many deserts, trait diversity goes up.
Trait diversity in arid regions may actually be enhanced due to the isolation of plants.
Trait diversity in arid regions may actually be enhanced due to the isolation of plants.
Arid climates reduce competition between plants, leading to the emergence of unique traits. Harsh environments require adaptive strategies that allow plants to survive and promote high trait diversity.
We know that plants in extreme environments are highly specialized in form and function (such as succulents like cacti and euphorbia, baobabs, and plants that germinate and flower only once in their lifetime in the driest deserts on Earth). These specialized plant communities have the highest levels of trait diversity.
The study found that areas with greater grazing pressure also supported higher trait diversity, as did areas with higher aridity, suggesting that plant avoidance and tolerance strategies in response to water stress and herbivory (being grazed) can also increase trait diversity.
The surprising results of this study suggest that plants use “alternative strategies” to cope with increasing environmental stresses brought on by climate change, drought and intensified land use.
Drylands, though arid, are also a global repository of adaptability and diversity.
Read the paper:
Gross, N., Maestre, FT, et al., 2024. Unexpected plant phenotypic diversity in a drought and pastoral world: Nature, Vol. 632, No. 8026, pp. 808-814. https://doi.org/10.1038/s41586-024-07731-3.
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