Drylands: unexpected plant diversity enables adaptation to extreme climates
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Trees in the Namib Desert
view moreCredit: Lixin Wang
The Earth is home to a diversity of plants with highly varied forms and functions. This extraordinary morphological, physiological and biochemical diversity determines how plants adapt and respond to ongoing global changes, with significant consequences for the functioning of ecosystems. Yet, 90% of current knowledge on the functional diversity of plants concerns only agricultural ecosystems and temperate zones. By contrast, drylands (see inset), making up 45% of the Earth’s terrestrial area, remain underrepresented in the data. These important zones are now directly threatened by increases in aridity, grazing pressure and desertification. We need to understand how plants respond to such pressures before we can establish the possible future evolution of these fragile ecosystems in terms of their biodiversity and functioning. To meet this urgent need, an international team of 120 scientists from 27 countries has carried out the first worldwide investigation of the functional diversity of plants in arid zones.
Having developed a standardised sampling protocol, the scientists collected and processed samples from the 301 plant species found across 326 representative plots from all continents (other than Antarctica) to characterise the functional diversity of the zones, generating a total of 1347 full sets of trait observations for analysis. Particular attention was paid to the characterisation of the plant elementome, that is, the diversity of chemical elements and trace elements (such as nitrogen, phosphorus, calcium, magnesium and zinc) found in plants, as these often-unrecorded traits exert a strong influence on how the latter function. Overall, the study involved more than 130 000 individual plant trait measurements.
A key hypothesis at the start of the study had been that aridity would reduce the diversity of plants through selection, leaving only those species capable of tolerating extreme water scarcity and heat stress. However, we found the opposite to be the case in the most arid rangelands of the planet, where plants instead exhibit a wide range of individual adaptation strategies. For example, some plants have developed high calcium levels, strengthening cell walls as a protection against desiccation. Others contain high concentrations of salt, reducing transpiration. Although fewer species are observed at local scale than in other regions of the planet (in temperate or tropical zones), plants in arid zones display an extraordinary diversity of forms, sizes and functioning, double that in more temperate climatic zones. This increase in trait diversity occurs abruptly at the point where rainfall volumes drop below the annual threshold of 400 mm. This is also the threshold for a pronounced decline in plant cover and the appearance of large areas of bare soil. To explain this phenomenon, the study’s authors suggest that the loss of plant cover leads to ‘plant loneliness syndrome’, where increased isolation and reduced competition for resources produces high degrees of trait uniqueness and functional diversity that are globally exceptional. This adaptive diversity could equally reflect complex evolutionary histories dating back to the initial colonisation of terrestrial habitats by plants more than 500 million years ago, when these habitats presented extreme conditions for living organisms.
This study reveals the importance of drylands as a global reservoir of functional diversity in plants. It provides a fresh lens through which to view plant architecture, the adaptation of plants to extreme habitats, historical plant colonisation of terrestrial environments, and the capacity of plants to respond to current global changes.
What are drylands?
Drylands are defined as tropical and temperate zones with an aridity value below 0.65. They cover 45% of the Earth’s terrestrial area and are home to a third of the global human population. They include sub-humid, semi-arid, arid and hyper-arid ecosystems such as the Mediterranean landscape, steppes, savannahs and deserts.
Journal
Nature
Article Title
Unforeseen plant phenotypic diversity in a dry and grazed world
Article Publication Date
7-Aug-2024
Plants show surprising diversity in arid landscape
King Abdullah University of Science & Technology (KAUST)
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An international research team including KAUST's Fernando Maestre conducted a field survey to investigate the impacts of aridity and grazing pressure on the chemical and morphological trait diversity of perennial plants across drylands worldwide.
view moreCredit: © 2024 KAUST; Sergio Velasco Ayuso.
Understanding how plants cope with climatic extremes and grazing pressure is important for reliable prediction about future biodiversity and the functioning of dryland ecosystems[1].
An international team, coordinated by KAUST’s Fernando Maestre, has assessed how 20 chemical and morphological plant functional traits jointly respond to changes in aridity and grazing pressure across global drylands. Increasing aridity and grazing pressure could be expected to reduce the level of plant diversity. However, the diversity of plant traits — including key traits linked to nutrient cycling such as specific leaf area and foliar chemical composition — actually increased above an aridity threshold of 0.7 (close to the transition between semiarid and arid zones). This trait diversity similarly increased with increasing grazing pressure.
“Global initiatives aiming to describe plant trait diversity have focused on plant morphology and leaf carbon economy, but neglected the diversity of chemical elements that sustain plant survival and growth,” says Maestre, who co-led this study as part of the BIODESERT global survey, which he designed.
“The elemental concentration in plant leaves has major implications for plant development and determines how plants respond to grazing pressure and water scarcity,” he says.
The team conducted a field survey to investigate the impacts of aridity and grazing pressure on the chemical and morphological trait diversity of perennial plants across drylands worldwide. They selected 98 sites from 25 countries representing the aridity gradient over which dryland rangelands can be found globally. Each site included several plots spanning local gradients of grazing pressure (from ungrazed to high grazing pressure) with a total of 326 plots surveyed.
Variations in plant functional traits reflect shifts in plant adaptation strategies under changing environmental conditions. The results indicate that aridity and grazing have a similar effect on plant trait diversity by promoting a wide range of strategies to cope with water shortage and grazing.
They measured traits related to the concentration of 14 chemical elements in plant leaves, the leaf and plant size, and the leaf carbon economy (leaf area and leaf dry matter). The results provide valuable information to explore how aridity and grazing shape the covariations and trade-offs observed among multiple morphological and chemical plant traits across global drylands.
High aridity levels also promoted functionally contrasting strategies: for example, tall species with fast-growing leaves following stress-avoidance strategies (high N-P-K and low leaf dry matter content) and small conservative species following stress-tolerance strategies (low N-P-K and high leaf dry matter content) with either low or high Mg-Ca and Zn-Na concentrations in leaves.
“These elemental strategies can reflect the contrasting role of chemical elements in plants, either as a way to tolerate high aridity levels or as base elements for defensive compounds against grazers,” Maestre explains.
The study delivers novel insights into how vascular plants respond to biotic stressors and environmental extremes and sheds light on how plant traits may be shaped by joint increases in aridity and grazing pressure. A key result was that more than half the trait diversity observed only occurred in the most arid and grazed drylands, highlighting the phenotypic uniqueness of these extreme environments.
The findings indicate that drylands act as a global reservoir of plant phenotypic diversity, challenging the pervasive view that harsh environmental conditions reduce plant trait diversity. “Our results also highlight the importance of considering a plant’s chemical composition (the elementone) to understand dryland biodiversity responses to ongoing climate change,” says Maestre. “Plants could have many alternative strategies to cope with increases in environmental stress induced by climate change and land-use intensification.”
Maestre, who joined KAUST in February 2024, carried out the study in his previous role at the University of Alicante. He plans to expand the BIODESERT survey into the arid and hyper-arid ecosystems of Saudi Arabia. “Such research would contribute to the monitoring of Saudi terrestrial ecosystems, offer important insights on how Saudi plant diversity can respond to ongoing climate change and guide the selection of the most suitable species for each Saudi region to be used in ongoing and future greening programs,” he says.
Journal
Nature
Article Title
Unforeseen plant phenotypic diversity in a dry and grazed world
Article Publication Date
7-Aug-2024
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