Deep-rooted plants, key to preventing desertification, at risk from climate change and overexploitation of aquifers

What is important is invisible to the eye. And in the world of plants, the most hidden and often least valued part is their roots. However, deep-rooted plants, those species capable of accessing water located more than 20 meters underground, are an essential ecological pillar for arid ecosystems around the world . Now, an article published in the prestigious journal Trends in Plant Science by CREAF researcher Josep Peñuelas , shows that climate change and human activity are harming these plants , a fact that could accelerate the desertification of vulnerable regions around the world, including large areas of the Iberian Peninsula.

According to the review article, rising temperatures, reduced rainfall, and falling water tables in aquifers make it difficult for seedlings or young plants to establish and for adult plants to survive . Under these conditions, plants expend much of their energy extending their roots to find water, an effort that compromises their ability to grow and reproduce. Furthermore, when the water table drops below the root zone, plant communities become entirely dependent on rainfall and become much more vulnerable to drought.

The study also shows that human disturbances such as overexploitation of aquifers, overgrazing, fires and intensive agriculture further aggravate this degradation . Currently, two billion hectares of the Earth, 15% of the world's land surface, are already degraded; of these, 260 million hectares have been lost to overgrazing. In Mongolia, for example, 76% of the territory has become desertified due to overgrazing and excessive extraction of water from aquifers. Given this scenario, the authors argue that deep-rooted plants are the unknown guardians of arid lands, to which much more conservation efforts must be devoted. Their loss is already being associated with changes in vegetation composition, more erosion, more sandstorms, greater aridity and less capacity to retain carbon in soils.

“We need to maintain the benefits that deep-rooted plants offer us, and for this we need a much more integrated approach,” explains Josep Peñuelas , research professor at the CSIC and researcher at CREAF, author of the study. “This involves protecting aquifers with strict regulations on groundwater extraction, applying rotational grazing to avoid both overgrazing and soil compaction, and reinforcing soil health with practices such as crop rotation. We also need to include the traditional knowledge of local communities.”

The article places deep-rooted plants at the heart of the SDGs, global commitments to combat desertification and the protection of biodiversity. What do they contribute that makes them so special? First, their network of filaments helps to fix the soil, protecting large areas of land from erosion . Some species reduce wind speed, fix dunes and accumulate fine particles that improve soil structure. Second, thanks to a process called hydraulic lift, these plants pump water from deep layers to the surface, making it accessible to other species that do not have such powerful roots . This creates “fertility islands” where underground communities of bacteria and fungi retain water, form humus and strengthen the soil's ability to resist erosion . In addition, the network of roots ends up forming a large invisible biomass that turns out to be a natural store of carbon sequestered underground . This turns deserts and arid areas, despite having little vegetation, into very important carbon reserves on a global scale.

In the Peninsula and in the driest areas of Catalonia, shrubs such as the broom ( Retama sphaerocarpa ) are perfect examples of this strategy : they maintain a small, almost leafless aerial part to avoid losing moisture, while their roots go down tens of meters to find water reserves. Also the tamarisk ( Tamarix spp. ), common in dry streams and saline soils, acts as a natural hydraulic pump , capable of connecting directly with deep aquifers to survive the hottest summers. The jujube ( Ziziphus lotus ) is the perfect example for the south of Spain (Almería, Murcia), the driest area in Europe. It creates the famous fertility islands , piles of sand and nutrients trapped under its branches, while the roots seek water far below. These species, along with others such as the mastic, are vital for fixing the soil and preventing the land from becoming an inert desert.

Deep-rooted plants have developed surprising strategies to survive in places where rainfall is almost non-existent. One of the most important is that, right after germinating, they invest all their energy in growing a powerful vertical root that quickly descends to find groundwater. There are species capable of reaching more than two meters deep in a few weeks. This main root is narrow, robust and with a very active tissue that allows it to pierce the soil and explore it vertically. Unlike other plants with many superficial roots, these reduce the number of branches to concentrate resources in a few very efficient roots. Their anatomy is also prepared: wide conductive vessels, thick tissues and a vascular system capable of transporting large quantities of water from the subsoil to the aerial part.

Another key feature is that roots can redistribute water: when the surface soil is dry, the hydraulic pump of the deep roots pushes it up; when there is surface moisture, they can do the reverse process and send it down to fuel their own growth.

Deep-rooted plants have also been a key resource for many communities living in arid areas for millennia. In some places, such as the oases of Central Asia, they are harvested as highly nutritious fodder. At the same time, governments and international organizations are using them to slow desertification and boost local economies. Projects such as Africa's Great Green Wall have generated millions of dollars in forest products and have boosted new green economies. In China, major corridors such as the Desert Highway have created jobs and tourism thanks to vegetated strips that protect the road from wind and sand.

"Arid and semi-arid Mediterranean ecosystems, such as those in Spain, Catalonia and other regions of southern Europe, may be especially sensitive to these processes. For this reason, knowing and better managing our deep-rooted plants will be key to containing desertification and strengthening climate resilience in the coming decades," concludes Peñuelas.