Fuerteventura: soil, erosion, colors and flora

Bare landscapes reveal a wide range of colors, the hues of rusted earth. With no vegetation to conceal the soil and underlying geological layers, the land's rugged splendor is laid bare. This is especially true in Fuerteventura, the oldest of the Canary Islands and the one closest to Africa, only 94 km away. Fuerteventura's dry soils, despite their inherent frailty, are valuable and inspiring. They provide a lifeline to xerophilic plants, hardy organisms, and, not least, goats. In the past the indigenous people, the Majoreros, lived off the land, but today subsistence farming has become nearly impossible due to intense erosion. It is a Martian dryland where the earliest soil-forming processes can be seen transforming bare rock.

Text and photos: Kathelijne Bonne.

Fuerteventura's landscapes intrigue with their colors. Shades of red, rusty brown and ochre blend into sandy beige, pink and mauve and disappear on the horizon in a haze of blue. Ash-grey volcanic debris on slopes is abruptly interrupted by black rocky basalt from the most recent lava flows that run through valleys. But it's only the rocks and beach pebbles that are black. Due to absence of soil organic matter (humus), there are by no means natural dark or black soils.

The arid climate of Fuerteventura and of Lanzarote as well is a result of its desert latitude and low relief. Unlike on Tenerife and the western Canary Islands, the trade winds do not rise against a steep wall of mountains, and do not cool enough to condense into rain. On Tenerife, rain does form, and the moisture feeds dense laurel forests that grow on deep black soils.

Soil: a non-renewable resource

I've written about soil already a few times, that thin outer, living layer of the planet. Or, more functionally and less poetically, the arable part of the earth's crust (the anthropocentric view) that consists of loose soil and in which plants can grow. Soil is virtually invisible, ubiquitous and inconspicuous, a seemingly inexhaustible commons hidden under the ground. But this is an illusion. Soil is as non-renewable as it is indispensable, a resource at the mercy of erosion processes.

That was the focus of my message when I gave a talk in Torrelodones (Madrid) on 1 March this year, organized by Asociación TorreVerde, the movement that formed in response to predatory construction projects and nature being paved over with concrete. The diversity of soils is immense, I (hopefully) conveyed to the audience, ranging from rocky 'Regosols' (*) in the mountains of central Spain and on Fuerteventura, to black 'Chernozems' in Ukraine and similar 'mollic' soils in the US, to the extensive soil repertoire in Catalonia and swamp soils in Andalusia. And that is only a sliver of the global soil mosaic. Fuerteventura offered a new look at soils on rugged volcanic soil.

(*) Regosols: young soils with little soil development (one of the 32 soil types of the WRB (the globally used soil classification system, see post on Catalonia).

Weathered land

Fuerteventura's rolling landscape, much smoother and spacious than mountainous Tenerife, and less black and rugged than Lanzarote, clearly shows that it is geologically ancient – although the soil itself is young (this apparent paradox briefly shows up again below). The Roman name of the island, Planaria – flat land, tells something about the advanced degree of weathering too. For me, native of an absolutely flat land, Flanders, Fuerteventura is of course anything but flat, but it is so compared to the other Canary Islands of which La Palma with the recently erupted Cumbre Vieja volcano is the youngest, and Tenerife with the Teide volcano the highest (3715 m).

Millions of years of erosion have worn down the sharp edges of the volcanic morphology, like a geological file working relentlessly. The solidified lava has crumbled into loose sediment. The elements have moved this debris downslope, and redistributed sand and gravel over large debris fans that spread out at the base of massifs, that in turn also consist of magmatic rock. Wide riverbeds and deeply incised canyons, dry at the time I visited Fuerteventura, suggest that large volumes of water ran through them. These dramatic landforms hint at a greener past shaped by powerful forces of erosion and intermittent, intense rainfall, leaving behind a striking contrast between arid expanses and the ancient pathways of water. 

Now the little rainwater (80 to 200 mm a year) drains to the ocean. A small part of the precipitation percolates into porous volcanic rock and sediment. Since rainwater is slightly acidic due to the presence of carbon dioxide, it corrodes the rock, releasing minerals, in a first step towards soil formation. Moisture is stored in incipient soil, forming small pockets from which drought-resistant plants (xerophytes) draw. But there is no dense vegetation cover (anymore) to capture and hold run-off water like a sponge.

Flora of Fuerteventura

A closer look reveals that this barren wasteland is full of life. Microorganisms are ubiquitous, enveloping the entire planet, but there are also visible signs of life such as snails, tiny flowers, cacti and succulents. The terracotta-colored slopes are speckled with steppe-like shrubs with spines such as aulaga (Launaea arborescens). The aulaga is characteristic enough of Fuerteventura that Miguel de Unamuno (1864-1936) dedicated several sonnets to it during his exile on the island.

Small dragon blood trees sprout from underneath bushes and loose stones, benefiting from some moisture that accumulates in the shade. Dragon trees with their primitive-looking trunks and geometric branches are not 'real' trees (it is possible to get into a discussion about what a tree actually is) – they have no tree rings. They are plants of the Dracaena genus endemic to the Canary Islands and the rest of Macaronesia (the other islands west of southern Europe and North Africa: Azores, Madeira, Cape Verde).

Curiously, dragon trees are also found on the island of Socotra in the Indian Ocean. This particular botanical distribution is known as the 'Rand Flora' – a remnant of a much larger past habitat. Indeed, dragon trees have a fascinating history dating back to the Oligocene. They truly are living fossils.

Soil erosion on Fuerteventura

Long before we evolved into humans, during the Tertiary, Fuerteventura was green and forested. In synchrony with drought phases of the Sahara, especially since the last 10,000 years, this island also got drier. When prehispanic people set foot there, drought was not uncommon and there were times of scarcity, often due to overgrazing by the goats they had brought along. But overall, there used to be a richer vegetation with succulents, cacti, 'tabaibales' and 'cardonales' (Euphorbia species), tamarisk and palm groves along rivers and wild olive groves higher up in the mountains. Heather, laurel, mastic tree and 'peralillo' (Maytenus canariensis) were also common. A lichen species, Roccella tinctoria, attracted attention for its purple pigment, once very much in demand.

The trend of desertification intensified from the time of the Spanish Conquista (1402) onward, even though grain continued to be widely cultivated well into the last century. However, the growing demand for timber proved detrimental for the remaining patches of forest and shrubland.

A combination of a few long droughts, clear-cutting, overgrazing by too abundant herds of goats led to irreversible soil degradation. At a given point, farming and self-reliance were no longer possible. Fuerteventura had become a desert, the landscape we see today. Ancient, beautiful windmills scattered in the landscape testify to the former grain industry.

Today, the goat cheese, el queso majorero, is the most important and probably the only local product. Although goats are low maintenance animals, that make do with tough grasses and dry plants, good grazing management and import feed are crucial to stop further erosion. Several experimental farms in the interior suggest that efforts are being made to protect and restore parts of the land.

This history of erosion is reminiscent of Iceland, also an intensely degraded volcanic island but colder and wetter. And the more exposed, the more vulnerable the soil becomes to erosion, both by wind and rain – a vicious cycle that is difficult to break as soil is not renewable, at least not within human time scales.

To revisit the paradox of young soils on ancient land: The continuous erosion of the entire island (a process known as denudation) perpetually exposes deeper, 'fresh' rock to the surface. Soil-forming processes begin anew each time, as though the reset button is being pressed repeatedly. This is why soils on Fuerteventura are young even though the rock on which they lie is old (Fuerteventura rose above the sea 20 million years ago).

Soilscapes of Fuerteventura

Despite its desert-like allure and apparent monotony, Fuerteventura does have a modest repertoire of soil types created by the combined action of air, precipitation, moisture, soil and microorganisms and plant roots on mineral-rich volcanic rock.

Fuerteventura has the following soil types, using the WRB's nomenclature (see article on Catalonia): Arenosols, Cambisols, Regosols, Leptosols, Calcisols and Solonchaks, with a few small inclusions of other soils.

Arenosols are the sandy soils formed in coastal dunes, including those near Corralejo. Cambisols are widespread on Fuerteventura, they are young soils with incipient profile development, probably the 'best' soils on the island. Less developed and shallower than the Cambisols are the Regosols, which show no clear profile development; and Leptosols even less so, which are the very thin soils on rocks and ridges or bare rock on where soil-forming processes are not yet visible or absent. Calcisols contain calcium, often in hard layers and crusts (calcrete, see photo below) that precipitate from calcium present in the volcanic subsoil, from marine deposits and sometimes from Saharan dust. The presence of snail shells is a signal that the soil is calcareous. Solonchaks are soils with excess salt (sodium). The salt comes from the sea but groundwater extraction, fertiliser use, irrigation with saline water and other human activities are the main contributors to soil salinization, a severe form of degradation.

However, due to the dry climate, there are no Andosols (dark volcanic ash soils) on Fuerteventura. Andosols are the 'best-known' volcanic soil type, forming on ash but in a wetter climate. They are ubiquitous in Japan and Iceland. On Fuerteventura, the last volcanic eruption dates back thousands of years (Malpaís de la Arena), so any ash or ash-derived soil has long been weathered away.

Although ancient, volcanic rock is rich in minerals and gives Fuerteventura's soils a certain potential for recovery to more stable and vegetated landscapes. The idea is to unlock this potential. Cambisols, Regosols, and Calcisols in particular, under proper management, can become a fertile ground for vegetation regrowth and a much-needed increase in organic matter.

It would be intriguing to consider what type of soils Fuerteventura possessed before they degraded over the last centuries, before Fuerteventura's palette turned ochre, brown and mauve. Were there grassland soils like Phaeozems and Kastanozems where grain was grown? Were there once Andosols? Are they all in the ocean now? Have they become a breeding ground for marine life? The answer is yes, large chunks of Fuerteventura lie in the ocean. It could be explored in a post on the geology of this unforgettable rock in the sea.

Miguel de Unamuno on the aulaga plant:

'The aulaga is a disarming and intimate symbol; it speaks, beneath the sky and barely above the ground, surrounded by the sea, of a thirst for life, a thirst for immortality grown from the volcanic bowels of the earth."

(Translation from original in Spanish: K.B.) 'La aulaga es una expresión entrañada y entrañable; la aulaga dice, frente al cielo y a ras de la tierra, ceñidas de mar, la sed de vida, la sed de inmortalidad de las entrañas volcánicas de la Tierra.'

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Contrast all this brownness with blue reading my post on the cyanotype prints of marine algae. Or read about the Cumbre Vieja, which erupted in 2021 and made us acutely aware of La Palma's other dangers: namely the risk of a tsunami. Or delve into Europe's other volcanoes, about Etna, Mt Saint Helens in 1980 and the supervolcano Campi Flegrei. Or about soil erosion in Spain, soils of Catalonia, and the fertile Chernozems and how they endure war. Soil erosion also exacerbates the magnitude of floods, read more about links between the Jet Stream, climate, deforestation and soils in Spain and Valencia.

Kathelijne: I am intrigued by how earth, life, air, ocean and societies interact on geological and human timescales.

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