Juniper, the plant that holds the dunes.

A wood that grows low

Formentera's juniper is not a tree you look up at. On the exposed coast it is a low, dark cushion of foliage, trimmed by the wind into shapes that follow the ground rather than rising above it. In sheltered hollows behind the beach it can grow taller, and a few old specimens read as small trees. The species here is Juniperus phoenicea, known in Catalan as the savina, the same word that names the island's port.

Seen from a path, the juniper scrub can look monotonous: the same low green repeated along the shore. Seen close, it is varied. Some plants are wide and flat, others have a single trunk that leans away from the prevailing wind, and the foliage keeps the smell of resin after rain. The variety is a record of how much shelter each plant has had, how long it has been there, and what the coast has done to it.

The best place to see the plant is not a garden but the back of a beach, where the sand stops being sand and starts being soil. There the juniper is joined by other coastal scrub, and the ground underfoot is a mix of shell fragments, worn stone and leaf litter that has taken years to accumulate. It is worth slowing down for: the whole wood is often no taller than the people walking through it, and just as easy to step over without seeing.

How a dune is built

A dune is not a heap of sand that happens to be there. It is a structure built by plants, in stages. Sand blown inland from the beach is caught first by low annuals near the tideline, then by tough grasses whose roots run down through the loose sand. Once the surface is stable enough, shrubs arrive. The juniper is one of the last, and it is the one that makes the sand stay.

The order matters. Break the first zone and the sand starts to move; break the last and the whole system loses its anchor. A single path cut straight through a dune can widen into a gap that the wind empties out over a winter. This is why the signposted routes across the island's dune belts are not simply a suggestion, and why the edges of the beaches are best crossed on foot where others already walk.

• TidelineSea rocket and other annuals catch the first blown sand and die back each year.
• Yellow duneDeep-rooted grasses hold the growing mound together while it is still loose.
• Fixed duneMosses, lichens and low shrubs close the surface and begin to build soil.
• Juniper scrubSlow-growing juniper takes over the stable sand and shelters everything beneath it.

Sand, pine and shade

Inland from the shore, the juniper gives way to the island's woods. The transition is gradual and easy to miss: the low green cushions thin out, the soil darkens by a shade, and pines begin to appear. Under them the ground is cooler and the wind is broken, so plants that could not survive on the open coast find a place. Walking from the beach to the interior of Formentera is, in miniature, a walk from the sea's edge to shelter.

This band of transition is where the island is most easily damaged without anyone noticing. It is not dramatic scenery; it is scrub and shade and thin soil. But it is the buffer that keeps blown sand off the fields and gives shelter to everything planted behind it, and it has been managed, cut and grazed for centuries. What looks wild on Formentera is usually a landscape that people have used and then partly left alone.

Salt, wind and patience

The juniper survives where little else does because it is built for the coast. Its foliage is scaled and waxy, which limits water loss and shrugs off salt spray. Its roots go down and out, finding fresh water well below the surface and gripping sand that would otherwise move. It tolerates being buried by a drift and it tolerates being cut back by wind, which is why the same plant can look battered and healthy at the same time.

What it does not do is grow quickly. Juniper wood is dense, and the plant puts on very little each year, which is why a stand that is cleared does not come back in a season or a decade. Grazing, charcoal burning and building have removed many of these woods around the Mediterranean. On Formentera the remaining stands are a small inheritance, and they are slow to renew, which is the whole argument for leaving them alone.

A juniper that reaches the height of a person has spent a long time getting there.

What the banquettes do

The dunes are not the only thing that holds the beach together. In late summer and autumn, the sea delivers heaps of dead seagrass to the shoreline: the leaves of Posidonia oceanica, the meadow that grows offshore and gives the island its clear water. These brown banks, the banquettes, look like debris and are sometimes treated as such. In fact they are part of the structure of the beach, trapping sand and softening the force of the waves.

The meadow itself is protected, and anchoring over it is regulated by a Balearic decree; in some bays mooring buoys are laid in summer for boats. From the shore, the practical point is simple. A beach that has been swept clean of its seagrass is a beach that will lose sand faster, and a dune with no beach in front of it has nothing to feed on. Check with the natural park office before anchoring or mooring.

Walking the green routes

Formentera is small and mostly flat, and the signposted green routes, the circuits verds, are the easiest way to cross it without cutting new paths. They follow old tracks and minor roads between the beaches, the villages and the lagoons, and they are marked for walking and cycling. Using them keeps feet and tyres off the dune edges, which is where damage starts. The island council maintains them; surface and signing vary from year to year.

Two practical notes. In high summer the middle of the day is punishing, and the shade on these routes is thin, so early morning is better. And if you are bringing or hiring a vehicle, the Consell de Formentera caps the number of visitor vehicles in high season, so the rules may affect your plans before you arrive. Check the current position with the council rather than assuming last year's arrangements still apply.

• Green routesFollow the signposted circuits verds rather than improvising across the scrub.
• Soft groundStay on the track where it crosses sand; a widened edge becomes a hole.
• Shade and waterCarry water and start early; the middle of the day offers little cover.
• VehiclesConfirm the Consell de Formentera rules before bringing a car in high season.

Wood that was worked

Juniper wood is hard, close-grained and strongly scented, and on an island with limited timber it was never wasted. It went into fuel and charcoal, into fence posts, tool handles and small pieces of carpentry, and into the everyday repairs of a farming coast. The scent that survives in old wood is one reason the shrub was valued. The same density that makes it useful also makes it slow to replace.

The coast you see now is partly a result of that economy ending. As farming, grazing and charcoal burning declined through the twentieth century, scrub began to close in and the juniper spread back over ground it had lost. That sounds like good news, and in part it is. But a juniper wood that has grown back on abandoned fields is not the same as the old coastal stands, and it is more fragile than it looks.

Look at the low woods behind the north beaches and the edges of the lagoons and you are seeing the survivors of that long use, plus what has grown since. The distinction matters when you are deciding where to walk. Land that has been left alone for a few decades can look established and still be thin, and one season of pressure can set it back by more than one season will repair.

Keeping the sand in place

There is no single dramatic threat to the dunes of Formentera. There is a long list of small ones: a car parked on the sand, a path widened by a shortcut, a plant pulled up because it smelled good, a beach raked clean in spring. None of these matters much alone. Together they are how a dune field becomes a flat strip of sand with a car park behind it, and the island has already lost ground in that way.

The habits that help are unglamorous and easy. Walk on the paths and on the wet sand rather than through the scrub. Do not pick the juniper, which will not grow back from a cutting and does not transplant well. Keep vehicles on made surfaces. If a beach has a boardwalk, use it. And when the posidonia arrives in autumn, walk over it or around it rather than moving it. The dunes will do the rest, slowly.

• Stay on pathsWidening a track by a step is how a blowout begins.
• No pickingJuniper does not root from cuttings and will not survive being moved.
• VehiclesKeep cars and scooters on made surfaces and in permitted parking.
• Autumn seagrassWalk over the banquettes or around them; they are holding the beach.