As the alders mature and die, they’ll make room for conifers to sprout and thrive in soils enriched by the nitrogen-fixing alder.

Without the nitrogen-fixing bacteria, for example, plants would starve for want of nitrogen, though surrounded by a sea of nitrogen-containing air.

But the process involved — the nitrogen-fixing Haber–Bosch process — is energy intensive: it accounts for a few per cent of the world's annual energy use.