Brittle Bladder-fern

Army green and a ministers’ quarrel

The bladder fern is a small fern whose leaves die back in the winter. In the summer it has a fresh green colour and the plant is sometimes mistaken for a young specimen of a larger fern species. In the wild bladder ferns can be found growing throughout the world. In the Netherlands it occurs rarely on walls and then often only as a single specimen.

Early this century the largest habitat of this fern in the Netherlands was discovered on a military site in Soest. It was growing amongst hundreds of army trucks, which had stood unused for decades. When the military wanted to sell the vehicles it was pointed out that permission to sell had to be obtained under the Nature Conservation Act. When permission was refused this led to a fight at ministerial level. Eventually, some of the plants were saved and now specimens from this camp can be found in the Blijdenstein Pinetum and the Botanical Gardens of Utrecht.

Concealed by deserted army trucks

Brittle bladder-fern are not very spectacular in appearance - they are only 10 cm high making them look like young ferns. It gets its name from the sori located on the underside of the frond, these sori have an arched, oval covering that looks like a little bladder/blister. The ferns are vulnerable to being overgrown and to eutrophication. In the Netherlands brittle bladder-ferns are rare and protected by law.

Ecology and habit

Brittle bladder-ferns were, until recently, most commonly found in South Limburg, for example in the ‘t Rooth quarry on a wall comprised of piled flint (waste product in limestone quarries). In the rest of the country they grow almost exclusively on old walls, mostly in Amsterdam. In each location only a few specimens occur, with a total population in the Netherlands estimated at 750 individuals at the beginning of the century. In 2006 however another population was found on a military site. Soesterberg Camp was a holding bay where, for decades, 350 DAF army vehicles had been parked. Beneath these trucks, which were standing very close together, a dark and humid microclimate had developed that was favourable for fern growth. Ten species of fern were discovered, but by far the most unexpected was the population of brittle bladder-ferns which, totalling about 10,000 individuals, immediately increased the Dutch population by 15x.

Threat

Unfortunately the holding bay with 10,000 individuals had to evacuated. The fern population is however secured by hundreds of individuals being replanted in four fern-rich sites spread across the country: a quay in Amsterdam, the Kuinderbos forest, the Botanic Garden in Utrecht and the Blijdenstein Pinetum. A small population may be able to continue growing in Soesterberg Camp.