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Last year, biology student Harold Timans started making an inventory of lichens in the Hortus as part of an Honours course. The inventory showed that there are 111 species of lichen in the garden, some of which are rare or even very rare. Harold wrote an article about it, which was published in Buxbaumiella, the magazine of the Bryological and Lichenological Working Group (BLWG).

An extensive inventory of all lichens found in the garden had not been made before, while the variation in ( old ) microclimates in the Hortus suggests that many species can grow there. However, the few single observations that exist soon gave rise to curiosity about more. For example, in several years André Aptroot found unusual species such as Aspicilia verrucigera (rimmed lichen), Xanthoparmelia loxodes and Xanthoparmelia mougeotii on granite boulders in the Siebold Garden. Also in 2017, Maarten Costerus found the unusual Hypopterygium tamarisci growing rampantly on a few limestone boulders, a species that so far has only been found in greenhouses in the Netherlands or has fledged from them. 

Intrigued by these finds, Harold set out to make a complete inventory. He divided the garden into five sections and carefully examined each section for all the lichens it contained. This resulted in a list of 111 species, although the actual number is probably even higher. A total of 19 rare to very rare species were noted. Many of these species are seldom found in Zuid-Holland, because the necessary niches do not or hardly occur in the province.

A selection of the finds:

Thelocarpon lichenicola is most common on acidic sandy soil in the Pleistocene regions, rarely near the coast. This species was last found in 1979 in Zuid-Holland. In the Hortus it was found on peat bogs.

Hymenelia prevostii is found in southern Limburg on limestone but rarely elsewhere. In the Hortus, it grows on limestone in an old planter.

Cladonia cariosa is a very rare Cladonia that is normally only found in calcareous sand drifts, heaths and calcareous grasslands. In southern Limburg, it grows on the zinc-polluted banks of the Geul. In the Hortus, it grows in a planter where the typical zinc flora is imitated. The fact that the species has managed to find this spot is extraordinary. It may have been brought from Belgium with the zinc-contaminated soil that was brought in for the construction of the planter and has been there ever since. The species is entirely new to Zuid-Holland.

In the same planter grows Diploschistes muscorum, which grows parasitically on the present Cladonias, and the rare Stereocaulon nanodes.

Verrucaria elaeina grows preferably on alkaline rocks and in the Hortus grows in large numbers on coarse limestone. The species is new to South Holland.

The entire article and the complete list of inventoried lichens can be read below.

Photograph cover: Cladonia cariosa, by Henk-Jan van der Kolk.

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