De mosflora op Iepen en andere bomen in Noord-Groningen

An inventory has been made of the bryophyles growing as epiphytes on trees in the northern part of the Dutch province of Groningen. Generally speaking, Ulmus x hollandica proved by far the richest tree from a bryological viewpoint, but near the sea-coast sometimes Populus x canadensis and Fraxinus excelsior may be fairly rich (table 1). Eight species were found that in a wider context can be termed more or less rare, viz. Frullania dilatata, Leucodon sciuroides, Orthotrichum lyellii, Radula complanata, Tortula laevipila, T. papulosa, Ulota phyllantha, and Zygodon viridissimus. Most of them are strongly declining, at least in other parts of the Netherlands. In the investigated area Ulota phyllantha, Frullania dilatata and Orthotrichum lyellii appear to be still rather common, except in the polders SE of the industrial town of Delfzijl. In comparison with Barkman’s data from c. 1950 the first species even has become more frequent. On the other hand, Leucodon, both Tortula species, and Zygodon have declined and ‘retracted’ to a belt of maximally 7 km from the sea-shore (fig. 4). A comparison is made with two other areas with similar bryophyte communities on Ulmus: 1) Kennemerland (more to the SW, province of Noord-Holland) and 2) Ostfriesland and adjacent parts of NW Germany (lying NE of Groningen). While these areas contain some species ( Metzgeria furcata and Orthotrichum pulchellum, respectively) wanting in N Groningen, the latter area is distinguished still by a generally better developed epiphyte vegetation. Elm disease and a too rigorous battle against this disease are its main enemies near the sea-coast in the northernmost part of the Netherlands.