Die Gattung Opuntia in Südtirol (Provinz Bozen, Italien)

The genus Opuntia in South Tyrol (Province of Bolzano, Italy)
Until the early 2000s there was known only one Opuntia species from literature as being
naturalized in South Tyrol, i.e. O. humifusa. In the framework of the ongoing overall
floristic mapping and of private excursions, the authors recognized the presence of
other species, which prompted them to start a project dealing with the genus Opuntia
in South Tyrol. Aim of the project was to record all Opuntia species either naturalized
or showing at least a tendency of spreading into wild habitats, as well as their distribution
and state of naturalization. In this paper, we present the results of this project.
Further, we offer a detailed determination key of the treated Opuntia species in order
to ensure a correct identification in the field.
With 7 documented species South Tyrol is among those Alpine provinces with the highest
amount of naturalized Opuntia species: O. engelmannii, O. humifusa, O. lindheimeri, O.
phaeacantha, O. polyacantha, O. scheeri und O. tomentosa. A recently published record
of O. stricta has to be considered as uncertain for the time being, since the so identified
plants need a reevaluation but could not be found anymore at the respective site. Well
established species are, in order of decreasing abundance, O. humifusa, O. engelmannii
and O. lindheimeri. The degree of naturalization of the remaining species, O. phaeacantha,
O. polyacantha, O. scheeri, and O. tomentosa, represented by locally restricted
plant groups or even by single plants, is partly difficult to assess. It is conceivable that
the records of the latter species either include old and today totally isolated plantation
relics or cases of a spontaneous colonization of unknown origin as well as plantations
with a starting tendency of naturalization.
As a primary source where the mentioned species started to spread from, Opuntia
stands planted along public promenades around 1900 may be taken into account, further
orangeries which at the same time were very popular particularly in the surroundings
of Bolzano, and private gardens. Among possible dispersal vectors for seeds are birds,
lizards and ants. Gravity has to be considered as an important dispersal vector for fruits
and stem segments, which once broken off and fallen down slope contribute efficiently
to a rapid spread of the species. This is evident on the rocky slopes around Bolzano.
While Opuntia humifusa is shown to have been naturalized in South Tyrol since the
beginning of the 19th Century, it is not fully clear when naturalization and colonization
of wild habitats, respectively, took place in the other species, especially in the
now well established O. engelmannii and O. lindheimeri. Missing literature references
and personal observations, however, suggest a spread in recent times.