differences in total N (t-value ¼ 1.78; d.f. ¼ 28; P ¼ 0.086) and zinc content
(t-value ¼ 1.72; d.f. ¼ 28; P ¼ 0.098) were close to significance.
Until now, the occurrence of A. squamatus has been restricted by the low number
of available habitats. In Slovenian coastal habitats A. squamatus was found
restricted to semi-saline habitats, which are represented only by a narrow strip
near the coast between the typical halophile vegetation and coastal grasslands or
arable land farther from the sea. The habitat types where A. squamatus thrives
belong to halophytic scrubs (1420), halophilous reeds and rush salt marshes
dominated by Juncus maritimus (1410; Fig. 19.3).
19.4 As an Engineering Species Aster squamatus
Could Potentially Become Invasive
Since the first record of A. squamatus in Slovenia, the species has had a constant
presence. Its abundance and distribution is limited by habitat availability, which is
scarce along the Slovenian coast, since coastal grasslands are poorly represented on
the Slovenian coast because of considerable human impact and because they belong
to low-productive grasslands. Such specific habitat properties do not enable
A. squamatus to become abundant or highly invasive, even though the species has
an enormous seed production and fairly high germination rates. However, these
low-productive habitats could be altered by species’ decomposing biomass, as
discussed later. On the other hand, in several European countries, coastal grasslands
are common habitats and could potentially be invaded by A. squamatus. It has been
observed before that A. squamatus, sometimes together with Conyza canadensis –
another alien species, invades vegetation with coarse perennial grasses and sedges
or rushes (Bassett 1980). Along with such invasions, it was mainly the abundance of
small annuals that declined: for example, Catapodium rigidum and Parapholis
incurva. We made similar observations in Slovenian habitats for Spergularia
marina and Parapholis strigosa, rare plants from semi-saline habitats. If in any
way the preferred semi-saline habitats become more common, A. squamatus could
rapidly invade favourable habitats. Our results show that A. squamatus has some of
the characteristics that have been recognised by statistical models from Knapp and
Ku ¨hn (2012) to be significant for non-native species. One of them is a high level
of seed production, which may be explained by non-native species being more
frequently able to self-pollinate (Knapp and Ku ¨hn 2012).
The study site is located in the Sub-Mediterranean part of Slovenia, and between
1951 and 2010 the temperature has risen and precipitation shifted to autumn. There
are more sunny days between May and August; all these consequences could be
attributed to global warming (Anonymous 2006). There is already a slight increase
in autumn temperatures, and predictions for the years 2036–2065 by various models
(C4I, ETHZ, KNMI, MPI, SMHI_BCM, SMHI_HadCM3Q3, DMI_ECHAM5,
DMI) based on climate data from 1971 to 2000 do show an increase in mean
temperature especially in the middle of the vegetation season (the ENSEMBLES
284
N. S ˇ ajna et al.
(t-value ¼ 1.72; d.f. ¼ 28; P ¼ 0.098) were close to significance.
Until now, the occurrence of A. squamatus has been restricted by the low number
of available habitats. In Slovenian coastal habitats A. squamatus was found
restricted to semi-saline habitats, which are represented only by a narrow strip
near the coast between the typical halophile vegetation and coastal grasslands or
arable land farther from the sea. The habitat types where A. squamatus thrives
belong to halophytic scrubs (1420), halophilous reeds and rush salt marshes
dominated by Juncus maritimus (1410; Fig. 19.3).
19.4 As an Engineering Species Aster squamatus
Could Potentially Become Invasive
Since the first record of A. squamatus in Slovenia, the species has had a constant
presence. Its abundance and distribution is limited by habitat availability, which is
scarce along the Slovenian coast, since coastal grasslands are poorly represented on
the Slovenian coast because of considerable human impact and because they belong
to low-productive grasslands. Such specific habitat properties do not enable
A. squamatus to become abundant or highly invasive, even though the species has
an enormous seed production and fairly high germination rates. However, these
low-productive habitats could be altered by species’ decomposing biomass, as
discussed later. On the other hand, in several European countries, coastal grasslands
are common habitats and could potentially be invaded by A. squamatus. It has been
observed before that A. squamatus, sometimes together with Conyza canadensis –
another alien species, invades vegetation with coarse perennial grasses and sedges
or rushes (Bassett 1980). Along with such invasions, it was mainly the abundance of
small annuals that declined: for example, Catapodium rigidum and Parapholis
incurva. We made similar observations in Slovenian habitats for Spergularia
marina and Parapholis strigosa, rare plants from semi-saline habitats. If in any
way the preferred semi-saline habitats become more common, A. squamatus could
rapidly invade favourable habitats. Our results show that A. squamatus has some of
the characteristics that have been recognised by statistical models from Knapp and
Ku ¨hn (2012) to be significant for non-native species. One of them is a high level
of seed production, which may be explained by non-native species being more
frequently able to self-pollinate (Knapp and Ku ¨hn 2012).
The study site is located in the Sub-Mediterranean part of Slovenia, and between
1951 and 2010 the temperature has risen and precipitation shifted to autumn. There
are more sunny days between May and August; all these consequences could be
attributed to global warming (Anonymous 2006). There is already a slight increase
in autumn temperatures, and predictions for the years 2036–2065 by various models
(C4I, ETHZ, KNMI, MPI, SMHI_BCM, SMHI_HadCM3Q3, DMI_ECHAM5,
DMI) based on climate data from 1971 to 2000 do show an increase in mean
temperature especially in the middle of the vegetation season (the ENSEMBLES
284
N. S ˇ ajna et al.