4.4. Regional distribution of the species
The strongest effect of temperature on establishment success
was found for L. major that showed a substantial decrease in growth
rate, root/total DW and main stem length with increasing temperature.
The strong relationship between growth and temperature
for L. major suggest that temperature plays an important role for
the regional distribution of this species. In New Zealand where this
study was conducted L. major is frequently occurring on the South
Island (de Winton et al., 2009) where summer temperature in many
lakes is close to 20 ◦C. L. major is also frequent on the North Island
(de Winton et al., 2009), but mostly in sites that do not frequently
reach 30 ◦C during summer. It is not likely that the species is controlled
by winter temperature because it is frequent in lakes on
temperate South Island in New Zealand, and our results instead
suggest that the species’ distribution is controlled by high summer
temperature. Distribution of E. densa in New Zealand is also
strongly corresponding to the temperature response found in this
study. E. densa is almost exclusively present on the North Island
and only few observations are made on the South Island. It is most
likely that this species is controlled by low winter temperatures in
South Island.
The more widespread distribution of E. canadensis in New
Zealand compared to E. densa and L. major is supporting that E.
canadensis is a generalist. However, the present distribution of
these three species in New Zealand might reflect the recent arrival
of two of the three species (L. major and E. densa), neither of which
has yet had time to spread to all suitable habitats, and both being
more common further north where they were initially introduced
(de Winton et al., 2009). On the other hand E. canadensis have
had roughly 100 years to spread around the country. Attempting
to explain distribution patterns from our study is therefore
complicated by dispersal history and human activities being the
sole important vector for their spread. However, the study allows
clear predictions of the habitats likely to be dominated by the
three species as dispersal continues and a climax distribution is
eventually realized. In conclusion we suggest that there is clear
evidence that E. densa will dominate warmer, shallower waters,
whereas L. major will be the dominant species in colder, clear-water
lakes, while E. canadensis continues its established role as a pioneer
species that is quickly replaced by the two taller species after their
arrival.