| Summary: | Despite the negative impacts invasive plants can impose on forest communities and
the potential for these impacts to increase with global change, little has been done in
New Zealand to examine what facilitates the establishment and spread of these
species within native beech (Nothofagus solandri) forest. This study examines the role
which forest canopy gaps and deer herbivory have on the dispersal, germination,
establishment and spread of invasive plant species within the Nothofagus forests of
the Hopkins and Huxley valleys, near Twizel, New Zealand. Propagule dispersal was
enhanced by the presence of a gap in the above canopy, resulting in an increased
number of seeds being caught on the sticky traps in these plots (159 compared to only
2 in the control plots). Seed bank samples were assessed by germination in a
glasshouse for one year. The invasive seed bank was larger in the fenced plots and
conversely to the other results of this study, the presence of a canopy gap had no
effect on the size of the seed bank. Aboveground cover of these invasive plants was
primarily driven by the existence of a canopy gap, suggesting that canopy gaps are the
main cause of plant invasions into Nothofagus forest (9% mean cover in the gap plots
compared to 0% in the control and fence plots). However, there was a further increase
in the aboveground cover of these invasive plants where a deer exclusion fence was
situated around the perimeter of an open canopy plot (27.5% mean cover in the gap
fence plots compared to 9% in the gap plots). Thus, suggesting that deer herbivory
could be decreasing the abundance of invasive plants in areas of forest where gaps
have been created. The canopy gaps allowed invasive plants to spread back into the
forest away from the openings but only for a short distance, as in all cases invasive
plants were absent by 6-7m from the plot edge. This localised establishment and
spread of invasive plants where canopy gaps have been created could pose a threat to
New Zealand’s native Nothofagus forests, as many small populations spread out
across a landscape can cause more ecosystem adversity than larger single populations.
If not kept in check these invaders could increase further with global change however,
if herbivory is assisting in the control of plant invasions management strategies for
both invasive plants and introduced deer may need to be revised.
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