Low Richness of Invasive Non-Native Plants in New Zealand Indigenous Forests May Not Reflect Low Impact

Aim

Identifying habitats vulnerable to plant invasions is essential for developing efficient management programmes. We assessed trends in richness and cover of non-native plants in indigenous shrublands and forests across New Zealand. We investigated whether species classed as invasive species exhibited higher levels of plant invasion than naturalised species and the extent to which this reflected plant life form.

Location

New Zealand.

Time Period

From January 2009 to March 2014.

Major Taxa Studied

Plant.

Methods

We analysed 839 permanent 20 × 20 m plots spread across New Zealand that could be classified to a recognised forest type: mānuka-kānuka shrubland, beech, beech-broadleaved, beech-broadleaved-podocarp and broadleaved-podocarp forests. Generalised additive models were run with native canopy richness or cover and spatial coordinates as co-variables in order to compare non-native plant richness and cover across forest types in relation to their invasive status and growth form.

Results

Overall, 35% of the plots had at least one non-native species. Mānuka-kānuka shrubland exhibited the highest mean non-native richness (11 species) and cover (32%) with broadleaved-podocarp forest presenting the next highest invasion level but to a much lesser extent (1.7 species and 3% cover). Despite presenting overall greater non-native richness, naturalised species had lower cover than invasive species (4.2%, 13.5%, respectively). This pattern was mainly related to non-native woody species that, despite their low richness, can reach greater cover than herbaceous species once established.

Main Conclusions

Despite half the New Zealand flora being composed of non-native plant species, relatively few were found in forest habitats. However, indigenous shrubland and early successional forests showed higher vulnerability to plant invasion. Woody species, which are overrepresented among invasive species, had higher cover than herbaceous species and were less limited by native canopy cover. Such findings highlight the threat posed by non-native woody species and the need for more targeted management programmes.