Naturalisation of the tree of love Aichryson laxum (Crassulaceae) and its implications for biosecurity surveillance in New Zealand

Abstract

ABSTRACT Aichryson laxum is documented as newly naturalised in Aotearoa New Zealand. Although tree of love is its colloquial name, it is a small, succulent herb in the Crassulaceae, and is indigenous to the Canary Islands. It has been cultivated in New Zealand for at least several decades. The species has recently been collected wild at multiple sites around Wellington, with collections also from Napier and near Ōtaki Forks, and an additional report of wild plants from Christchurch. Despite its small size, A. laxum can grow densely, adding to the weeds smothering surfaces and displacing indigenous species from habitats such as open, rocky banks. Within Wellington, A. laxum was first noticed to be growing wild in 2019, but it has now been found to be established at multiple sites throughout the city. Either it is spreading aggressively via its tiny, wind-dispersed seeds, or it has lain undetected for some time, even though it is a distinctive species among Wellington’s flora. Both scenarios are concerning, and A. laxum provides another example of the shortcomings of the nation’s current surveillance – recently described by the Parliamentary Commissioner for the Environment as largely passive and serendipitous – for emerging weeds newly naturalising from the tens of thousands of exotic plant species cultivated in New Zealand.