The tree of love – another biosecurity failure

The tree of love – another biosecurity failure

Exotic species that spread into the wild are one of the pressures impacting the uniquely special biodiversity of Aotearoa New Zealand. Biosecurity measures are intended to mitigate this. But one aspect of the country’s biosecurity was recently criticised as patchy, limited, passive, and serendipitous.  Curator of Botany Leon Perrie and Genetics Researcher Lara Shepherd introduce a recent example of this problem.

We have just published a paper describing the naturalisation in Aotearoa New Zealand of the tree of love, Aichryson laxum. Despite its colloquial name, the tree of love is not a tree but a short-lived, succulent herb in the crassula family. It’s indigenous to the Canary Islands. It’s distinctive for its rounded, hairy leaves, and many-parted yellow flowers.

Link to our paper about tree of love in New Zealand Journal of Botany.

A shrub with small leaves that are mostly green except for some in the centre which are pinkish with yellow flowers.
A cluster of many individuals of tree of love, Aichryson laxum. Photo Leon Perrie CC BY
Yellow flowers with pointy petals and bobbles on the end of the stamens.
The yellow flowers of tree of love have many parts, usually with 10-11 petals.  Flowering in New Zealand occurs at least from August through December.  Photo Leon Perrie CC BY
A split photo with the top half showing three stems of a plant with leaves at the end of them lying on the ground. The second half of the photo shows a small shrub with rounded leaves.
Each individual tree of love grows from a single stem that repeatedly forks.  The leaves are rounded and hairy.  The stems are also hairy.  Photos Leon Perrie CC BY.

Specimens of tree of love, Aichryson laxum in Te Papa’s herbarium collection.

The problem with tree of love

Tree of love has been cultivated in New Zealand for at least several decades. However, in the wild, tree of love can grow densely so that, despite its small size, it smothers the rocky banks that it seems to favour. This leaves less room for the native species of this habitat, including the seedlings of bigger species.

A photo of a small-leafed plant growing on the side of a tree that is also covered in lichen.
Tree of love (and other weeds) overtopping the lichens and bryophytes on a rockface before native vascular plants have had the chance to establish.  Photo Leon Perrie CC BY

The tree of love invasion

Self-sown wild plants of tree of love have been found in Wellington, Christchurch, and Napier. Amazingly, tree of love became widespread through Wellington city before anyone seemingly noticed it!

A map of Wellington harbour with the streets mapped out in red and black dots and circles showing locations of where the plant is found.
Recorded distribution of tree of love, Aichryson laxum in Wellington. Black circles are specimens in Te Papa’s herbarium. Open circles are verified iNaturalist observations. All records made since 2019. Map by Leon Perrie. Te Papa

Tree of love was first recorded growing wild in 2019, by a contributor to iNaturalist, a website for engaging with biodiversity, including recording and identifying species. We started seriously looking for tree of love when we found it on our walk to work in late 2021.

It’s possible that tree of love is spreading very quickly. Or maybe it has been in Wellington for a long time but overlooked until recently. Both scenarios are concerning – it’s either a super weed and/or we (the country) are bad at noticing new weeds.

The broader weed problem

It may be astonishing that in ‘clean, green’ New Zealand, our 2400 or so indigenous vascular plant species are outnumbered by some 2900 exotic species self-propagating in the wild.

In a recent report on weeds in Aotearoa New Zealand, the Parliamentary Commissioner for the Environment (PCE) pointed out that almost all New Zealand’s new weeds come from our gardens. It’s thought that every year roughly 10 species of plants that we cultivate ‘jump the fence’ to establish in the wild for the first time, with some of these going on to become serious environmental problems. More than another 20 000 exotic plant species are estimated to be in cultivation, so the reservoir of potential new weeds in the country is already massive.

Concerningly, no one is systematically monitoring the new establishment of garden escapees. That is why the PCE bemoaned:

“…the current patchy and limited nature of a largely passive surveillance system that is too often dependent on serendipitous sightings.

That’s a problem for Aotearoa Zealand when the cost of a problematic weed is lower the earlier it is dealt with – both economically and ecologically.

Read the Parliamentary Commissioner for the Environment’s report: Space invaders: A review of how New Zealand manages weeds that threaten native ecosystems.

What can you do?

Like the PCE, we think Aotearoa New Zealand needs to do a better job of tracking and tackling new weeds within its borders. Ideally this would have some central coordination by agencies with a biosecurity mandate. Regardless, everyone can play a part by getting to know the plants growing wild in our neighbourhoods.

A good way to learn plants is joining the field trips of the regional botanical societies, which exist in many population centres.

Additionally, anyone can upload photos of plants (and other living things) to the iNaturalist website. There, a community of nature enthusiasts will suggest identifications.

If you notice a plant that seems to be new to your area, or is increasing in abundance, report it to your local council. They will have local weed experts.

To get hands-on with tackling weeds, the Weedbusters website has many tips. Or lend a hand to a community group; many restoration groups also do weeding. There are also community groups that specifically target weeds, such Old Man’s Beard Free Wellington.

Small rounded leave on a succulent growing on the side of a tree.
Tree of love (at bottom of image) is yet another addition to the exotic flora matting windy weedy Wellington’s hillsides. It’s similar to the already well-established Aeonium haworthii (or hybrid derivatives; at top of image); the latter has hairless leaves that are widest above their middle and have a short apical spine. Photo Leon Perrie CC BY

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