Wednesday Weed: gorse

A couple of my favourite blogs and websites have regular weekly features.  Pharyngula has Mary's Monday Metazoan, Friday Cephalopod, and the wonderful Botanical Wednesday, whereas Why Evolution is True has Caturday Felid.  So I'm going to steal their idea and have a Wednesday Weed, but maybe not every week.  Sometimes I might simply post a picture.  This week: gorse.

Gorse, Ulex europaeus.

When my family arrived in Porirua, where we lived for a few months in 1956, my mum picked some gorse in flower from waste land nearby and put it in a vase to brighten up the house.  Her family had had an ornamental gorse bush in their garden in Stoke-on-Trent, England.  The neighbors thought she was mad, because gorse is probably New Zealand's worst weed.  It was brought here quite early (the first published record is from 1867), probably as a hedge plant, and quickly established and spread.

What makes gorse so weedy?  The young shoots are palatable to livestock, but once the prickles have hardened off they won't be grazed.  The seeds are ejected explosively from the pods, but they only travel a few metres.  Then they can remain dormant for a long time.  We recently took up a concrete path that was laid in the mid-1960s (dated by an old newspaper lodged in the cavity of a concrete block in the adjacent wall) and within months several gorse seedlings had appeared in the newly-exposed soil.  Over a very few years, a few scattered bushes can turn into a dense closed impenetrable canopy.

Gorse is a legume, and legumes have an association with nitrogen-fixing bacteria that enables them to acquire nitrogen easily.  Not a single one of the millions of eukaryotes (organisms that have membrane-bound nuclei and mitochondria in their cells, i.e., including all animals, plants, fungi, and protists) is able to fix nitrogen.  All living things need nitrogen—it's absolutely essential for proteins and nucleic acids—and it's abundant in the atmosphere (about 80%), but it's not available to eukaryotes without being processed first by bacteria.  That's not an intelligent design, but we have to live with it.  Legumes live with it very well by having a permanent association with such bacteria in nodules in their roots, and that can give them a competitive advantage on some sites, particularly in the presence of phosphate (it's worth noting that both superphosphate application and scrub weed clearance were subsidized by the New Zealand government until the mid-1980s).

Gorse at Hinewai reserve, with a fire in the distance.

Burning gorse will kill it (it's highly flammable), but new seeds will germinate to restore the gorse canopy.  But leave it alone and the bushes quickly grow tall and leggy, allowing native plants to establish underneath.  Gorse won't re-establish in this semi-shade, and soon the native forest canopy overtops the gorse one, and a new forest is born.  However, while gorse has been successfully used as a "nurse crop" in this way, such as at Hinewai Reserve on Banks Peninsula, Jon Sullivan, Peter Williams, and Susan Timmins (Sullivan et al. 2007) caution that succession under gorse is different from succession under a native shrub, kānuka (Kunzea ericoides).  They found forests that originated under a gorse canopy had lower species richness, fewer small-leaved shrubs like mingimingi (Leptecophylla juniperina and Leucopogon fasciculatus), and less kāmahi (Weinmannia racemosa) than in forests grown under kānuka.

As alternatives to spraying, burning, and waiting until gorse reverts to forest, Landcare Research Ltd have trialled a range of agents for biological control of gorse, including several insects (moths, thrips, a weevil) and a mite.

Reference

Sullivan, J.J.; Williams, P.A.; Timmins, S.M. 2007.  Secondary forest succession differs through naturalised gorse and native kānuka near Wellington and Nelson.  New Zealand Journal of Ecology 31: 22–38.