|Gorse, Ulex europaeus.|
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.|
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.
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.