Beating Weeds with Science
Alder with dense native understory. Photo credit: Kate McAlpine.
More than 50 species of woody weeds occur in large, dense patches throughout New Zealand. Most of these weeds can displace native vegetation and disrupt ecosystem processes, so predicting which species are likely to persist and which are likely to be replaced by native vegetation is critical for informing management tactics. The idea that leaving woody weeds in the environment until they are outcompeted by native species has the advantage of reduced control and management costs.
Kate McAlpine from DOC is leading research into this question. “Our theory is that weed species that regenerate strongly under their own canopy are most likely to persist, because the next generation is already present in the understory, ‘lying in wait’ to take over when the current adult cohort dies,” explained Kate. On the other hand, woody weed species that don’t regenerate under their own canopy (presumably because of shade intolerance) are less likely to persist in the long term. Instead, they are likely to be replaced by more shade-tolerant species that do establish in the understory. If those species are native, then the weeds may be replaced by native plant succession, provided no further disturbance occurs. Some species, such as gorse (Ulex europaeus), are known to act as ‘nurse’ plants, and facilitate native succession. However, the likely persistence and long-term impact of most woody weed species in New Zealand is poorly documented.
Kate and her colleagues (Susan Timmins from DOC, and Shona Lamoureaux and Sarah Jackman from AgResearch) have surveyed the understory vegetation in 132 populations of 41 woody weed species around New Zealand. “We found that 27 weed species had little or no regeneration under their own canopy, and thus appear to have potential to be replaced by native succession − particularly at sites where there is a dense native understory. This is good news and many more species than was previously thought. However, 14 woody weed species did regenerate strongly under their own canopy, and thus appear more likely to be able to persist in the long term,” said Kate. Of those 14 species, Chinese privet (Ligustrum sinense) appeared to be the ‘worst’, having consistently high weed regeneration in the understory and very little native vegetation. Some species, such as sycamore (Acer pseudoplatanus) and tree privet (Ligustrum lucidum) were highly variable in terms of regeneration, with very high numbers of seedlings and saplings in the understory at some sites but none at all at others.
Kate also found that many woody weed stands had a dense native understory (50−90% cover), and suggests that this should give the natives a head start over re-invading weed species when the canopy plants die. “In fact, natives far outnumbered non-native species in the understories of these weed stands, both in terms of percent cover and the number of species,” Kate said. In total, more than 170 native species were recorded, including canopy-forming species such as tawa (Beilschmiedia tawa), tītoki (Alectryon excelsus) and kahikatea (Dacrycarpus dacrydioides). Māhoe (Melicytus ramiflorus) was the most commonly recorded species, present at 67% of sites. Sites close to a native forest remnant were more likely to have a dense native understory, which shows that distance to a native seed source is important. The rate at which a woody weed will be replaced through the process of succession is likely to be at least as long as the life span of the weed, but can also be influenced by site-specific factors such as the presence of browsing animals, and temporal factors such as the rate at which the weed naturally thins out, allowing more light to enter the understory. “But we are talking in terms of decades or even centuries for native species to become dominant,” agreed Kate. “And, if a major disturbance such as a fire or flood occurs, the successional clock may be set right back to zero”.
Kate is now planning to investigate what happens next when the adult weed plants die: do the natives in the understory take over, or does the weed re-invade? “My aim is to provide regional councils, DOC, restoration groups, and anyone else dealing with plant invasions on a large geographic scale with better information about which species are likely to persist, and which might die out naturally. This will help weed managers to prioritise species and sites for control,” Kate said.
This project is funded by the Ministry of Business, Innovation and Employment as part of Landcare Research’s Beating Weeds programme, with additional support provided by the Department of Conservation.
Contact: Kate McAlpine (kmcalpine@doc.govt.nz)