New Zealand has as many native as invasive plant species. A number of woody invasive species have been rapidly increasing their distribution in recent decades. In many places they are chemically or mechanically controlled often with the implicit, rarely achieved, aim of advancing the restoration of native woody vegetation. However, there may be another option: leave these exotic shrubs – they may be ‘nurse-crops’ that aid the recovery of native vegetation. This seems to be particularly effective in moist environments and where nitrogen-fixing species are involved, such as gorse (Ulex europeaus). In the drier parts of New Zealand, broom (Cytisus scoparius, another nitrogen-fixer) is a real problem, and here this species was thought to be fairly useless as a nurse-crop, until recently (see this post).
In an article that is now available online, we describe a field experiment that tested how five different management treatments of broom cover affected the germination, survival and growth of native tree and shrub species.
We imitated different management techniques that are applied to broom in the dryland zone: in some plots the broom shrubs were sprayed with weedkiller, elsewhere we had bull-dozers drive over the shrubs and leave the debris, or tractors rake all the shrubs (including their roots) and take the debris away, in other plots the shrubs were mulched and the mulch was left, and in the last type of plot, we just didn’t touch the broom shrubs and just left them standing. In all plots, we sowed seeds and planted seedlings of six native tree and shrub species. There was no evidence of unassisted regeneration of native shrubs from plants nearby during our experiment.
We sowed thousands of seeds, and very few seeds germinated and even fewer germinants survived until the end of the experiment (3.5 years after sowing), but some species had much higher germination than others (particularly the hard seeded Kowhai, Sophora microphylla, germinated well). Plots where the broom cover was mulched, crushed or root-raked had very low seed germination and high mortality of planted seedlings, which was apparently due to the soil disturbance and harsh conditions of the open sites that were created. Under the living broom canopy germination and survival rates were significantly higher. This indicates that the positive (facilitative) effects of the living canopy, such as the provision of shade and a moister cooler environment in the dry summer, outweighed any negative effects (probably particularly the increased competition for moisture by the living broom shrub and the native seedlings).
This study suggests that at this dry site, compared with the chemical and mechanical treatments of this woody weed, retaining a live broom canopy was most beneficial for the germination and establishment of planted native woody seedlings. Importantly, it was also the cheapest management option by far, and it may thus be an important strategy to advance the succession of indigenous woody species in these dryland weed communities.