Scotch broom facilitates indigenous tree and shrub germination and establishment in dryland New Zealand

It is common practice in New Zealand dryland areas to chemically or mechanically control invasive woody weeds, including Scotch broom (Cytisus scoparius). Such weed control is not always effective in achieving the often implicit aim of advancing the restoration of indigenous woody vegetation. We used a field experiment on a braided river terrace on the Canterbury Plains to test how five different management treatments of broom cover affected the germination, survival and growth of six indigenous tree and shrub species in a dryland setting. Mulched, root-raked and crushed treatments resulted in low seed germination and high mortality of planted seedlings, which was apparently due to the associated soil disturbance and microsite conditions. Significantly higher germination and survival rates of indigenous woody species under the living broom canopy indicated that the facilitative effects of the living canopy outweighed any negative effects. With no evidence of unassisted regeneration of indigenous plants from local sources during our experiment, our results suggest that retaining a live broom canopy was most beneficial for the germination and establishment of planted indigenous woody seedlings at this site. Compared with sowing and planting after mechanical or chemical broom control, sowing seeds and planting seedlings under the living broom canopy was also the cheapest management strategy to advance the succession of indigenous woody species in these dryland weed communities.