The potential impact of Scotch broom on softwood forestry in Australia.

A CLIMEX model of Scotch broom, (Cytisus scoparius (L.) Link) indicates the potential for range expansion of this weed in Australia. Over 97% of the Australian softwood plantation area was found to occur in areas of suitable or optimal climatic suitability for C. scoparius. Trees growing in competition with C. scoparius, across 24 New Zealand sites, average 51.7% (range 32.8–64.8%) and 23.6% (range 11.0–37.8%) of the height and diameter of trees growing in the absence of plant competition, respectively. Annual rainfall exhibited a moderate positive correlation with relative tree height (height of trees growing in competition with C. scoparius compared with those growing weed-free) (r = 0.37). If these reductions in P. radiata growth and diameter under competition with C. scoparius were experienced in Australia, further spread of this weed could seriously impact on site productivity and would add a significant burden to the industry by means of control costs.