The effect of shade upon seedling growth in groundsel bush (Baccharis halimifolia L.)

Abstract

Analysis of seedling growth of groundsel bush (Baccharis halimifolia L.) under different glasshouse light conditions indicated that seedlings growing in light shade (24% daylight) maintained relative growth rates equal to those of seedlings grown unshaded (57% daylight) for up to 11 weeks after germination. Compensation for reduced light was effected through increased leaf area rather than through increased photosynthetic efficiency. Although markedly less than in unshaded or light shade treatments, growth was maintained throughout the experimental period under the lowest radiant flux employed (17% daylight).

Root systems developed more slowly and were smaller in shaded than in unshaded seedlings. Increased stem allocation (height growth) followed a peak in root allocation, and did not occur until 11 weeks after germination in any treatment.

Comparisons of seed weight and seedling relative growth rate with associated pasture species indicate that groundsel is at a disadvantage in terms of both parameters. It appears that the success of this woody weed is due largely to a combination of high reproductive output and effective dispersal, which enables it to colonize relatively open microsites which occur in cultivated as well as natural field situations.