Köchy, M., and S. D. Wilson (1999)
Differences in competitive effects among species may be related both to differences in mass and differences in growth forms. I measured the effect of shrubs on grass aboveground net primary productivity (ANPP) and the effect of grasses on shrub ANPP in a reciprocal removal experiment for two years. In grassland where shrub abundance was low, shrubs suppressed grasses as much as grasses suppressed shrubs, even though shrubs had 6 times more standing crop. Shrubs reduced available soil nitrogen more strongly than grasses did, but shrubs and grasses did not differ in their effects on light or soil water. In brush, however, where shrub standing crop was 37 times grass standing crop , shrubs suppressed grasses strongly, whereas grasses did not suppress shrubs. Thus, as shrub abundance increased, the interaction between shrubs and grasses changed from symmetric to asymmetric. On a per-gram basis, however, shrubs had smaller effects on light consumption, nitrogen uptake and water uptake than grasses, suggesting that the competitive effect of shrubs is related mostly to shrub size. Secondary growth in shrubs, however, allows them to accumulate more mass and height, and to eventually displace grasses in spite of the grasses' greater per-gram effects on resources.