PARENT SESSION
Oral Session #19: Plant Competition. Presiding: N. Slade.
Monday, August 6, 2001. 1:00 PM to 5:15 PM. Hall of Ideas J.

Timing is of the essence: Intra-annual variation in root and shoot competition.

Cahill, James¹, ¹

ABSTRACT- Lacking from studies of competition-productivity relationships is an understanding of how root/shoot competition develop during a growing season. In an old-field, Achillea millefolium was used as the focal species to test how (1) root/shoot competition (2) root/shoot interactions, and (3) the per-gram effects of neighbors vary during the growing season. Competition was modified using exclusion tubes and tie-backs. Both the strength of competition and the direction of the root/shoot interaction varied through time. Root and shoot competition were strongest in the second half, with competition having a greater proportional effect on plants grown for half the season than those grown for the full season. Within a single time treatment, root/shoot competition were additive in their effects on growth in the first half of the study, were less than predicted in the second half, and were greater than expected over the full season. There was a greater negative per-gram effect of neighbors when grown for the full season then when grown only for half, with competition unrelated to neighbor biomass in the second half of the summer. These results suggest that it is not absolute neighbor biomass that determines the strength of competition. Instead, an individual's position in the size hierarchy may influence the strengths of the various components of plant competition. This has implications for the evolution of plant traits and the community level effects of competition.

KEY WORDS: plant competition, per-gram effects, non-additive interactions, root competition