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Title: Tiller organization within the tussock grass Schizachyrium scoparium: A field assessment of competition-cooperation tradeoffs

Author
item Derner, Justin
item BRISKE, DAVID - Texas A&M University
item Polley, Herbert

Submitted to: Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2012
Publication Date: 4/1/2012
Citation: Derner, J.D., Briske, D.D., Polley, H.W. 2012. Tiller organization within the tussock grass Schizachyrium scoparium: A field assessment of competition-cooperation tradeoffs. Botany. 90:669-677.

Interpretive Summary: Competition among individual subunits of plants can affect performance and growth of plants. We assessed this competition among individual subunits in a model grass species, little bluestem (Schizachyrium scoparium) in a field setting for two years. We grew constructed plants with 1, 4, 8 or 12, individual subunits and collected measurements of biomass, numbers of tillers (both reproductive and vegetative) and numbers of tillers per unit area (density). As the number of subunits per plant increased, it did not affect tiller density or biomass of individual vegetative or reproductive tillers. However, we did observe a large reduction in tiller mass and number per subunit, suggesting high levels of competition among the subunits. This suggests that there is a division of labor within plants that is exhibited as competition among the individual subunits that is compensated by cooperation among tillers within the individual subunits. This division of labor functions to optimize the competitive ability of the entire plant by coarse-scale resource preemption by the individual subunits, while resource sharing among tillers within subunits supports new tiller establishment and growth within this highly competitive microenvironment.

Technical Abstract: Tussock grasses are characterized by a compact spatial arrangement of tillers that contributes to intense intra-tussock competition. This investigation was designed to directly assess the magnitude of competition among autonomous subunits of tillers within tussocks (i.e., integrated physiological units) to further define the mechanisms of tiller organization within this successful growth form. Tussocks of Schizachyrium scoparium were grown in the field for 2 years with 1, 4, 8 or 12 autonomous subunits to span the range observed in naturally occurring local populations. Increasing numbers of subunits per tussock did not affect tiller density or mass of individual vegetative or reproductive tillers, but it did intensify intra-tussock competition as evidenced by a large reduction in tiller mass and number per subunit. This pattern of tiller organization is indicative of a division of labor within the tussock that is manifest as a tradeoff between competition among autonomous subunits and cooperation among tillers within these physiologically integrated subunits. We conclude that an increasing number of autonomous subunits associated with tussock basal expansion contributes to coarse-scale resource preemption and competitive ability, while resource sharing among tillers within subunits supports new tiller establishment and growth within this highly competitive microenvironment.