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Tragedy of the commons in intraspecific plant competition.
Laird, Robert*,1, Aarssen, Lonnie1, 1 Department of Biology, Queen's University, Kingston, Ontario, Canada
ABSTRACT- A common plant response to increased below-ground (root) competition intensity is to increase the relative allocation to root biomass at the expense of shoot (stems, leaves, and seeds) biomass. However, since root tissue is non-photosynthetic and thus cannot 'pay' for the cost of its own construction, the decrease in shoot tissue mass should exceed the (relative) increase in root tissue mass, decreasing total plant biomass per plant per unit soil volume. This reduced yield has been identified recently as a 'tragedy of the commons' effect, a situation that can be predicted to occur whenever competing parties (plants or otherwise) share a common resource. It is a tragedy in the sense that while these plant allocation patterns are evolutionarily stable, overall yield would be greater if only plants could 'agree' to not shift allocation below-ground in the face of competition. We examined the effects of intraspecific competition on root and shoot biomass allocation in five species of Brassica, grown in a greenhouse. Within species, genetically uniform individuals were planted either alone or in groups of four, with soil volume per plant kept constant between the two treatments (i.e. same nutrient availability per plant). Plants that shared soil volume with neighbours had lower total biomass (per plant) than plants that had their own exclusive soil volume. For all but one species this was accompanied by an increased root-to-shoot biomass ratio in competing versus non-competing plants, providing evidence for a tragedy of the commons. These trends persisted - and indeed became slightly stronger - through time, as evidenced by the analysis of allocation patterns of plants of different ages (i.e. harvest times). It is important to account for shifts in allocation patterns to above- and below-ground biomass when assessing the yield of competitive plant systems.
KEY WORDS: above- vs. below-ground allocation, Brassica, plant yield, tragedy of the commons