Plants adjust their size and reproductive effort in response to numerous selection pressures and constraints. The self-thinning rule describes a well-known tradeoff between size and density. Plants also trade off investment into growth versus sexual reproduction. We examined how these tradeoffs affected allometry and flowering across space and time using the salt marsh grass Spartina alterniflora. We collected data on plant height, density and flowering probability over 16 years at different elevations at 8 different sites in Georgia, USA.
Results/Conclusions
Plants at different elevations, in different sites, and in different years varied tremendously in size and shoot density. However, all this variation could be explained by a single allometric relationship consistent with the -2/3 self-thinning law. Flowering was size-dependent, and the threshold for flowering varied across elevation, sites and years. Plants that were stressed reproduced at a smaller size than plants that were growing under good conditions, and this contrast was consistent across elevation, among sites, and among years. Finally, reproductive biomass and the proportion of shoots flowering increased with increasing vegetative size (plant height or shoot biomass). Combining these two patterns, S. alterniflora plants growing high density are small, and reproduce at a smaller size than large plants growing at low density. Although there is tremendous spatial and temporal variation in S. alterniflora growth and reproductive patterns, all this variation can be understood as resulting from two simple allometric tradeoffs.