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GCE IV - Key Finding in 2019

    Allometric rules describe variation in salt marsh plant size and flowering

    Allometry is the study of how an organismís shape changes as it grows. The best known allometric rule for plants is the -3/2 self-thinning law, which describes a well-known trade-off between size and density. Plants also trade off investment into growth vs. sexual reproduction, as described by life-history theory. Liu and Pennings (2019) took advantage of 16 years of GCE monitoring data to examine relationships between Spartina alterniflora height, shoot density, and flowering. Although plants in different landscape positions and years varied tremendously in size and shoot density, all this variation could be explained by a single allometric relationship consistent with the self-thinning law, but with a lower slope (Fig. 1a). The size at which plants had a 50% probability of flowering also differed among habitats, sites, and years, but was consistently smaller under conditions in which average plant size was smaller (Fig. 1b). 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 at high density are small and reproduce at a smaller size than large plants growing at low density. Historically, ecologists have dealt with variation in size of marsh plants like S. alterniflora by dividing the marsh into different habitat types. This work shows that it may be possible to understand differences in growth across the marsh as simply different parts of the same allometric relationships, and thus provides a more synthetic understanding of salt marsh ecology. Moreover, because saltmarsh plants often occur in monospecific stands, they may serve as simple, model systems for studies of plant life history.


    Fig. 1 (a) Relationships between shoot mass and shoot density, and (b) binomial regression models predicting the probability that an individual shoot of Spartina alterniflora will flower based on shoot height in creekbank versus mid-marsh habitats for all sites and years combined. Source: Liu and Pennings 2019.


This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959, OCE-1237140 and OCE-1832178. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.