Scaling laws for biomass allocation in desert annual plants

Background/Question/Methods

The laws that govern how plants allocate their biomass remain largely unknown.  Knowledge of these laws could potentially have a great impact on the fields of ecology, evolution, and botany.  One possible explanation is Metabolic Scaling Theory (MST).  One prediction of MST is that in large plants, leaf mass will be proportional to total mass to the ¾ power.  However, for very small plants, MST predicts that leaf mass should scale at a rate closer to isometry.  Empirical tests have generally shown support for MST, however, the data has come predominantly from large plants.  For this study, we used data gathered for another project to examine whether small desert annual plants support the predictions of MST.  We examined whether the scaling relationships for annual plants are significantly different than for larger woody perennials.  Additionally, we examined how different taxa differed in their biomass allocation patterns, in order to see whether differences are a result of phylogenetic constraint, recent adaptation, or phenotypic plasticity.  

Results/Conclusions

Our new data tended to confirm the predictions of MST, in that the leaf mass of small plants tends to scale isometrically with total mass.  Annual plants had similar allocation patterns to woody perennial plants, but tended to invest more in leaves and less in roots per unit biomass, and to have slightly different scaling exponents.  We found that each family of annual plants varied in their absolute level of stem and leaf mass, but that all of these patterns scaled across sizes at the same rates.  Additionally, we found no difference in root allocation between families.  This suggests that although leaf mass and stem mass may be somewhat interchangeable, there is a physiological constraint in the amount of root mass required to support a given amount of above-ground biomass.  We found that much of the size-corrected variation in our dataset was due to within-species variation, and that between-family variation had the smallest effect.  This suggests that in the annual plants considered, variation is due more to phenotypic plasticity than genetic constraint.