On the Allometry of Biomass Partitioning and Light Harvesting for Plants with Leafless Stems

Prior explicit allometric models are extended to predict the scaling relationship between the ability of plants with leafless stems to harvest sunlight H and total standing plant biomass MT (which equals the sum of standing stem and root biomass, MS and MR). Provided that H scales in a directly proportional manner (isometrically) with respect to either stem surface area (i.e.H∝SAS ) or total stem biomass (i.e. H∝MS), the allometric model presented here predicts thatSAS∝MT3/4 orMS∝MT3/4 , respectively. These alternative predictions are tested empirically using data for standing stem and root biomass gathered for the large columnar cactus species Pachycereus pringlei. Statistical comparisons between observed and predicted scaling relationships indicate thatSAS ∝MT3/4, whereasMS ∝MT3/4 is mathematically inconsistent with the observation that stem biomass scales nearly isometrically with respect to root biomass. The contention that the H of leafless stems scales isometrically with respect to stem surface area is thus reasonable both theoretically and empirically.