This dataset contains individual species size data in vegetation plots that have had various plant functional groups or species experimentally removed at the Jornada Basin LTER site in southern New Mexico, USA. This data was collected with the objective to distinguish the differential effects of plant community biomass, functional groups, and biodiversity within functional groups on ecosystem and plant community function. To make these distinctions, treatments were established by the selective removal of plant species or functional groups within experimental plots. There are eight treatments: control (C, no removals); four functional group removal treatments (PG, perennial grass removed; S, shrubs removed; SSh, subshrubs removed; Succ, succulents removed), and three species richness manipulation treatments. Richness manipulations included a simplified treatment (Simp), where only the single most abundant species of each growth form is preserved and all other species in the growth form are removed, a reduced‐Larrea treatment (rL), where the Larrea is assumed to be the dominant and is removed while minority components remain, and a reduced-Prosopsis treatment (rP), where Prosopis rather than Larrea is removed as the shrub dominant. In 1999, this pilot study attempted to assess individual species responses of representative individuals in these treatments. Ten randomly selected individuals of eight plant species were measured in each experimental plot, and this dataset reports volumetric data (diameters and height) for each. The study was designed as an individual-based complement to the transect data in EDI dataset knb-lter-jrn.210121001 but was not continued past 1999. This dataset is complete.