The allometric relationships of trees in temperate and tropical forests are relatively well known, but not those of woody shrubs or transitional (shrub/tree) life forms. We explored the transition of Prosopis glandulosa var. torreyana from tree to shrub along a semi-arid topographic sequence comprising of six landforms (hillslope, footslope, upper and lower bajada, playa and dune) with varying soil texture and water availability. In each landform, we measured P. glandulosa shoot pre-dawn water potentials (Ψ) in one ‘dry’ and one ‘wet’ year. We also measured plant height, widest basal stem diameter, crown area and number of basal branches. Total basal stem area was calculated. We used simple (Model II linear regression) and expanded (incorporating an asymptote to height or crown area) allometry models to compare height with widest basal stem diameter and crown area with total basal stem area. There were significant correlations between maximum plant size and inter-year Ψ means suggesting that soil water availability decreased down the topographical sequence. The height asymptote was statistically significant in all landforms and lower toward finer-textured soils. On the other hand, crown area was a linear function of total basal stem area and was also site specific. Our results suggest that more basal branches are produced as plant height decreases in more stressful environments, effectively increasing crown area with a minimum investment in supporting tissues. The polymorphic characteristics of Prosopis may partly explain their occurrence in many arid and semi-arid environments.

Nomenclature: Correll & Johnston (1970); for families: Anon. (1998).