Contributions to the Biology of Larrea Species

Seeds were collected from three taxa: L. divaricata from Arizona in the Sonoran desert of North America (LDNA); and L. divaricata and L. cuneifolia from northwestern Argentina, in the monte of South America (LDSA and LC). These three desert shrubs are generally not sympatric, and LDSA appears to occupy the most mesic sites. Seedlings were grown to the age of 3—7 mo in a greenhouse and compared as to root— and shoot—growth rate, photosynthesis and transpiration under high and low water stress, pathway of photosynthesis employed, leaf stomatal density, and leaf anatomy. Herbarium specimens collected from 55 sites along moisture gradients were also analyzed for path of photosynthesis, leaf stomatal density, and leaf anatomy. Under mesic conditions, LDSA had a higher growth rate, yet maintained the lowest biomass fraction in leaves and the highest in roots. There were no differences among taxa in net photosynthesis, though LDSA had a significantly higher transpiration rate. Under xeric conditions (plant pressure potential about —45 atm), the photosynthetic rates of LDSA were depressed the most, but variability within taxa prevented conclusions on differences in rates of photosynthesis and transpiration. Stomatal density was greatest for LDSA and least for LC, and these differences were maintained in the greenhouse. Reduction in stomate number seems to be a major mechanism for transpiration reduction between Larrea taxa, but within a taxon there was no relationship between site precipitation and stomatal density. The path of photosynthesis (from 13C/12C and enzyme—activity ratios) was consistently C3. Leaf cross—sections appeared to reveal mesophyll differences between LC and the two LD taxa. Two ecotone sites, where LDSA and LC share dominance, were sampled for community structure and the presence of putative hybrids. One site, with a one—to—one ratio of Larrea taxa, showed no evidence of biotic interaction between the taxa or between them and associated species, and hybrids accounted for less than 1% of all shrubs. Analysis of mass—collected random leaf samples also showed little evidence of hybridization in leaf shape or stomatal density. A second ecotone site, 600 km to the south, differed in some respects from the above, but, again, hybridization was a rare event. The basis of the mesophytic nature of LDSA might lie with its shrub architecture as well as with its physiological properties.