Water Relations of a Perennial Grass and Seedling vs Adult Woody Plants in a Subtropical Savanna, Texas

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Abstract
Over 52 .+-. 16% (Mean .+-. SE) of the seeds of the arborescent legume Prosopis glandulosa var. glandulosa germinated within two weeks of dissemination in plots dominated by a perennial grass (Chloris cucullata) in July 1984 and 63 .+-. 7% of those germinating survived through September 1985. Our observations spanned a period of normal temperatures and below-normal precipitation, suggesting the apparent increased abundance of Prosopis on this site in recent times has probably not been episodic with regard to moisture. Over 60% of the herbaceous root biomass occurred in the upper 30 cm of soil. In contrast, tap roots of Prosopis seedlings had penetrated beyond 40 cm within 4 months of germination and their mean proportion of total biomass belowground increased from 0.27 .+-. 0.09 in May to 0.52 .+-. 0.15 in August. Net photosynthesis (Pn) and conductance (g) of Chloris were closely coupled to fluctuations in moisture in the upper soil horizons (< 30 cm), whereas Pn and g of one-year old P. glandulosa seedlings were correlated with soil moisture at depths between 30 and 90 cm. Among mature Prosopis plants, Pn and g varied as a function of soil moisture at depths > 90 cm. We conclude that rapid development of roots of Prosopis seedlings during their first year of growth apparently enhanced survival by enabling them to access soil moisture beyond the zone effectively utilized by grasses. On sites with a history of grazing, competition for water from herbaceous vegetation may not play a significant role in limiting establishment of P. glandulosa seedlings or the growth of mature plants in most years. Partitioning of soil moisture between P. glandulosa and grasses, achieved early in the life cycle of Prosopis, helps explain how this woody plant can successfully establish in grasslands.