Small-scale variability in water storage and plant available water in shallow, rocky soils

Background and aims We quantified spatial variability in water storage and plant access to water in the rocky soils of a karst savanna dominated by Ashe juniper (Juniperus ashei) and honey mesquite (Prosopis glandulosa). Methods In a 25x25 m grid with 5-m node spacing, water content and bulk density profiles were measured to a depth of 1.6 m by a combination of time domain reflectometry, neutron thermalization and gamma ray densitometry. Changes in water content were used to infer recharge and plant uptake of soil water. Predawn water potentials of trees were sampled periodically to evaluate individual differences in water access. Results Pore volume and maximum water storage varied between 0.24 and 0.42 m(3) m(-3), and 198 to 431 mm, respectively, across the 36 individual profiles. Porosity accounted for 19 and 20% and depth for 35 and 61% of the variation in uptake and recharge, respectively. Predawn water potentials were consistently different among individual trees over multiple dry seasons. Conclusions Unequal water status among trees was consistent with the variability of recharge and uptake in the rooting zone, suggesting that trees cannot fully compensate for spatial variability in soil properties by roots foraging for water.