Models of dryland ecohydrology focus on plant community and productivity responses to precipitation variability and associated processes affecting water storage and redistribution. Contemporary models explicitly address how ecological memory, through interactions between past abiotic and biotic conditions, shapes ecohydrological responses to precipitation. The role of water storage in plant tissues and the reshaping of the distribution, magnitude and timing of ecosystem water flows and productivity responses to precipitation by massive succulent plants in desert landscapes have not been investigated. Here we provide evidence that the evolution of gigantism and massive water storage in plant stems has significantly disrupted the timing and magnitude of ecohydrological processes in environments where these plants achieve high densities. We focus on the giant saguaro (Carnegiea gigantea) and the organ pipe cactus (Stenocereus thurberi) that attain high community dominance and population densities in the Sonoran Desert. We illustrate, using data from irrigation experiments and allometric scaling in natural populations, how ecosystem water storage and flows are shifted by giant cacti and how variation in stem functional traits shape ecohydrological responses. Our irrigation experiment in Arizona involved geophysical, photogrammetric and physiological approaches to quantify residence time, uptake and loss of water on plots with and without occupation by giant water storing cacti.
Results/Conclusions
Measurements of stem geometry in 16 populations of saguaro in the Sonoran Desert and allometric scaling of stem moisture content show that the amount of water stored on the landscape by giant cacti represents a sizable proportion of the effective average annual precipitation (10-20% after runoff, evaporation, and deep infiltration). The average residence time of water in stems of giant cacti at maximum transpiration rates is 50 d in organ pipe and 120 d in saguaro, much longer than that of shallow soil water during summer periods. High resolution dendrometers, sap flow, and time-lapsed 3D Electrical Resistivity Tomography of soil reveal that water is removed quickly from around giant cacti and is routed to and stored in stems starting 24 h after irrigation. Taken together, our approach to quantify the timing, fate and redistribution of precipitation inputs and residence times of water captured in succulent tissues of giant cacti illustrates how presence of these massive stem succulents disrupt ecohydrological storage and flows in desert environments and may greatly extend ecological memory of antecedent conditions.