Woody plants modulate the temporal dynamics of soil moisture in a semi-arid mesquite savanna

Background/Question/Methods

In arid and semi-arid ecosystems (drylands), soil moisture abundance limits biological activity and mediates the effects of anthropogenic global change factors such as atmospheric CO₂ increases and climate warming. Moreover, climate variability and human activities are interacting to increase the abundance of woody plants in drylands worldwide. How woody plants interact with rainfall to influence patterns of soil moisture through time, at different depths in the soil profile and between neighboring landscape patches is poorly known. In a semi-arid mesquite savanna near Tucson, Arizona we deployed arrays of sensors located in distinct microsites (beneath a mesquite canopy and in an open area) to measure volumetric soil water content (θ) every 30 minutes at several depths between 2004 and 2007. In addition, to quantify temporally dynamic variation in soil moisture between microsites and across soil depths we analyzed θ time-series using Fast Fourier Transforms (FFT).

Results/Conclusions

FFT analyses confirmed the prediction that by reducing evaporative losses through shade and reducing rainfall inputs through canopy interception of small rainfall events, the mesquite canopy was associated with a decline in high frequency (hour-to-hour and day-to-day) variation in shallow θ.  Finally, we found that in both microsites, high frequency θ variation declined with increasing soil depth as the influence of evaporative losses and inputs associated with smaller rainfall events declined.  These findings are consistent with, and may help to explain, the results of other investigations that have examined the role of woody plants in enhancing nutrient cycling and altering carbon cycling in dryland ecosystems.