Response of Root Depth to Climate - a Hydrologist's View of the Critical Zone

A variety of processes in the critical zone are modulated by the presence and extent of plant roots. While it is challenging to observe and measure roots in the field, a number of investigations indicate that climate, characterized by precipitation and potential evapotranspiration, has significant explanatory power with respect to root morphology. Building on these findings, this work focuses on hydrologic drivers to determine a water-optimal root depth. A stochastic model of soil-moisture dynamics is coupled with a carbon cost-benefit analysis to determine root depth as an analytical function of the frequency and intensity of precipitation, along with soil, vegetation, and climate characteristics.

Consistent with empirical evidence, the predicted root depth is greatest when precipitation and potential evapotranspiration are in balance. For wetter environments, deeper roots are not needed to meet demand; in drier environments, there is no water to warrant deep roots. In wet environments, root depth is more sensitive to rainfall frequency than intensity, and in water-limited ecosystems, the opposite is true. In both wet and dry conditions, recharge decreases and evapotranspiration increases with increasing precipitation frequency and concomitant decreasing intensity. These results can be used to inform both paleoclimate investigations and predictions of the impacts of climate change on critical-zone processes.