Influence of Lateral Flow on the Predisposition of Aspen Mortality during Drought

Abstract:

Lateral subsurface flow can be critical to understanding the spatial soil moisture availability to plants, and when, where, and how drought are influencing individual plants. The concentration of intensive aspen damage in certain hillslopes with higher temperature and lower soil moisture suggests that soil augmentation/reduction from lateral redistribution could help explain the survivability of some aspen through its influence on soil water availability during drought. It remains unclear how lateral water redistribution helps to limit hydraulic impairment of aspen located in different topographic positions during a drought event. This study employed an integrated ecohydrology model, TREES, combining plant-water balance and canopy physiology, to examine the potential effects of lateral flow on hydraulic and metabolic performance of aspen, by exposing trees to a set of soil water conditions associated with different levels of water stress. Sap flux, soil moisture, meteorological and plant hydraulic data from aspen trees in Colorado that died (SAD) and those that lived were used to parameterize the model. Our goal was to quantify the extent to which lateral flow explained sudden aspen dieback. The results indicate that the predisposition of tree mortality is related to the level of soil water augmentation. A reduction of 30% soil water content could introduce 21.55% increase in the loss of hydraulic conductivity (PLC), 23.6% loss in canopy transpiration, 21.7% loss in GPP. It would also cause the frequency of greater than 50% PLC to increase from 42.1% of the time to 51% of the time, and the frequency of hitting the 88% PLC pressure to increase from 11% to 14% of the time. On the other hand, an augment of 30% soil water content could introduce 20.2% reduction in PLC, 16.4% gain in canopy transpiration, 16.5% gain in GPP. The frequency of greater than 50% PLC is reduced to 31% of the time and the frequency of hitting the 88% PLC pressure is reduced to 6% of the time. This result suggests that water augmentation or reduction due to topographic control can be critical for survival or mortality of individual under extreme conditions. The result of this study has implications for coupling lateral flow with plant hydraulics and extending tree mortality simulations to landscape scale.