Waterlogging Effects on Soil and Vegetation in Mediterranean Forests Used for Runoff Attenuation

One proposed approach for reducing downstream flood hazards is the implementation of runoff attenuation facilities upstream. These facilities are often based on natural floodplains and may be located adjacent to agricultural lands, natural reserves, and forests. While their hydrologic benefits are evident, the effects of flooding frequency and duration on soils and vegetation are more difficult to predict and may be either beneficial or detrimental, depending on climate, soil properties, and land use. Our case study site is in Horshim Forest, a small Mediterranean forest intersected by Wadi Qana, a tributary of the Yarkon River, which flows through the densely populated Tel Aviv metropolitan area, Israel. The site is part of a planned network of upstream runoff attenuation facilities designed to mitigate downstream flood risk. To examine the impacts of waterlogging on soils and vegetation during active attenuation, we conducted a controlled flooding experiment in a forest plot dominated by 35-year-old Aleppo pine trees (Pinus halepensis). Mature pine trees were selected due to their abundance and the limited data on their flood tolerance. Flooded and control plots were continuously monitored using soil sensors measuring water content, oxygen concentration, and redox potential, along with tree sensors measuring stem diameter changes (dendrometers) and sap flow. Flooding events lasting 48–72 hours were applied during winter in the flooded plot, while the control plot remained under natural conditions. Plant performance indicators, including stem girth, needle length, and greenness index, were measured monthly. Waterlogging led to declines in soil oxygen and redox potential, in some depths reaching anoxic conditions, with responses dependent on flood duration and antecedent soil moisture. In contrast, the control plot remained under oxic to suboxic conditions. Tree responses were variable and appeared to depend on pre-flood soil water availability and the timing of flooding during the winter season. Our study demonstrates how short-term flooding alters soil aeration conditions and tree responses, with implications for the ecohydrologic design of runoff attenuation facilities.