Physiological effects of mature tree transplanting characterize the roles of the soil-root interface in the field

Abstract The intimate connection between roots and soil particles is a prerequisite for the continuous flow of water and nutrients from soil to trees, and carbon flow from roots to the rhizosphere. The soil-root interface has been studied in multiple laboratory and greenhouse experiments. Yet its multiple roles and their differential contributions to tree health have rarely been experimentally studied on mature trees in the field. We took advantage of mature olive tree transplanting to test the physiological effects of breakage of the soil-root interface in situ. Eight olive trees were monitored following transplantation into a site located 4 km from their native site, along two years. Additional eight trees were monitored simultaneously at the native site, as control. To decrease mortality risk, transplanted trees were heavily pruned before transplanting, and were irrigated and fertilized in their new site. Transplanted trees had ~50% lower rates of leaf photosynthesis and transpiration; ~80% lower root starch content; and ~30% higher loss of xylem conductivity, than native trees. Leaf water potential (LWP) was similar across trees, becoming more negative in the transplanted trees only in the second year following transplanting. While starch content and xylem conductivity recovered in the second year, leaf gas exchange and LWP remained significantly lower than in native trees. Soil P and K were higher under transplanted trees that remained stressed than under trees that recovered. Breakage of the soil-root interface caused a multi-system stress to trees. The lack of persistent LWP response might indicate that loss of hydraulic conductivity was driven by root, rather than aboveground, embolisms. Degradation of starch in the roots indicates an increase in belowground sinks. In the long run, recovery of starch content means no carbon limitation; yet the prevailing effects on leaf activities suggest a long-term stress unrelated with water or carbon supply.