PS 36-193 - No evidence of hydraulic lift in a longleaf pine (Pinus palustris) savanna during an abnormally wet year

Background/Question/Methods

Hydraulic lift has been documented in many ecosystems, including longleaf pine sandhills in the southeastern United States. Nocturnal hydraulic lift can account for nearly a third of the daily water transpired by longleaf pine trees. Understory plants in longleaf pine sandhills may take up a portion of the water lifted by the pines, but the effects of hydraulic lift on understory species remain unknown. The objective of this study was to determine if, and to what extent, hydraulic lift affects the physiology and growth of understory plants in longleaf pine sandhills. We targeted four species from differing functional groups throughout the 2021 growing season. We compared predawn and midday water potentials and gas exchange between plant individuals in trenched plots designed to exclude pine roots and individuals in untrenched control plots. We also tracked the growth of each individual plant over the course of the growing season. Soil moisture probes installed in each plot collected volumetric water content at depths of 10 and 50 cm throughout the study.  

Results/Conclusions

Based on the soil moisture measurements, we did not find evidence of hydraulic lift occurring during the 2021 growing season. Conditions were unusually wet, with rainfall 53% above normal from July-November. Comparing plants in pine root exclusion and control plots, we found no difference in plant stress or performance based on gas exchange, water potential, or growth. In fact, preliminary results suggest that plant final biomass was greater in the root exclusion plots than in the control plots for certain species, pointing to competition between the pines and some understory plants when soil water is abundant. These data, combined with the evidence of hydraulic lift from previous years in the same ecosystem, suggests that this process may occur patchily - both spatially and temporally – in these ecosystems. Hydraulic lift may not occur at all in wetter years, even in these excessively well-drained, sandy soils. Thus, the amount and relative importance of hydraulic lift for understory species may depend heavily on growing season precipitation.