Longleaf pine (Pinus palustris) has a historically wide range across the southeastern US and grows across many habitats and soil types. The species exhibits a unique growth form as a seedling in which it typically does not grow a stem for 10 or more years. In this so called “grass-stage” the seedling grows root tissue and persists in the understory. Renewed interest in longleaf restoration, and how it compares to other species under changing climate, suggests a need to know current variation in physiological adjustments the seedlings make as we predict what might happen in the future. This study aims to understand physiological adjustments of longleaf pine seedlings on different soil types. Field work was conducted at the Jones Center at Ichauway, an ecological preserve located in southwestern Georgia, US, on naturally regenerated longleaf pine seedlings. Pre-dawn and mid-day leaf water potentials along with gas exchange were measured on seedlings occurring on two soil types of different drainage classes: well-drained sandy loam (Bonneau series) and excessively well-drained sand (Lakeland series). We hypothesized that seedling leaf water potentials would become more negative on less well-drained soils during water stress conditions due to the water-holding capacity of the soils.
Results/Conclusions
Seedling mid-day leaf water potentials did not drop below -1.6 MPa before droughty conditions developed. Slight differences between sites were expected based on the water-holding capacities of the soils and were reflected in the differences in mid-day water potentials at each site. Stomatal conductance was higher on sandy loam soils but both sites decreased over time as available soil water diminished. Seedlings under drought experienced significantly lower pre-dawn water potentials, approaching values seen during the mid-day under normal conditions (-1 to -1.5 MPa). Seedlings showed more negative water potentials for pre-dawn and mid-day on sandy soils than on sandy loam. Whole plant conductance decreased with decreasing pre-dawn water potentials across both sites and soil moisture conditions with the less well-drained soil class declining more steeply with decreasing water potentials. Future work will incorporate a third, intermediate soil drainage type and throughfall exclusion and trenching to push the range of soil moisture conditions beyond what we observed under natural climate variations.