Longleaf pine (Pinus palustris) seedlings show possible decoupling from water in upper soil layers during in situ artificial drought

Background/Question/Methods

Longleaf pine (Pinus palustris) is the dominant tree in the now rare woodland savanna ecosystem of the southeastern US. It is well-adapted to frequent low-intensity fires which maintain low tree density and a rich groundcover community. While there is a wealth of knowledge around burning to maintain these ecosystems, there are many unanswered questions about longleaf physiology and ecology. Physiology data are especially lacking for seedlings outside a nursery or greenhouse setting. This is an important distinction, as longleaf exhibits a morphologically distinct “grass-stage” seedling in natural settings. The effects of episodic drought on grass-stage seedlings are unclear. Our objective was to determine what level of hydraulic isolation impacts longleaf pine seedling physiological processes? We hypothesized that seedlings where water inputs from rainfall and hydraulic lift were eliminated would show lower leaf water potentials and transpiration rates than controls. Rain-exclusion shelters were installed in situ, in July 2021 and root-exclusion trenches were dug in half of sheltered and unsheltered plots. Soil moisture probes were installed on all plots at 15cm and 50cm depth. Physiological data were collected throughout the summer and fall of 2021. Root collar diameter measurements were taken before treatment and in November 2021.

Results/Conclusions

Volumetric water content (VWC) on sheltered plots in these excessively well-drained sandy soils decreased to about 3% at both depths in August 2021, about a month after shelters were installed. VWC in unsheltered plots fluctuated with rainfall but reached highs of over 15% and never decreased below 5% for both depths. Trenching did not cause a significant change in water availability with or without shelters. Gas exchange data are not significantly different between plots, but follow, in parallel, a diurnal and then seasonal pattern. Root collar diameters did not change in sheltered plots but increased in non-sheltered plots (P< 0.1) over the measurement period. Turgor loss points (TLP) decreased as the season changed from summer to fall in sheltered and open plots in tandem. These results suggest that seedlings may have access to deeper water and are decoupled from soil water down to at least 50 centimeters on this site. With climate change threatening water availability and threatening flash droughts, these seedlings might be resilient to these types of events when naturally regenerated in a longleaf pine ecosystem.