Water use of co-occurring loblolly (Pinus taeda) and shortleaf (Pinus echinata) in a loblolly pine plantation in the Piedmont

Measuring water use in co-occurring loblolly pine (Pinus taeda L.) and shortleaf pine (Pinus echinata Mill.) enhances our understanding of their competitive water use and aids in refining watershed water budget model parameters. This study was conducted in a 12-ha forested headwater catchment in the Piedmont of North Carolina, southeastern U.S., from 2018 to 2019 (pre-thinning) to 2020 (post-thinning). Sap flux density (J(s)), species-level transpiration (T-s), and watershed-level transpiration (T-w) were quantified. Water use efficiency (WUE) in loblolly and shortleaf pines was compared, alongside an investigation into how both species' J(s) and T-s responded to atmospheric vapor pressure deficit (VPD). Loblolly pine had 19%-36% higher Js than shortleaf pine. Daily T-s for loblolly pine ranged from 15.0 to 29.0 L/day while T-s in shortleaf pine ranged from 3.0 to 6.8 L/day. The T-s was significantly higher in loblolly pine when compared to shortleaf pine likely due to higher canopy position and higher growth rates of the former. WUE, defined by annual tree biomass growth per tree water use, was not significantly different between the two. Daily J(s) and T-s in both species responded nonlinearly to VPD, with loblolly pine being more sensitive and variable. Species-specific water use should be considered when quantifying T-w and developing reliable models to predict the effects of forest management practices on water resources.