There is an increasing interest in the restoration of longleaf pine throughout its historical range in the southeastern United States. It is widely assumed that longleaf pine is a drought resistance species, but whether drought resistance differs from other southern pine species is unclear, especially since seedlings may exist in a grass-stage for many years. An important component of drought resistance in tree seedlings is the degree of xylem water vulnerability to cavitation. Hydraulic failure due to xylem embolism during drought can result in high seedling mortality. Our objective was to examine hydraulic vulnerability of roots of longleaf pine seedlings representing six different seed sources from across its native range. Between September and October 2015, a modified Sperry Apparatus was used to generate root hydraulic vulnerability curves and determine pressures corresponding to 12, 50, and 88 (P12, P50, and P88) percent loss of conductivity in lateral roots of one-year-old seedlings planted in an outdoor research facility. Leaf predawn and midday xylem water potential and midday leaf transpiration were measured over three days in October 2015 to calculate leaf-specific hydraulic conductance (LSC).
Results/Conclusions
Seed source did not have a significant effect on root hydraulics and LSC. Average P12, P50 and P88 was -1.5, -2.2, and -2.8 MPa, respectively and mean LSC was 2.7 mmol m-2 s-1 MPa-1. Root P50 ranged from -2.01 MPa in the Georgia seed source to -2.3 MPa in a Louisiana seed source. Leaf-specific hydraulic conductance ranged from 1.0 to 5.7 mmol m-2 s-1 MPa-1 and declined nonlinearly with decreasing midday water potential. Interesting, 50% loss of LSC occurred near root P50, indicating that root embolism has significant control on grass-stage whole plant hydraulics. Mean P50 of longleaf pine seedlings was lower than reports for mature longleaf and slash pine trees (-1.3 MPa) but similar to ranges reported for mature loblolly pine (-1.0 to -2.3 MPa), mature ponderosa pine (-1.8 to -2.2 MPa) and mature pinyon pine (-1.4 to -2.9 MPa). Average minimum leaf water potential was -1.6 MPa. The hydraulic safety margin was 0.5 MPa at P50 and 1.2 MPa at P88 and margins were comparable with other pine species. These results also suggest little plasticity in hydraulic architecture and integrated traits such as P50in longleaf pine.