Characterization of Optimum Physiological Responses of Field-Grown Loblolly Pine
Photosynthetic photon flux density (PPFD), air temperature (Ta), needle net photosynthesis (Pn), vapor pressure difference (VPD), stomata1 conductance (gw), transpiration (E), and predawn and daytime xylem pressure potentials (XPP) were measured in a loblolly pine (Pinus taeda L.) plantation in 1995 and 1996. Boundary-line analyses were conducted to determine optirhum physiological responses and critical levels of Ta, PPFD, VPD, and XPP that restricted gas exchange. Our results showed that under field conditions, the maximum Pn, gw and E were 6.0 µmol m-2 s-1, 180 mmol m-2 s-1, and 3.5 mmol m-2 s-1, respectively. Irradiance less than 1100 µmol m-2 s-1 was the most significant variable limiting Pn. Threshold values of Ta, VPD and XPP that resulted in a decrease in Pn and gw were 33 °C, 1.3 KPa and -1.3 MPa, respectively. Our findings suggest that the boundary-line technique is an alternative for assessing and predicting responsive physiology of trees without artificial constraints on the environment.