Gas exchange characteristics and water relations of Larix occidentalis

Photosynthetic, transpirational, and stomatal responses to light, temperature, humidity, and plant moisture stress were measured for greenhouse-grown seedlings of Larixoccidentalis Nutt. Light saturation was 550 μmol photons•m −2 •s −1 ; light and CO 2 compensation points were 26 μmol photons•m −2 •s −1 and 59 μL CO 2 •L −1 , respectively. Light-saturated photosynthetic rate was over 7 μmol CO 2 •m −2 •s −1 with a temperature optimum between 18 and 23 °C. Photosynthesis, transpiration, and stomatal conductance to water vapor declined as xylem pressure potential decreased from −1.5 to less than −2.5 MPa; above −1.5 MPa no effect was observed. Stomatal conductance declined with increasing leaf to air vapor density difference. Stomatal conductance increased with increasing irradiance. Nighttime stomatal conductance was about 50% of the daytime maximum conductance regardless of xylem pressure potential. When plants were well watered, the ratio of xylem pressure potential to transpiration (XPP/E) decreased by 1.5 × 10 −3 MPa•(μg H 2 O•cm −2 •s −1 ) −1 with each mg H 2 O•cm −2 that had been transpired. After 7 days of drought, however, XPP/E decreased at 7.9 × 10 −3 MPa•(μg H 2 O•cm −2 •s −1 ) −1 per mg H 2 O•cm −2 . These characteristics are compared with other conifers and are used to suggest differences between growth patterns of L. occidentalis and sympatric species.