INFLUENCE OF VAPOR-PRESSURE DEFICIT AND SOIL-MOISTURE ON GAS-EXCHANGEOF JUNIPERUS-OCCIDENTALIS

An unusual combination of meteorological events allowed comparison of gas exchange for juvenile and adult Juniperus occidentalis under contr asting conditions of soil moisture and vapor pressure during August 19 87 and August 1988. Responses of carbon dioxide assimilation, leaf con ductance, transpiration, and intercellular carbon dioxide concentratio ns to changes in photosynthetic photon flux density, leaf temperature, and xylem water potential were measured for juvenile and adult trees. Carbon dioxide assimilation was significantly higher for juveniles wi th high soil water. Adult assimilation and conductance for both juveni les and adults were not significantly different in 1987 and 1988. Simi lar diurnal curves of leaf conductance in 1987 and 1988 were associate d with different amounts of transpiration because of high vapor pressu re deficits in 1987 and low soil water in 1988. Although stomata of ju venile J. occidentalis were more responsive than adult stomata, juveni les did not restrict transpiration when vapor pressure deficits were h igh; adults had higher water-use-efficiencies. With vapor pressure def icits > 3.5 kPa, fluctuations in leaf conductance of juvenile J. occid entalis indicated stomatal closure and reopening in approximately 20 m inute cycles from 1000 to 1630 hr. Increases in air temperatures and v apor pressure deficits can be expected to have less effects on physiol ogical processes of J. occidentalis than would variations in available soil moisture.