PHOTOSYNTHETIC CHARACTERISTICS AND POTENTIAL MOISTURE STRESS FOR THE ACTINORHIZAL SHRUB, MYRICA CERIFERA (MYRICACEAE), ON A VIRGINIA BARRIER ISLAND

Seasonal variations in photosynthesis and water relations parameters were quantified for Myrica cerifera, the dominant woody species on the barrier islands along the eastern shore of Virginia. From June through September of 1989, maximum values were 35 μmol m−‐2 sec−‐1 for net CO2 assimilation, 10.5 mm sec−‐1 for stomatal conductance to water vapor diffusion, and –0.3 MPa for xylem pressure potential at the field site on Hog Island. Midday minimum xylem pressure potential often was less than –1.5 MPa. Data from the field and measurements on glasshouse plants indicated that stomatal opening and photosynthesis were sensitive to leaf water potential (<–0.8 MPa) and the leaf‐to‐air humidity deficit (>1.5 kPa). Using meteorological data and derived photosynthetic responses, predictions indicated that M. cerifera photosynthesis would have been limited at the field site due to nonoptimal air temperatures and humidity deficits on at least 90% of the days during the relatively wet summer of 1989. By comparison, these parameters were expected to limit photosynthesis on all but 2 d, or more than 98% of the time during the relatively dry summer of 1990. The sensitivity of Myrica cerifera to atmospheric humidity and plant moisture status may explain the distributional preference for the more mesic swale sites of barrier islands.