Photosynthesis and stomatal conductance of Juncus effusus in a temperate wetland ecosystem

Net photosynthesis and stomatal conductance of the emergent macrophyte Junc us effusus L. was measured in six plots in the Talladega Wetland Ecosystem (Hale County, AL) over diel and annual periods to evaluate the effects of w ater table fluctuations on photosynthesis and water losses. Half of the plo ts were in the areas where sediments remained saturated annually (stable wa ter table) and the remaining plots were in the areas where water table heig hts fluctuated and sediments (upper 20 cm) did not remain continuously satu rated. Annual patterns of average net photosynthesis were similar among all sample plots at the wetland (p > 0.05; repeated measures ANOVA), which sug gested that no differences in net photosynthesis resulted from the observed differences in sediment saturation. A marked seasonal pattern of net photo synthesis occurred with the greatest average photosynthesis during winter, summer (1997), and spring (11.0, 10.2, and 9.03 mu mol CO2 m(-2) s(-1), res pectively) compared to autumn and summer (1996) (6.64 and 5.93 mu mol CO2 m (-2) s(-1), respectively). Average apparent photosynthetic efficiency of J. effusus was 1.43% and varied seasonally with the highest values in winter (1.86%) followed by spring (1.69%), autumn (1.34%), and summer (0.80 and 1. 13%; 1996 and 1997, respectively). Although significant differences (p < 0. 05) in stomatal conductance occurred among sample plots within the wetland, no correlation was found between stomatal conductance and sample plot loca tion or sediment saturation within the wetland. Seasonally, greater transpi ration by J. effusus occurred during the summer (1996) and autumn months (0 .43 and 0.28 mol H2O m(-2) s(-1), respectively) compared to winter, spring, and summer (1997) (0.22, 0.20, and 0.16 mol H2O m(-2) s(-1), respectively) . Seasonal measurements of diel photosynthesis and stomatal conductance sho wed that daily net photosynthesis was variable among sample plots and among the seasons at the wetland, with maximum photosynthesis from 1000 to 1400 hours. Diel measurements of stomatal conductance showed significantly highe r transpirative water loss during evening hours compared to high light peri ods. (C) 1999 Elsevier Science B.V. All rights reserved.