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

Abstract

Net photosynthesis and stomatal conductance of the emergent macrophyte Juncus 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 water table fluctuations on photosynthesis and water losses. Half of the plots were in the areas where sediments remained saturated annually (stable water table) and the remaining plots were in the areas where water table heights fluctuated and sediments (upper 20 cm) did not remain continuously saturated. Annual patterns of average net photosynthesis were similar among all sample plots at the wetland ( p > 0.05; repeated measures ANOVA), which suggested that no differences in net photosynthesis resulted from the observed differences in sediment saturation. A marked seasonal pattern of net photosynthesis occurred with the greatest average photosynthesis during winter, summer (1997), and spring (11.0, 10.2, and 9.03 μmol CO 2 m −2 s −1, respectively) compared to autumn and summer (1996) (6.64 and 5.93 μmol CO 2 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 location or sediment saturation within the wetland. Seasonally, greater transpiration by J. effusus occurred during the summer (1996) and autumn months (0.43 and 0.28 mol H 2O m −2 s −1, respectively) compared to winter, spring, and summer (1997) (0.22, 0.20, and 0.16 mol H 2O m −2 s −1, respectively). Seasonal measurements of diel photosynthesis and stomatal conductance showed 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 higher transpirative water loss during evening hours compared to high light periods.