SEASONAL PATTERNS OF CO2 AND WATER VAPOR EXCHANGE OF JUNCUS ROEMERIANUS SCHEELE IN A GEORGIA SALT MARSH

CO2 and water vapor exchange studies of intact plants of black needle rush (Juncus roemerianus Scheele) were conducted in an undisturbed marsh community on Sapelo Island, Georgia. The seasonal patterns of the light and temperature responses of net photosynthesis, transpiration, leaf diffusive conductance, water-use efficiency and respiration were determined five times over the year. Internal resistances to CO2 uptake were also evaluated. Net photosynthesis was highest in early spring, but declined only slightly through the year. A distinct and moderate temperature optimum of net photosynthesis was observed with decreasing rates above 30 C. Leaf conductances to water vapor were similar at all seasons and were high at cooler temperatures and decreased with increasing temperature. Transpiration was relatively high and constant during all seasons. The water-use efficiency of photosynthesis was high below 25 C, but decreased sharply above that temperature. Dark respiration was relatively low. Seasonal changes reflected changes in leaf density. Decreasing stomatal conductances and increasing respiration rates reduced net photosynthesis at higher temperatures. The stomatal resistance increased and internal resistances to CO2 uptake decreased over the year, but the total resistance remained constant. The internal resistance to CO2 uptake was consistently higher than the stomatal resistance. These seasonal response patterns show that J. roemerianus is well adapted to the seasonal changes in ambient temperature and irradiance and other microenvironmental factors in the high marsh. These physiological characteristics permit this C3 species to maintain a high productivity in a seasonally hot and stressful environment. THE SALT MARSHES of the Atlantic and Gulf coasts of the U.S. are among the most highly productive natural ecosystems (Odum, 1961; Whittaker, 1975). These marshes are characterized by an assemblage of few species of vascular plants in distinct zonation patterns (e.g., Cooper, 1974). Juncus roemerianus Scheele is an important vascular plant species in salt marshes from New Jersey to north Florida on the Atlantic coast and from south Florida to Texas on the Gulf coast (Cooper, 1974; Eleuterius, 1976a). In the Georgia marshes, J. roemerianus generally dominates the upper or high marsh regions which are not flooded with each tidal cycle. There are several estimates of the productivity of J. roemerianus (e.g., Keefe, 1972; Williams and Murdoch, 1972; Gabriel and de la Cruz, 1974), and the high productivity of this species in the Georgia marshes (2,156 g DW m-2 yr-t) may contribute significantly to the overall productivity of the 1 Received for publication 5 July 1977; revision accepted 6 March 1978. Research was supported in part by the National Sea Grant Program, U.S. Dept. of Commerce, Grant No. 04-5-158-4, and by the National Science Foundation, Grant No. DEB7706632. The assistance of A. E. Antlfinger, Dr. J. L. Gallagher, and the staff of the University of Georgia Marine Institute is gratefully acknowledged. Contribution No. 357 from the University of Georgia Marine Institute, Sapelo Island, Georgia 31327. marsh ecosystem of Georgia (Gallagher et al., 1978). However, there is little information concerning the physiological ecology of this species. Gallagher (1975) applied a single pulse of ammonium nitrate to J. roemerianus in a Georgia salt marsh and found no significant changes in live or dead aerial biomass or elemental composition either 10 weeks or 1 year after treatment. He suggested thatJ. roemerianus was growing near the asymptote of its nitrogen response curve. Rozema (1976) studied the responses of four European Juncus species to a range of salt concentrations in solution culture and found differences in the response to salinization as well as different physiological mechanisms by which the adaptations were achieved. This study was designed to begin investigations on mechanistic explanations for the variations in productivity and distribution of J. roemerianus along environmental gradients in the southeastern salt marshes. The seasonal temperature and light responses of photosynthesis, respiration, transpiration, leaf conductance, CO2 diffusion resistance and water-use efficiency measured on in situ J. roemerianus plants were investigated in this study. The daily patterns of photosynthesis and transpiration and estimates of productivity extrapolated from these gas ex-