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

Abstract

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.

Salt Marsh Light Use Efficiency is Driven by Environmental Gradients and Species‐Specific Physiology and Morphology

“…Differences in photosynthetic pathways, J. roemerianus, a C3 species, and S. alterniflora, a C4 species, may cause species‐specific sensitivities to temperature gradients in marshes where these species typically grow in monoculture. Measurements of net photosynthesis have shown optimum temperatures for S. alterniflora between 30°C and 35°C (Giurgevich & Dunn, 1979) and <30°C for J. roemerianus (Giurgevich & Dunn, 1978). Ishtiaq and Abdul‐Aziz (2020) found optimal soil temperatures for S. alterniflora LUE (NEE/PAR) above 17°C in gas chamber experiments.…”

Carbon cycling in a tidal freshwater marsh ecosystem:a carbon gas flux study

“…As an alternative to harvest methods, fluxes of CO2 and CH, have been used to estimate gross and net macrophyte productivity under ambient field conditions (Blum et al 1978, Giurgevich & Dunn 1978, Howes et al 1984 or under experimental conditions such as elevated atmospheric CO2 concentrations (Drake 1984, Curtis et al 1989, Azc6n-Bieto et al 1994 or manipulated soil salinities (Pezeshki 1991, Hwang & Morris 1994. The carbon gas flux technique integrates processes that occur within and between aboveground and belowground compartments (e.g.…”

Biological Flora of Coastal Mid- and High Marshes: Juncus roemerianus Scheele

“…Juncus roemerianus is a C 3 species in carbon fixation in light-dependent reactions of photosynthesis (Haines and Hanson, 1979). Giurgevich and Dunn (1978) reported that net photosynthesis is highest in spring and only slightly declines through the year with decreasing rates above 30° C. Transpiration is relatively high throughout the year with higher respiration and is accompanied by reduced photosynthesis at higher temperatures (Giurgevich and Dunn, 1978). They concluded that unique physiological characteristics permit high levels of productivity in hot, salt-stressed environments.…”

“…Productivity in coastal marshes is high with biomass usually greater in warm temperate regions of the Atlantic and Gulf Coast. Juncus roemerianus is noted for its high productivity in hot stressful environments (Giurgevich and Dunn, 1978). Aboveground biomass is most often recorded because of the relative ease of data collection.…”