Juncus

Abstract: Summary 1 We investigated the factors producing zonation patterns of the dominant plants in south-eastern USA salt marshes where Juncus roemerianus dominates the high marsh, and Spartina alterniflora the middle and low marsh. 2 Juncus did not occur naturally in the Spartina zone and performed poorly when transplanted there, irrespective of whether neighbours were present or removed, indicating that its lower limit was set by physical stress. 3 In contrast, although Spartina occurred naturally at low densities in the Juncus zone, it performed well if transplanted there only if neighbours were removed, indicating that its upper limit was set by competition. 4 Parallel laboratory and field manipulations of flooding, salinity and competition indicated that the lower limit of Juncus was mediated by both flooding and salinity, but not by competition. 5 The general mechanisms producing zonation patterns of vegetation in coastal salt marshes may be universal, as suggested by previous studies, but the importance of particular factors is likely to vary geographically. In particular, salinity stress probably plays a much more important role in mediating plant zonation patterns at lower latitudes. 6 Our results suggest that the nature of ecological interactions is likely to vary geographically because of variation in the physical environment, and this variation must be taken into account in order to successfully generalize the results of field studies across geographical scales.

Abstract: Species composition affects the carbon turnover and the formation and emission of the greenhouse gas methane (CH4) in wetlands. Here we investigate the individual effects of vascular plant species on the carbon cycling in a wetland ecosystem. We used a novel combination of laboratory methods and controlled environment facilities and studied three different vascular plant species (Eriophorum vaginatum, Carex rostrata and Juncus effusus) collected from the same wetland in southern Sweden. We found distinct differences in the functioning of these wetland sedges in terms of their effects on carbon dioxide (CO2) and CH4 fluxes, bubble emission of CH4, decomposition of C-14-labelled acetate into (CH4)-C-14 and (CO2)-C-14, rhizospheric oxidation of CH4 to CO2 and stimulation of methanogenesis through root exudation of substrate ( e. g., acetate). The results show that the emission of CH4 from peat - plant monoliths was highest when the vegetation was dominated by Carex (6.76 mg CH4 m(-2) h(-1)) than when it was dominated by Eriophorum (2.38 mg CH4 m(-2) h(-1)) or Juncus (2.68 mg CH4 m(-2) h(-1)). Furthermore, the CH4 emission seemed controlled primarily by the degree of rhizospheric CH4 oxidation which was between 20 and 40% for Carex but > 90% for both the other species. Our results point toward a direct and very important linkage between the plant species composition and the functioning of wetland ecosystems and indicate that changes in the species composition may alter important processes relating to controls of and interactions between greenhouse gas fluxes with significant implications for feedback mechanisms in a changing climate as a result. (Less)

Abstract: Carbon, sulfur, and nitrogen stable isotope ratios were measured in an irregularly flooded Mississippi, USA, salt marsh to evaluate the relative importance of vascular plants and algae in the food web. Primary producers had l s t inc t 6I3c values: Spartina alterniflora (hereafter Spartina) (-13%), Juncus roemerianus (hereafter Juncus) (-26Y~), and edaphic algae (-21 %o). A pure zooplankton sample, which should closely approximate the phytoplankton, had a 6I3C value of 2 3 % ~ . Values for 6 3 4 ~ ranged from 0 to +2 7~ for vascular plants, while those for edaphic algae and zooplankton were + 14 and + l 1 %I, respectively; b I 5 ~ for all primary producers ranged from +S to +6%. For 49 of the 56 (88 %) consumers sampled, 613C fell w i t h a range of -22 to -18 %o; this range centered around edaphic algae and zooplankton, but was dstlnct from the b13C of Spartina and Juncus. Values of b 3 4 ~ for 48 of the 56 (86 %) estuarine consumers ranged from +9 to +16%0, which included edaphic algae and zooplankton but was 8 to 15 36o more enriched in 3 4 ~ than Spartina or Juncus. Dual isotope plots showed the fauna to tightly cluster around values for edaphic algae and zooplankton; no consumer even moderately resembled Spartina or Juncus. This strongly suggests that the ultimate food sources for this marsh's invertebrate and fish fauna are the benthic and planktonic algae; direct contributions from vascular plants appear to be minor.