Vascular Plants and Bryophytes

To survive in fast-flowing water, vascular plants must have adventitious roots, rhizomes (stolons), flexible stems, and streamlined narrow leaves (Westlake 1975). Podostemum ceratophyllum Michx. exemplifies the morphology necessary to occupy high-gradient streams. Attaching to rocks with disk-like processes and giving rise to linearly divided leaves, Podostemum is usually found in clear streams with good aeration. Although Podostemum is seldom reported since it occurs in swift, usually "white water" (Fassett 1966), Meijer (1975) documented its broad distribution in the southern Appalachian Mountains and suggested that it is an indicator of clean streams in the region. Podostemum is the dominant macrophyte in the New River and contributes significantly to the rivers organic matter budget (Hill and Webster 1984), generally entering the food chain as an autumnal pulse of rapidly decomposed detritus (Hill and Webster 1982, Hill and Webster 1983).

The water willow, Justicia americana (L.) Vahl., is also an important macrophyte of southeastern streams. Unlike Podostemum, Justicia roots in sediments (Schmalzer and others 1985), and is absent from the more turbulent habitats. However, Justicia is the dominant emergent plant in the New River, contributing 12 percent of the aquatic macrophyte biomass (Hill 1981).

Mosses and liverworts are the dominant macrophytes in environments with the highest turbulent flows (Westlake 1975). This situation may be partially due to the fact that bryophytes are able to use free C0₂ as a carbon source (Gessner 1959) and turbulent water insures CO₂ saturation. Messer and others (1986) and Kaufmann and others (1988) reported that most small streams in the southern Blue Ridge are probably supersaturated with C0₂. Substrate stability is probably another factor influencing local moss distribution, since mosses tend to be most abundant on bedrock and large boulder substrates (Gurtz and Wallace 1984, Huryn and Wallace 1987). Gilme (1968) surveyed the bryophyte flora of high-gradient Appalachian streams extensively and found that four bryophytes dominate. Fontinalis dalecarlica is the most ubiquitous aquatic moss, occurring in first- to third-order streams in depths of 10-100 cm. The gametophyte typically forms mats with "streamers." Hygroamblystegietum fluviatile dominates relatively shallow, first- and second-order streams, forming thick mats on submerged rocks, but may occur as a subdominant in larger streams. The distributions of two other potentially dominant bryophytes, Sciaromium lescurii and Scapania undulata, are poorly known (Glime 1968). An aquatic species of Fissidens, a largely terrestrial moss genus, seems to occur in habitats with high levels of NO₃ and PO₄ as well as C0₂, such as those in Doe Run in Kentucky (Minckley 1963).