Disjunct Populations of Isoetes macrospora in Southeastern Tennessee

Abstract

macrospora Dur. is a northern species widely distributed throughout eastern Canada and the northeastern United States west to Minnesota (Pfeiffer, 1922; Wherry, 1961). Recently, populations of this species were found in the Little Tennessee and Hiwassee Rivers of southeastern Tennessee. The only other report of this species from the southeastern United States is that of Svenson and Griscom (1935), who cite it from northern Virginia. This report was accepted by Massey (1944, 1960) and by Harvill, Stevens, and Ware (1977) for the Virginia Flora, but was not mentioned by either Fernald (1950) or by Gleason and Cronquist (1963). There had been some confusion concerning the county of this Virginia collection. The voucher located in the Brooklyn Botanical Garden Herbarium is labeled Isoetes macrospora Dur., Shallow Water. Passage Creek. Macrospores Average 650 /. Frederic Co., VA. L. Griscom and H. K. Svenson 5561, July 2, 1933. Massey (1944) cited the collection as being from Passage Creek in Shenandoah County. This is verified by C. E. Stevens (pers. comm), who corresponded with Svenson, and indicated that Svenson still recalls the locations along Passage Creek as being in Shenandoah County. Recently, this location was examined by A. M. Evans, and plants of . macrospora were found to occur frequently in the shallow, cobble-bottomed pools of the Elizabeth Furnace area. Both the 1935 and 1978 collections from Passage Creek and the recent collections from the Little Tennessee and Hiwassee Rivers represent significant disjunctions from the glaciated, northern, contiguous range of I. macrospora to the unglaciated Ridge and Valley province of Virginia and Tennessee. The Passage Creek population is ca. 300 miles from the northern range, and the Tennessee populations are ca. 450 miles from the Virginia population. In the Little Tennessee and Hiwassee Rivers, the known populations of I. macrospora are limited to sections of river between upstream dams and impounded waters of mainstream reservoirs of the Tennessee River. The habitat is essentially riverine and consists of an alternating series of pool and riffle areas. Water depth is generally shallow (<2 m), but varies depending on releases from upstream dams. These releases also help to maintain a cool water temperature throughout the year (15 to 19? C in July, 1978). Individual plants ofI. macrospora were found rooted in sand-filled crevices of the rocky, cobble substrate in the moderateto fast-flowing water of pool areas that are 0.1 to 1.5 m deep. Plants growing in areas of weak currents have slightly erect to recurved leaves which spread from the corm in a symmetrical ring. In areas of strong current, the leaves are prominently recurved and fixed by the downstream current, giving the plants a striking, swept-back aspect.