Species: Rorippa columbiae

RORIPPA COLUMBIAE is perennial. The plants spread by rhizomes (Stuckey, 1972) and possibly by rooting at the nodes of above-ground stems (Pam Camp, personal communication). The plant is often found growing in clusters of above-ground stems (Gehring 1991, 1992; Kaye and Massey 1991; Pam Camp, personal communication; Lois Kemp, personal communication; Ginger King, personal communication; Nora Taylor, personal communication). Presumably these clusters are clones and represent only one genotype. The clusters range in size and extent of areal coverage from thousands of stems covering large areas in dense colonies to one or only a few above-ground stems. <br><br>Growth of RORIPPA COLUMBIAE is largely determined by water availability. On Pierce Island, RORIPPA COLUMBIAE population vigor and size has decreased between 1984 and 1990/1991 on Pierce Island (Lynn Cornelius, personal communication). This trend may be due to weather patterns, since the early 1980's were high runoff years while the late 1980's were low runoff years (Scherer 1991). However, annual growth can be decreased by too much water as well. On Pierce Island, plants growing at lower elevations begin seasonal growth later than those higher on the island, produce few flowers or do not flower at all, mature few if any seeds, and are probably dependent on vegetative spread for population maintenance in most years (Gehring 1990, 1991). At Featherbed and Foley Lakes in south-central Oregon, population frequency has been correlated with annual precipitation since 1983 (Kaye and Massey 1991). In 1991, a large snowstorm occurred in late spring presumably causing population size and density at Featherbed and Foley Lakes to increase; frequency in monitored plots increased from 0.25% in 1990 to 43.3% in 1991 (Ginger King, personal communication). <br><br>Most populations found on the Columbia River below Bonneville Dam are relatively small, usually having fewer than 500 individuals but sometimes having only one or two plants. The number of stems in populations of RORIPPA COLUMBIAE on Pierce Island ranges from 50 to 500. At Hamilton Creek in 1992, there were 300-500 stems of RORIPPA COLUMBIAE (Gehring, personal observation). The Sandy River population had approximately 50 clusters of RORIPPA COLUMBIAE stems in May 1992 (Oregon Natural Heritage Program Data Base). At Sandy River there was 5-10% total vegetation cover on 13 May 1992. <br><br>The largest known populations of RORIPPA COLUMBIAE are located along the Hanford Reach of the Columbia River, between Vernita and Richland, Washington. The species is abundant along this stretch of the river (Sauer and Leder 1985; Camp 1992; Gehring, personal observation). Population size on the Hanford Reach is probably on the order of magnitude of millions of stems over approximately 40 river miles (Gehring, personal observation). <br><br>At Malheur Lake, (Harney County Oregon), reported population sizes vary considerably. Kaye and Massey (1991) reported five groups of plants covering an area over 3000 m2 in 1990. Nora Taylor (personal communication, 1992) described one large group of more than 500 stems and two smaller groups of plants. The records in the Oregon Natural Heritage Program Data Base indicate two large populations with more than 1000 stems each in 1991. Population size is apparently varying from year to year, probably depending on soil moisture. <br><br>Population sizes at most of the other Oregon populations are generally small. Approximately 200 stems were growing on the south and east shoreline of Featherbed Lake and in the lake bed near the north shore in 1990 (Kaye and Massey 1991). At Foley Lake, approximately 200 stems of RORIPPA COLUMBIAE were observed growing in the lake bed and in a nearby water hole in 1990 (Kaye and Massey 1991; Ginger King, personal communication). At Binkey Lake in 1983, there were one or two stems of RORIPPA COLUMBIAE growing in sand on the lake shore (Oregon Natural Heritage Program Data Base). At Silver Lake, there were 300 to 400 stems along the southeast shore in 1985 (Oregon Natural Heritage Program Data Base). The two RORIPPA COLUMBIAE populations at Paulina Marsh had approximately 30 stems and 60-80 stems (Oregon Natural Heritage Program Data Base). However, the plants at Paulina Marsh are reported to grow quite large; over two feet in diameter (Oregon Natural Heritage Program Data Base; Ginger King, personal communication). On Stuckel Mountain, the RORIPPA COLUMBIAE population was estimated to have between 100 to 125 plants in 1983 (Oregon Natural Heritage Program Data Base). <br><br>In California, the RORIPPA COLUMBIAE population at White Deer Lake was estimated to have between 51-100 plants in 1987 (California Natural Diversity Data Base). By 1991, Julie Nelson (personal communication) estimated there were only 30 to 50 plants. However, some of the plants flowered in 1991 (Julie Nelson, personal communication). At Clear Lake Reservoir in 1987, one population was estimated to have over 1000 plants (California Natural Diversity Data Base). In 1992, the plants averaged 10 cm in height when growing without interspecific competition and reached 2 dm when growing with other species. Some plants at Clear Lake were flowering by mid-June, 1992 (Mary Paezel, personal communication). <br><br>The timing of initiation of annual above-ground growth of RORIPPA COLUMBIAE differs between the populations on the Columbia River and in southern Oregon and northern California. Annual growth usually begins in April or May in southern Oregon and northern California, when melting snow begins to provide a source of water for vernal lakes and intermittent streams. In contrast, the Columbia River usually does not drop below the elevation level of most populations until late July or August (Gehring, 1990, 1991) and sometimes as late as September (Sauer and Leder, 1985; Pam Camp, personal communication) so the RORIPPA COLUMBIAE populations do not initiate growth until late summer or even early fall. <br><br>Based on herbarium samples, Sauer and Leder (1985) suggested three distinct morphs of RORIPPA COLUMBIAE may have evolved: (1) decumbent, trailing stems less than 30 cm long on the Columbia River; (2) erect, freely branching, spherically shaped, 30 cm in diameter and height on the lava beds of northern California; (3) stems of intermediate size and habit at the south-central Oregon sites. However, it is more likely that morphology is dependent on water availability and regime. For example, plants with little or no stem development have been observed both along the Columbia River (Pam Camp, personal communication) and on lake margins in south-central Oregon (Ginger King, personal communication). Additionally, decumbent, trailing stems have been observed on the Columbia River (Gehring, personal observation) and at Malheur Lake (Nora Taylor, personal communication) while relatively large, erect plants have been collected or observed at Lava Beds National Monument (Sauer and Leder 1982), in roadside ditches in south-central Oregon Ginger King, personal communication), and along the Columbia River (Sackschewsky, personal communication; Gehring, personal observation). One would expect that plants with an abundant water supply (e.g., high water table) and a long growing season could grow larger and branch more than plants which are inundated for much of the growing season or are limited in growth by water availability. <br><br>In summary, the distribution of RORIPPA COLUMBIAE indicates that the species requires moist or perhaps wet soil. However, it also seems apparent that annual growth can be decreased or eliminated by too much water during crucial portions of the growing season. The most favorable growing conditions and habitat appear to include spring flooding and a relatively high water table during the remaining portion of the growing season. However, there is no evidence to indicate that actual submergence of the plants is necessary in every habitat or in every year, as long as the soil is wet. A crucial aspect for the growth and long-term survival of RORIPPA COLUMBIAE may be the timing and/or duration of the spring flood, especially if flowering is sensitive to photoperiod.