Influence of Environmental Factors and Preliminary Demographic Analyses of a Threatened Orchid, Platanthera praeclara

Abstract

I n 1987, 160 individual Platanthera praeclara Sheviak and Bowles were permanently marked on 16 transects on the Sheyenne National Grassland, in southeastern North Dakota. They were located on five sites that represented five management regimes: (1) grazed-rotational; (2) grazed-season long (5.5 mo); (3) ungrazed; (4) ungrazed and burned, and (5) grazed and burned. By 1994, only 4% of the originally marked Platanthera praeclara individuals were observed; however, numbers did not differ (P = 0.13) among sites. Beginning in 1990, all Platanthera praeclara individuals in belt transects on each site were marked and counted, and demographic data were recorded. Total orchid density differed (P = 0.04) among sites, but not with a consistent pattern among years (P = 0.001). In some years, Platanthera praeclara density was positively correlated with surface soil moisture. Preliminary demographic analyses indicate that Platanthera praeclara may be short-lived and that absent plants rarely reappear. Cluster analysis indicated that the most common vegetative community supporting Platanthera praeclara was dominated by Poa pratensis L. and Juncus balticus Willd., although Platanthera praeclara density on the Sheyenne National Grassland was most highly correlated with cover of Calamagrostis stricta (Timm.) Koel.