The Origin of Populus deltoides and Salix interior zones on Point Bars along the Minnesota River

An investigation was made to ascertain the nature and origin of the separate and distinctive zones of Populus deltoides Marsh, and Salix interior Rowlee commonly occurring on point bars along the Minnesota River. These vegetation zones are primarily monospecific, have high stem densities, possess well-defined boundaries and are restricted to specific contour intervals on the point bars. A method was devised to determine past river elevations at ungauged locations between two gauging stations along the lower Minnesota River. Comparisons of calculated river elevations with sur- veyed vegetation-zone elevations have illustrated two mechanisms by which the zones originate. During years of zone establishment the river elevation remained fixed for periods ranging from 13-22 days near the contour line marking the lower boundary of the zones, thereby limiting the area available for colonization during the dispersal period of a pioneer species. The second and less common mechanism involves the stunt- ing, by a constant river elevation, of the lower portion of a zone previously established by the above mechanism. The plants in the upper portion of the zone initiate normal spring growth but the lower, submerged plants must await a drop in river elevation be- fore growth can begin. Thus the development of what appears to be two vegetation zones from one previously established zone may occur. Both mechanisms require the maintenance of an appropriate river elevation during the waning period of general flooding.