We compiled all quadrat data into a master database. For each site we determined the mean percentage of all woody stems that were buckthorn as a measure of dominance, the mean count of buckthorn stems per square meter (of buckthorn-occupied quadrats only) as density, and mean ground level diameter of all buckthorn stems for size. Golden Graphics Surfer enabled us to then create isoline maps for these summary indices, using each observation site as the centroid position for extrapolation.
We obtained measurements of 6155 individual buckthorn stems from 1258 quadrats, and recorded also 4665 stems of other woody species that we did not identify or measure for size.
Mean dominance, density, and diameter varied substantially between sites (Figure 5). Eleven sites had buckthorn exceeding 50% of all woody plants. Five sites averaged ten or more buckthorn stems per square meter. Stems were small (<4 mm diameter) at seven sites where buckthorn was numerically dominant, and were large at six sites where buckthorn was under 50% dominant.
Buckthorn dominance (mean percentage of all woody plants) for all sites was 52.85% (range 2.87 to 96.42, standard deviation 26.40), and comprised the greatest proportional dominance of woody vegetation on most non-wetland surfaces within the study area (Figure 6). There were three concentrations in excess of 60% buckthorn. One occurred to either side of Michigan Avenue at a trail crossing, in an aging jack pine (Pinus banksiana) stand (Figure 2). A second “hot spot” of buckthorn dominance was along an abandoned roadway at the western border of our Schmeeckle Reserve study area, in a mixed grove of white pine (Pinus strobus) and red maple (Acer rubrum). The third concentration occurred in red maple (Acer rubrum) and red oak (Quercus rubra) along a trail on the north shore of Lake Joanis.
Buckthorn had sub-majority (less than 40%) concentrations at three locations. All were at or alongside wetter ground, although not in true wetlands. The northeastern portion of the study area generally had much less buckthorn, especially among red maples (Acer rubrum) and red oaks (Quercus rubra) at Site M, where a plankwalk crossed ground that floods during springtime snowmelt. The other two locations of lesser buckthorn prevalence (Sites D and G) were at the start of boardwalks crossing adjacent wetlands. However, other such elevated walkways (Sites C, E, R, and U) did not exhibit similar lesser-dominance of buckthorn.
Mean stem density for our entire study area was 7.92/m2 (range 1.92 to 35.23, standard deviation 6.76). The standardized abundance of buckthorn stems (Figure 7), was especially high again at the Michigan Avenue trail crossing, but only on the west side (Site E) of this raised roadway. The numerous buckthorn stems at this site occurred mostly within a narrow (about five meter wide) strip of higher ground between the road to the east, and a sedge (Cyperacea) and willow (Salix spp.) marsh to the west. Sampling quadrats that occurred within that marsh had virtually no buckthorn present. The lesser density east of Michigan Avenue may be due to mechanical clearing in October 1999, although now much buckthorn has regrown (Figure 2).
Density within occupied quadrats was lower (less than six stems per square meter) at locations having fully mature (Site B) or very dense (Site V) white pine (Pinus strobus). Density was also low along the northern side of Lake Joanis (Sites I, J, and K), where mature upland broadleaf trees dominated, and also in a low-lying fern-dominated area west of the lake (Site H) where mechanical clearing had occurred within the previous year.
Mean stem diameter for all sites (Figure 8) was 3.24 mm (range <1.00 to 90.00, standard deviation 4.94). Buckthorn size (stem diameter at ground level) was largest (averaging in excess of eight millimeters) in two quite distinct situations. The first was on raised ground within wetlands. East of Michigan Avenue at Site R some exceptionally large (40 to 90 mm) buckthorn stems grew along the high banks of a small stream channel, and within the seasonally flooded area at Site Q buckthorn occurred almost exclusively as circular clusters of large (up to 80 mm) multiple stems on raised hummocks. The second situation was dry ground having full sunlight penetration for much of the day. At Site J there existed a right-angle bend in the trail and evidence of brush cutting to widen this trail, at which full sun reached the ground much of the day and buckthorn stem diameters were up to 60 mm.
Small stem diameters (less than two millimeters) prevailed in three other situations. One was at the extreme northeast corner of the study area (Site N), in mature upland broadleaf forest; much blackberry (Rubus spp.) occurred in this understory. Another occurred east of Site D and west of Site E, on the periphery of a sedge (Cyperacea) and willow (Salix spp.) hummock wetland. The third small diameter situation was in a mature stand of white pine (Pinus strobus) having thick needle litter north of the marker at Site B; south of the trail at this location the ground becomes lower and wetter, pine size and frequency diminished, and buckthorn became larger, denser, and dominant.