An island rookery of approximately 8000 Ardeidaean birds in west-central Minnesota was mapped and divided into three areas of use: heavy, light and none. Quadrats, 1 m on a side, were used to sample these three areas for floristic composition, soil nutrients and pH, light regime and great blue heron nest concentration. The results support previous work on oceanic rookeries showing a reduction in plant density in areas of high avian activity. The distribution of 21 plant species discussed, with relation to the three areas of activity, is limited neither by the summer light intensity nor the concentration of nutrients measured during the winter. INTRODUCTION The purposes of this investigation were to provide a quantitative description of the vegetation in an active fresh-water heron rookery and to investigate any changes in soil nutrients as possible limiting factors in the subsequent floral pattern. Alpheraky (1905), Guthrie-Smith (1914, 1925), Wilson (1959), Fineran (1964, 1966) and Gillham (1956, 1963a and 1963b) have all reported on vegetational responses to high bird concentrations in oceanic colonies, but very little information is available with respect to fresh-water nesting sites. The mechanisms of these vegetational responses are somewhat dependent upon the type of avian aggregation. A tree-roosting or nesting colony influences the ground vegetation mainly through the accumulation of the droppings. On the other hand, ground-feeding and burrowing or tunneling nesters also graze, burrow and trample the area. Herons represent a tree-nesting colony. The physiological effect of guano on plant life is reported to result from an unfavorable osmotic balance set up in the soil by the accumulation of salts; salt concentration in seabird guano is often very high (Gillham, 1963b). The macronutrient soil constituents of these oceanic rookery areas have been well established. The soils, are characterized by: 1. A decrease in the C: N ratio as a result of the high influx of nitrogen, usually in the form of nitrates (Blakemore and Gibbs, 1968). Oceanic rookery soil nitrate content sometimes shows a 100-fold increase over control areas (Gillham, 1956). 2. An increase in phosphate content, almost solely as inorganic phosphate. A 30-fold increase has been noted here (Gillham, 1956). 3. A high concentration of soluble ions, namely sodium, potassium, ammonium, chloride, sulfate, nitrate and phosphate (Blakemore and Gibbs, 1968; Campbell and Claridge, 1966). 4. A decrease in the amount of exchangeable calcium and magnesium. This is due apparently to a displacement by ammonium ions during the leaching process (Ward, 1961; Blakemore and Gibbs, 1968). The hydrogen ion concentration of guano varies widely (pH: 5.8