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Methods for Experiment 108 -

Disturbance method

This experiment is a disturbance gradient with 20 different levels of disturbance. The upper 5cm of sand was removed from each of 20 plots and replaced with blackdirt. Each plot was rototilled 20cm deep 15 to 20 times. Using baling wire, the inner square meter of each plot was divided into a grid with 25 20cm x 20cm squares. The plot with the lowest level of disturbance has one grid square disturbed, the second lowest has 2 disturbed, and on up to the highest disturbance with 20 grid squares disturbed. Each disturbance square is randomly chosen without replacement and the disturbance is made by digging and turning the soil with a trowel such that existing plants are uprooted. Plots are disturbed annually in late May beginning in May 1991.

General Design and Site Characteristics

These experiments are located at Cedar Creek Natural History Area, Anoka county Minnesota, on a large glacial outwash sandplain. In May 1990, a bulldozer was used to remove the upper 20cm to 30cm of soil from a 20m x 50m area of a 29 year old field. The exposed subsurface sand contained an average of 267 ppm nitrogen and had a pH of 7. The bulldozed area was divided into 215 1.5m x 1.5m plots with a 0.5m aisle between each plot on all sides. Plots are laid out in 22 rows of 10 plots each and each plot has been randomly assigned an experimental treatment. Every plot is surrounded by aluminum flashing that is driven approximately 20cm into the soil to inhibit movement of roots into neighboring plots. All plots receive an annual fertilizer treatment (20g/m2 of P2O5, K2O, and CaC03; 30g/m2 of MgSO4; 2 tsp/ m2 of trace elements) that is complete for plant macro- and micronutrients with the exception of nitrogen. To create initially identical plant communities, 59 species of plants (47 forbs and 12 grasses) commonly found on Minnesota tallgrass prairie reserves were planted in each plot (Table 1). Seeds for each plot were mixed with moist dirt in a bucket and then spread evenly over the entire plot. All plots received the same number of seeds of each species (see Table 1 for actual numbers). One monoculture plot for each species was also seeded with these same numbers of seeds to determine germination rates and plant allocation patterns. These experiments are unfenced. However, gophers are excluded from plots by trapping. Schizachyrium scoparium (little blue stem) is maintained (but mown before flowering) in the aisles between the plots in order to draw down nutrient levels within the aisles to eliminate edge effects.

Plot Treatments