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Methods for Experiment 086 -

Disturbance for E108, E086, and E103 - Methods

Three experiments are designed to look at disturbance factors and interactions that may vary plant access to resources. The first 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. The second experiment is a nitrogen x water x fire factorial with 3 replicates. There are four nitrogen treatments (300 ppm N, 1000 ppm N, 3400 ppm N and 3400 ppm N + 90g ammonium nitrate added annually), two water treatments (natural rainfall, 1/2 summer rainfall) and two fire treatments (spring burn, no burn). To create plots with 300 ppm nitrogen, the upper 0.6cm of sand were removed and replaced with 0.6cm of black dirt containing 3400 ppm nitrogen. To create plots with 1000 ppm nitrogen, the upper 5cm of sand were removed and replaced with 5cm black dirt. To create plots with 3400 ppm nitrogen, the upper 20cm of sand were removed and replaced with 20cm black dirt. The fourth level of the nitrogen treatment adds 90 g of ammonium nitrate yearly in sping to certain 3400 ppm nitrogen plots. All plots were rototilled 15-20 times to ensure the mixtures were homogeneous. Plots with a fire treatment are burned with a kerosene drip torch annually in late May. Plots with a "1/ 2 summer rainfall" treatment are covered by rainout shelters that stand approximately 1.3 m high and are made with a clear plastic roof. These shelters are moved every one to two weeks from late May to early September in order to protect plots from rain for half the summer on average. Both fire and water treatments were initiated in May 1991. A herbivory x nitrogen factorial with uneven replicates was created in August 1991 with 3 nitrogen levels (300 ppm, 1000 ppm, and 3400 ppm) and 2 levels of herbivory (insecticide and no insecticide). Twenty-seven plots are included in this experiment; there are four replicates of each nitrogen level that are sprayed with insecticide and five replicates of each nitrogen level that are not sprayed. Nitrogen levels were created in the same manner as above in the nitrogen x fire x water experiment however, there is no fourth level with added ammonium nitrate. Plots that exclude herbivory have insect screen around the sides to discourage insect migration into the plots and are treated with Malathion insecticide every 2 weeks during the summer.

Methods: 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.

Notes

1990 May 1 to June 2 2 : plot construction. June 20: fertilized all plots. June 23: plots Bromo-gassed. June 27: planted seeds in all plots. September 6 to October 21: percent cover and number of each species was recorded from each plot. 1991 May 10: burned the plots of E086 designated "high" fire treatment. May 17 : drought treatment begins for E086. Low group 1 cover dates: May 17 to May 2 3, June 1 to June 6, June 14 to June 20, June 2 8 to July 4, July 19 to July 25, and August 2 to August 15 Low group 2 cover dates: begin May 24 and continue alternate to Low group 1 until September 21. June 10 to June 14 : 9 soil cores per plot were taken and dried to be analyzed later for pH and total N. June 17 to June 21: Vegetation survey. The percent cover of each species was recorded in each plot. July 15 to July 17: 4 root cores were taken from each plot and combined. The combined soil samples were washed to isolate root biomass. Roots were dried in a drying oven and then weighed the following week. July 22 to July 25: Vegetation survey. August 19 to August 23: Three above- and below-canopy light meter readings were taken from each plot. The number and species of flowers were recorded in each plot. August 24-29: Vegetation survey. September 21: drought treatment ends. Gophers were trapped thoughout the season although only one was captured. 1992 May 17: fertilized all plots. May 28: burned the plots of E086 designated "high" fire treatment. June 12: Drought treatment begins for E086. Low group 2 cover dates: June 12 to June 25, July 9 to July 24, and August 11 to August 31 Low group 1 cover dates: begin June 26 and alternate with Low group 2 until September 10. June 15 to June 23: vegetation survey. June 24 to June 26: Three above- and below-canopy light meter readings were taken and the number and species of flowers were recorded in each plot. June 30 - July 9: soil samples taken from plots to determine ammonium/nitrate composition. July 6 to July 9: more soil samples. July 13: more soil samples. July 15: soil sampling ends. July 20 to July 27: vegetation survey. July 28 to July 29: light meter and flower records. August 17 to August 21: 4 root cores were taken from each plot and combined. The combined soil samples were washed to isolate root biomass. Roots were dried in a drying oven and then weighed the following week. August 24 to August 31: vegetation survey. August 31: deflorated little blue-stem in aisles. September 1 to September 3: light meter and flower records. September 10: drought treatment ends. Gophers were trapped thoughout the season although only one was captured.

Sampling

Each plot is sampled from within the inner square meter in order to remove edge effects. Each plant species is identified and the percent cover is visually estimated using a small grid with 64 4.2 cm x 4.2 cm squares. When possible, lightmeter recordings (PAR) are taken from beneath and above the vegetation in each plot. This is used in conjunction with the estimated average height of the vegetation in order to get an estimate of aboveground biomass and light availability. Four 60cm3 root cores are taken and combined to determine average below-ground biomass for each plot. All inflorescences are counted for each species to estimate regeneration success. Initial species identification and percent cover were recorded in September 1990 (at the end of the first growing season). Since then, these same data, along with lightmeter recordings, have been recorded in June, July and August of each year. Root samples and inflorescence counts are taken in August only. In June 1991, nine soil samples were taken from each plot in order to determine soil pH and total nitrogen means and variances. Since these measurements remain relatively stable over a period of years (D. Tilman, pers. comm.), the total nitrogen and pH measurements will be taken only once. In July 1992, 5-9 soil samples were taken from each plot in order to determine ammonium and nitrate availability.

Table of Species Addition

List of species and number of seeds planted in 1990. FORBS

GRASSES

Treatment layout : trmte103

Additional notes: Black Dirt = Amount (inches) of black dirt mixed with sand % Rainfall = Percent rainfall received from May 17 to Sep 21 % Burned = Percent of plot burned % Herbivores = Percent of plot exposed to herbivores % Seed Addition = Percent of plot with yearly seed addition Number squares = Number of 20cm x 20cm squares disturbed per plot Deviation = Standard deviation of the amount of black dirt Nitrate Addition = Ammonium Nitrate (34-0-0) addition (g/m2/yr)

Treatment layout : trmte104

Additional Notes: Black Dirt = Amount in inches of black dirt mixed with sand % Rainfall = Percent rainfall received from May 17 to September 21 % Burned = Percent of plot burned % Seed Addition = Percent of plot with yearly seed addition Number of Squares = Number of 20cm x 20cm squares disturbed per plot Deviation = Standard deviation of the amount of black dirt Nitrate Addition = Ammonium Nitrate (34-0-0) addition (g/m2/yr)

Treatment layout : trmte105

Additional Notes: Black Dirt = Amount in inches of black dirt mixed with sand % Rainfall = Percent rainfall received from May 17 to September 21 % Burned = Percent of plot burned % Seed Addition = Percent of plot with yearly seed addition Number of Squares = Number of 20cm x 20cm squares disturbed per plot Deviation = Standard deviation of the amount of black dirt Nitrate Addition = Ammonium Nitrate (34-0-0) addition (g/m2/yr)

Treatment layout : trmte86

Additional Notes: Black Dirt = Amount in inches of black dirt mixed with sand % Rainfall = Percent rainfall received from May 17 to Sep 21 % Burned = Percent of plot burned % Herbivores = Percent of plot exposed to herbivores % Seed Addition = Percent of plot with yearly seed addition Number of squares = Number of 20cm x 20cm squares disturbed per plot Deviation = Standard deviation of the amount of black dirt Nitrate Addition = Ammonium Nitrate (34-0-0) addition (g /m2/yr)