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Methods for Experiment 026 -

Experimental Design: Species

The five grass species we used are common prairie, grassland and old field species but differ in their successional status (Tilman 1988) and their response to nutrient addition (Tilman 1987). They are the five most abundant grasses during old field succession at Cedar Creek Natural History Area (CCNHA), where this work was performed. Agrostis scabra Willd. (a native bentgrass) and Agropyron repens (L. ) Beauv. (quack grass, introduced from Europe) are both early successional species at CCNHA. Poa pratensis L. (Kentucky bluegrass, introduced) dominates mid-successional fields at CCNHA. Schizachyrium scoparium (Michx. ) Nash-Gould (little bluestem, formerly Andropogon scoparius Michx. ) and Andropogon gerardi Vitm. (big bluestem) are native prairie species that dominate older successional fields at CCNHA. For brevity, we will refer to each species by its genus. Seeds of Agrostis, Agropyron, Poa, Schizachyrium and Andropogon were planted in the monoculture plots in late May, 1986, at a density, based on germination trials, to yield 3000 seedlings/m2. Seeds were covered with 0.5 cm of sand. There were 2 replicate monocultures of Andropogon, 3 replicate monocultures of Poa and Schizachyrium, and 4 replicate monocultures of Agrostis and Agropyron for each soil mixture, giving a total of 160 monoculture plots. This design was caused by limited space and the need for a balance between monocultures and competition plots in the various competition experiments that were performed simultaneously in these gardens. Monoculture and competition treatments were randomly assigned to the 64 plots within a soil mixture, with each soil mixture being independently randomized. For further information, see fertilization details.

Experimental Design: establishing initial range of soil Nitrogen

This research was performed in an experimental garden at CCNHA, which is located on a large glacial outwash sandplain in Isanti and Anoka counties of Minnesota. Each species was grown in both monoculture and mixed species plots across a range of total soil nitrogens. In August, 1985, we used a bulldozer to remove the upper 60 to 80 cm of soil from a 31m x 34m area of a 25 yr old field, thus exposing subsurface sand. This substrate was 93% sand, 3% clay, and 4% silt, had a pH of 6.6, contained 0.3% organic matter and had a total soil nitrogen, as N, of 90 mg/kg of soil. This area was divided into ten regions, 3 m x 12 m each, with 1 m walkways between them. Each of the ten regions was randomly assigned to be one of ten different soil mixtures. Nine of the soil mixtures were established by adding an amount of top soil per plot such that, once top soil and subsurface sand had been mixed to a depth of 22 cm, top soil would comprise approximately 0% (soil mixture 1), 10%, 20%, 40%, 50%, 55%, 70%, 90% or 100% (soil mixture 9) of the upper 22 cm of soil in the plot. To keep the surface of all soil mixtures level with their surroundings, the sand in each was excavated to a depth equal to the amount of top soil to be added. Soil mixture 10 was a duplicate of the 100% top soil treatment, but also received 6.55 g/m2/yr of additional N as NH₄NO₃. This was added monthly in proportion to normal N mineralization at Cedar Creek (Pastor et al. 1987; April = 9%, May = 33%, June = 28%, July = 15%, August = 17%). After top soil was added, each plot was thoroughly rototilled twice. This resulted in a uniform, 22 cm deep layer of soil that was underlain by unmixed sand. Approximately 60 m3 of top soil were used. This soil, obtained off-site, was a Duelm sandy loam of the Hubbard-Isanti-Duelm Association. It averaged 72% sand, 4% clay, and 24% silt, had 2.9% organic matter, total soil N of about 1200 mg/kg, and a pH of 7.2. It was chosen because it was a sandy outwash soil with total soil nitrogen content similar to that of undisturbed upland soils at CCNHA. To assure that no nutrients other than N would be limiting, we analyzed the soils, and based on these analyses, added 5.6 g/m2 of P, 8.9 g/m2 of K, 42 g/m2 of MgSO4, 8 g/m2 of Ca, 0.68 g/m2 of Zn, 0.28 g/m2 of Cu, 0.11 g/m2 of B, and 0.19 g/m2 of Mn in April 1986. These nutrients were thoroughly rototilled, to a depth of 22cm, into all plots. These nutrients were also added to the soil surface in subsequent years. Soil pH was stable, at 7.2, in all plots except the pure sand mixture and the 10% top soil mixture which received fine ground commercial lime in 1987 and 1988 to adjust their pH to 7.2. All plots were watered to receive at least 1.5 cm/wk in April, May, and October, and 2.5 cm/wk in June through September.

Field Operations: Burning

In 1990, plot 11 was burned.

Field Operations: Fencing

In April, 1986, galvanized sheet metal (26 gauge) was used to divide each of the 10 soil mixtures into 64 experimental plots, each of which was 0.75m x 0.75m. To do this, the sheet metal, which was 25cm tall, was driven into the soil to a depth of 23cm. The entire garden was then fenced to a height of 2m and to a depth of 1.3m belowground to exclude all mammalian herbivores.

Field Operations: Fertilization

Fertilizer is mixed and spread by hand once a year, in early May. Each plot gets the same fertilizer mixture except plot 5. Plot 5 receives NH₄NO₃, on the first of every month from April to August. Fertilizer added to plot 5 for the last time in April 1994.

Field Operations: Mammal Trapping

Small mammals are trapped in the fall to discourage overwinter inhabitants. Pocket gophers are trapped out of the experiment as soon as they appear (very infrequent).

Field Operations: Weeding

All plots and aisles are routinely weeded to keep the area free of unwanted plants and future seed sources.

Light Penetration

Light meter readings for E026 were taken in the same way as in E001, except that only one set of readings (one above and one below vegetation) was done per subplot. This was because the subplots are so small. In 1991, light profiles were taken in the monoculture subplots only.

Light Penetration Measurements Methods

From 1982 to 1988, light meter readings were taken using a Li-Cor, Inc. Integrating Quantum/Radiometer/Photometer, model LI-188B. Two people were needed to take light readings with this system. One would hold the control box and record data and the other would hold the light sensor. The batteries would be checked before starting each session. If the batteries were OK, they would proceed to take readings. The sensor was connected to the control box by a relatively short cord, so the two people taking readings were required to stay close together. When taking readings, it was necessary to get a range value for each light value entered. If the integrating time of 1 second was not sufficient, it was increased to 10 seconds. This was also recorded. Measurements are taken within a 4 hour period, 2 hours on either side of solar noon. (Solar noon is half way between sunrise and sunset; it is not 1200 hours). Solar noon is at 1315 hours, Central Daylight Time. Samples are taken between 1115 hours and 1515 hours. Measurements are not taken when the plot being sampled is shaded. Light readings are done when the sky is clear, whenever possible. If a cloud passes over the plot being sampled, assistants wait for the cloud to pass before taking the readings. If the sky is mostly cloudy, light meter readings are not taken. Two measurements are taken in each plot. Each measurement consists of one reading above the vegetation and a second reading at ground level. Both values are taken to get the percent of sunlight above the vegetation that reaches ground level. In taking the above vegetation reading, the sensor must be kept level, held high above all vegetation, kept out of the shade (of plants and people) and it must be clean. When taking the below vegetation reading, at ground level, the sensor must be kept level, out of the soil and out of the shade created by people. Light Data Transformations: Light readings are transformed to obtain percent light penetration which represents the percent of light above the vegetation that reaches the ground surface. In cases where the experiment involves shading, another variable is computed to reflect the percent of sun light that reaches above the canopy. This variable is called light available. In the case of absence of artificial shades, the latter is set to 1. percent light penetration = ( Light below canopy / Light above canopy ). percent light available = ( Light below shade / Light above shade ).

Treatment layout : trmte26

The following treatments are found in each of the ten different soil types. Each plot (soil type) contains 64 subplots: Species: 100 Ar = Agropyron repens, seed from Roseau, MN 117 Pp = Poa pratensis, seed collected at CCNHA 103 Ss = Schizachyrium scoparium, seed from Crex Meadows, WI; harvested by Prairie Rest, Inc 098 ArCCNHA = Agropyron repens, seed from CCNHA 102 Ag = Andropogon gerardii, seed from PRI 101 As = Agrostis scabra, seed from CCNHA Types of plots: Dewit = Plots are seeded at the same time with the ratio of the two species changing while the total density remains constant. Mono = Monoculture SeedInv. = Seed of a second species is sown into a high density stand of the first species which has grown for a year. Est.Inv. = Each subplot is divided into four units, two of whish are planted at high density with one species, two with the other species. After the first season the barriers seperating the four units will be removed. Total density: The values given are the estimated number of germinating plants per square meter; the actual germination rates for the different species in the different treatments are available elsewhere. Since the subplots are 0.75 meters by 0.75 meters, their actual area is 0.5625 m2. Nitrogen plots (P1 - P10)

Light plots (P12-P14)

Large monoculture (P11)

1985 SOIL NITROGEN DATA:

Vegetation Sampling Methods

E026 was sampled on an annual basis from 1986-1990. Each plot was sampled each year. A 7.8 x 55cm strip was clipped at the soil surface during August 28 - September 12 1986. The same size strip was harvested during July 25-August 5 1987. A 10 x 40cm strip was clipped during July 13-27 1988. These three strips were located at least 10cm from a plot edge in an area that had not been previously clipped. Sampling dates were chosen based on phenology, to be near the time of peak Agrostis flowering. Vegetation was sampled using the same clippers as described in above ground vegetation sampling methods except in 1986 when a hand-held clipper with a more narrow blade was used. Since the clip strips were so small, new metal frames were made to aid in clipping instead of using the 3m long aluminum poles. The metal frames were made so that after setting the frame down on the subplot, all the clipper needed to do was clip between the two edges of the frame. The distance from the edge of the subplot and the length of the strip were already set due to the frame shape. Clip strips are 10cm x 40cm unless noted otherwise below. 1986: 7.8 x 55cm strip. Clipped 10cm in from left side,10cm in from tag side, 5cm in from back side All treatments clipped except 30-41 and plots 12,13 & 14 1987: 7.8 x 55cm strip. Clipped 10cm in from right side, 10cm in from tag side, 5cm in from back side All treatments clipped except 30-41 1988: Clipped 17.5cm in from left and right sides, 5cm in from back side All treatments clipped 1989: Clipped 10cm in from tag side, 17.5cm in from left and right sides Only treatments 42, 43, 1, 5, 6, & 8 clipped 1990: Treatments 38-41 clipped 2, 30cm x 30cm squares (those sections not clipped in 1988). Treatments 30-37, 20cm x 55cm strips were clipped 10-30cm from right side. Treatments 1-29, 42 & 43, 10cm x 40cm strips were clipped 30cm from the back side of each subplot. 1991: Treatments 1-8, 42 & 43, 10cm x 40 cm strips were clipped 50-60cm from the front of each subplot and centered left and right. Treatments 9-37 were sampled in the same place, but only in plots 1, 5 & 7. 1992: Treatments 42 and 42 (?) - plots 1-10. Treatments 9-29 in plots 2, 3 & 6 (3 lowest N plots) = seed vs. seed treatments. 10cm X 40cm strip; centered left and right. 22-32cm from front. Also, monocultures (treatments 1-8) in plots 1-10. 1993: 10cm X 40cm strips; 23-33cm from back edge, centere left and right. Plots clipped were those with treatments 1-7, 9-20, 24-29, except for those which had been burned.

Watering Methods

All plots receive at least 1.5cm/week of water in April, May and October and 2.5cm/week in June through September. If the plots receive this water from natural rainfall, then no supplemental watering is needed. When watering is needed, it is done using a system of hoses set up in the field. A black irrigation pipe, 3 inches in diameter, is hooked up to the pump. This extends out to the E026 area where the 3-inch hose is divided into two 2-inch hoses. One part goes toward the E026 (and E071) plots (the other toward E055, E069, E070). From this hose, several regular garden hoses are connected. Gardena Excellent Oscillating Sprinklers, model 6968, are attached to these hoses. These sprinklers are purchased at Sears and made at Gardena, Inc., 6031 Culligan Way, Minnetonka, MN 55345. Three or four hoses/sprinklers are used at one time. Any more than this reduces the water pressure enough so the sprinklers will not work. The sprinklers are moved in order to reach all of the plots.

sexe026 - Reproductive output

Reproductive output

Reproductive biomass (seeds and flowering stems) was determined by counting the number of flowering stems within each plot at the time of peak reproductive biomass and subsampling flowering stems to determine mass per stem and seeds per stem. This was used to calculate the total reproductive mass per square meter, seed mass per square meter and seeds per square meter. These data were combined with germination rates to estimate the number of viable seeds produced per square meter. Plant height was determined by placing a 25 cm diameter, 5.2 g paper disk on top of the plants in a monoculture, and measuring the height that the center of the disk was held above the soil surface.