- Experimental Design
- Infrastructure
- Species Combinations
Experimental design
The experiment is a factorial design of two precipitation distributions at the shelter level (n=4), two warming treatments within each shelter at 100 W/m2 (5 plots per treatment, 10 plots per shelter), and 5 species combinations (see Plant species combinations), with one species combination per warming treatment.
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Figure 1. Layout of the shelters and plots within the shelters
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Simulated precipitation regimes include two patterns that vary in seasonal distribution and event size, but not in total annual precipitation (1018 mm) or total number of events (Fig. 2). The long-term (50 yr) precipitation pattern characteristic of the region is replicated within four of the shelters (Fig. 1). The frequency and intensity (amount) of precipitation events are also simulated from the long-term precipitation record of the region. In the precipitation redistribution treatment imposed beneath the other four shelters, 40% of the summer (May – September) precipitation is withheld from each event and evenly redistributed to the two preceding spring (March and April) and two subsequent autumn (October and November) months. The redistribution treatment effectively increases the intensity of the summer drought (redistribution dry phase) and the amount of precipitation that occurs during the cooler seasons of the year (redistribution wet phases).
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Figure 2. Monthly watering totals (mm) in the control (dark grey) and redistributed (light grey) precipitation regimes. The event sizes, frequencies, and monthly totals are based on the 50-year climate record. In the redistributed treatment, May-September event sizes are 40% reduced compared to the control (intensified summer drought phase) and this amount is redistributed to the spring (March + April) and autumn (October + November) months, where event sizes are 52% larger than the control (wet phase – light blue background). The number of events (91) and annual total rainfall (1018mm) are identical in each treatment.
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Infrastructure
The research infrastructure includes eight permanent 9 x 18 m x 5 m rainout shelters built with greenhouse frames covered with clear greenhouse poly film (Photo 1) in a design based on other precipitation studies. The ends and sides below 1.5 m are left open to maintain microclimatic conditions as near ambient as possible. Incident solar irradiance, photosynthetically active radiation (PAR), air temperature, and relative humidity are continuously monitored in each shelter and control plots by using Hobo dataloggers (Hobo U12, Onset Computer Corp., Bourne, MA). Sheet metal flashing 40 cm in width was inserted 30 cm into the soil (into the clay layer) to isolate each shelter from both surface and subsurface water flow. Ten 2 x 2 m plots, with five species combinations (see Plant species combinations), are located beneath each shelter in the native soil. An overhead irrigation system (17 spray nozzles per shelter) simulates precipitation regimes by supplying reverse osmosis water from four 11,500 liter holding tanks to each shelter. Infrared lamps (Kalglo Electronics, Bethleham, PA; model MRM-1208L) suspended over individual plots (at 1.5 m height) simulate warming. This enables us to evaluate plant response to altered precipitation patterns and warming both independently and in combination. In addition, two similar blocks of plants were left uncovered (without shelters, lamps and sheet metal barriers) to serve as a control of ambient conditions (20 additional
plots). A weather station (EZ Mount Groweather, Davis Instruments, Heyward, CA) on site records precipitation, air temperature, and other parameters. The areas between plots and shelters are maintained by mowing.
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Plant species combinations
Two sets of five species combinations are grown 2x2 m plots beneath each of the rainout shelters and shelterless controls (Figure 1 and see example plots in photograph). One set is warmed and one set is grown with a dummy lamp. The most abundant deciduous tree (post oak; Quercus stellata), perennial grass (little bluestem; Schizachyrium scoparium), and an invasive evergreen tree (eastern redcedar; Juniperus virginiana), are grown in monoculture. In addition, both of the woody plant species are grown in equal numbers with little bluestem at densities representative of native grasslands in order to investigate tree-grass competition under these global change scenarios. These species were selected on the basis of their dominance in the southern oak savanna and their contrasting functional traits. Plots are hand weeded to maintain the prescribed species combinations.
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