Droughts are forecasted to become increasingly frequent and intense in many regions worldwide, likely impacting community structure, ecosystem functioning, and ecosystem services. The International Drought Experiment (IDE) is a coordinated, multi-site drought experiment requiring only a moderate investment of time and resources by investigators. This coordinated, distributed experiment quantifies the impacts of extreme drought across a wide range of terrestrial ecosystems based on a common experimental design and a comparable suite of measurements. Two stand-alone experiments following the IDE design were established at Cedar Creek, E307 sIDE (savanna IDE) and E306 tIDE (trait IDE). Precipitation reduction shelters were established during the growing season (May through September) from 2017 to 2020 to simulate a 1 in 100 year dry event, which at our site corresponds to removing 43% of annual precipitation.
Experiment Design
The sIDE (savanna IDE, e307) study was established in Field D, an oak savanna, and consisted of 3 precipitation treatments (precipitation reduction shelter, inverted shelter as an infrastructure control, no shelter), fully crossed with two fertilization treatments (unamended control or NutNet NPK treatment), and with 5 replicate plots for each of the six treatment combinations, for a total of 30 plots, each 3 by 3 m (2 by 2 m inner area sampled), covered by 3 by 3 m shelters. Infrastructure controls help account for shelter effects that are unrelated to removing rainfall. Building on our previous related work, we are testing, for example, how nutrient enrichment alters the response to experimental drought and whether nutrient-induced biodiversity loss contributes to these effects.
The tIDE (trait IDE, e306) study was established as subplot treatments nested within a fertilization by irrigation study. Thirty-six 9 by 9 m plots were planted with the same mixture of 32 grassland species in early spring of 1994 and again in 1995 . Starting in 2007, plots were randomly assigned to one of the six treatments, which included all combinations of two water treatments (ambient and +25 mm/week during the growing season) and three nitrogen fertilization treatments (ambient, +7 g N m-2 yr-1, or +14 g N m-2 yr-1). Irrigation and N treatments were applied at the main plot (9 by 9 m) level. The three precipitation treatments (precipitation reduction shelter, inverted shelter as an infrastructure control, no shelter) were applied at the subplot level and fully crossed with the two irrigation treatments and with two of the three fertilization treatments (unamended or 14 g N m-2 yr-1), with six true replicate plots for each of the 12 treatment combinations (3 drought x 2 irrigation x 2 fertilization), for a total of 72 subplots. In 2017, when the tIDE experiment began, the irrigation treatment was discontinued. We are testing, for example, whether previous irrigation shifts the community away from drought-tolerant traits, which could then decrease resistance to subsequent experimental drought.
In both experiments, from 2017 to 2020, shelters removed about 43% of precipitation during the growing season (May through September). At our study site, a 43% reduction in precipitation on an annual basis would amount to a 1 in 100 year drought.