Basic hypotheses:

see Alba et al 2017 for details

  Present are two shelters and two control sites.  Based on a mycorrhizal associated framework, one treatment and control are dominated by Quercus rubra and Carya ovata; ectomycorrhizal associated trees.  The other treatment and control sites are dominated by Acer saccharum and Liriodendron tulipifera; arbuscular mycorrhizal associated trees.  

The objectives of this long-term study are: (1) to assess the effects of directional and prolonged changes in precipitation amount and variability on population, community and ecosystem processes; (2) to elucidate the mechanisms behind those changes, and finally (3) to predict the future state of drylands functioning under novel climate conditions.

This experiment is part of the Loma Ridge Global Change Experiment, established by Michael Goulden in 2007. There are two ecosystem types, grassland and shrubland, with more studies completed in the grassland. The plots in the shrubland are 4 times the area and twice the height of the grassland. In 2014, half of the manipulated plots were designated "recovery" plots and the precipitation manipulation was ended. This experiment is also split-plot factorial with nitrogen addition (60 kg/ha/yr).

Drought manipulations as described have been carried out starting 1999 until 2017 (18 years), while 1998 was a pretreatment year. Plans for the next future (starting 2019) are to transform the active manipulation into a passive, drought-net design. This would mean that the "old" manipulation of 3 months no precipitation (an average decrease of 20% on an annual basis), would change to a year long decrease in precipitation of 20% or any other percentage (not yet decided).

We use the Gherardi et al approach, passive drought shelter, collect water and pump on to irrigated plots.

We record community composition with individual-level mapping, which gives us demographic data, but means our community level data are a little "nonstandard" (some life forms represented as canopy cover, others basal cover, others just density).