Methodology and performance of a rainfall manipulation experiment in a pinon-juniper woodland

Climate models predict that water limited regions around the world will become drier and warmer in the near future, including southwestern North America. We developed a large-scale experimental system that allows testing of stand level impacts of precipitation changes. Four treatments were applied to 1600 m(2) plots (40 m x 40 m), each with three replicates in a pinon pine (Pinus edulis) and juniper (Juniper monosperma) ecosystem. These species have extensive root systems, requiring large-scale manipulation to effectively alter soil water availability. Treatments consisted of: (1) irrigation plots that receive supplemental water additions, (2) drought plots that receive 55% of ambient rainfall, (3) cover-control plots that receive ambient precipitation, but allow determination of treatment infrastructure artifacts, and (4) ambient control plots. Our drought structures effectively reduced soil water potential and volumetric water content compared to the ambient, cover-control, and water addition plots. Drought and cover-control plots experienced an average increase in maximum soil and ground-level air temperature of 1-4 degrees C during the growing season compared to ambient plots, and concurrent short-term diurnal increases in maximum air temperature were also observed directly above and below plastic structures. Our drought and irrigation treatments significantly influenced tree predawn water potential and canopy transpiration, with drought treatment trees exhibiting significant decreases in physiological function compared to ambient and irrigated trees. Supplemental irrigation resulted in a significant increase in both plant water potential and canopy transpiration compared to trees in the other treatments. This experimental design allows manipulation of plant water stress at the tree/stand scale, permits a wide range of drought conditions, and provides prolonged drought conditions comparable to historical droughts in the past-drought events for which wide-spread mortality in both these species was observed.