Climate research predicts that many of the arid and semi-arid regions around the world will become drier and warmer in the near future, especially throughout southwestern North America.  We have developed a large-scale experimental design that enables us to determine the long-term ecological impact of changes in precipitation.  The primary purpose of our manipulative study is to determine the hydraulic mechanisms of piñon (Pinus edulis) and juniper (Juniperus monosperma) survival and mortality during drought, but the experimental treatments are also being utilized in the investigation of plant and soil respiration, nutrient cycling, and mycological and entomological influences across changing precipitation conditions. Four different treatments covering 1600 m² (40m X 40m), each with three replicates, have been constructed to investigate predicted climate variations.  Treatments include: 1) water addition plots that apply 20mm of additional precipitation, 2) drought plots that prevent 45% of natural precipitation from reaching the soil through a series of large plastic troughs, 3) plastic cover control plots that do not prevent precipitation from reaching the soil but feature plastic covers like those in the drought treatments, and 4) ambient control plots. Five piñons and five junipers are being monitored within each of the 12 experimental plots. 

Results/Conclusions

After one year of treatment, there was significantly greater piñon mortality in the drought plots compared to the rest of the landscape. The drought treatment structures effectively reduced soil water availability for the trees compared to the ambient, cover-control, and water addition plots.  Drought and cover control treatments also result in a 2–3^oC increase in soil and air temperatures compared to ambient and water addition plots.  Three days following a water application before the onset of seasonal monsoons, plant water potential was 46–69% greater in water addition plots and 14–18% lower in drought plots, when compared to ambient controls.  Trees in the water addition plots were also the only trees in any treatment that were photosynthetically active at this time. During the summer monsoon when natural water availability was at its greatest, trees in the drought plots still had predawn plant water potentials that were 35–52% lower, and photosynthetic rates that were 14–33% lower than trees in control plots. With the continuation of the manipulative treatments, this experimental design will help determine the mechanisms that influence tree mortality due to changing precipitation patterns.