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 the ecosystem impacts of precipitation changes. Four treatments were applied to 1600 m2 plots (40 m × 40 m), each with three replicates in a piñon 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 air temperature at ground level of 1-4° 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, sap-flow, and net photosynthesis, 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 xylem sap-flow compared to trees in the other treatments. This experimental design effectively allows manipulation of plant water stress at the ecosystem 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.
A micrometeorological station was used to document the climatic conditions at the study site. Monitoring the ambient environment in this way allowed us to more easily determine which tree growth responses were driven by changes in the native climate as opposed to those resulting from the rainfall manipulation treatments. Environmental factors such as temperature, relative humidity, and photosynthetically active radiation (PAR) have a huge impact on the physiological processes that are being explored in this project. The data collected by the station created a local climatic record which was needed to provide the context in which the treatment effects can be examined and sensor readings can be interpreted.
A CR-10X datalogger was used to record data from a micrometeorological tower centrally located in an open intercanopy area of the study site. This tower recorded precipitation with a Series 525 rain gauge (Texas Electronics, Dallas, TX), net radiation with a Kipp and Zonen NK-LITE net radiometer (Campbell Scientific, Logan, UT), photosynthetically active radiation (PAR) with a LI-190SA sensor (Li-Cor, Lincoln, NE), windspeed and direction monitored with a 05103-L R.M. Young wind monitor (Campbell Scientific, Logan, UT), and air temperature and RH% with a Vaisala HMP45C sensor. During winter months the rain gauge was fitted with a snow adaptor to thaw snow and record the total amount in mm rain. All met-station measurements were made at a height of 1-3 m above ground depending on the sensor array in question.
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