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.
Knowledge Network for Biocomplexity (KNB) Identifier
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Location: Site situated on the eastern flank of Los Pinos Mountains, approx. 3 miles south of NM state route 60, directly adjacent to the eastern boundary of the Sevillets USFWS National Refuge. Landform: Located at the base of the eastern flank of the Los Pinos Mountains. Slope varies from 0-2% in experimental plots situated in level portions of the site, with steeper grades ranging from 6-18% for plots established on hill-slopes., Soils: Soil texture analysis across the site (both hill-slope and flat/level topography) revealed surface soils that are predominately silt loam with; (1) a transition to sandy loam texture at depth, and (2) a significant percentage of coarse fragments present both at the soil surface and throughout the profile. Soil depth across the site ranges from 20 to ≥ 100 cm, with shallower soil depths occurring on hill-slopes where depth to caliche and/or bed-rock is only 20-30 cm in some instances. , Hydrology: Well drained., Vegetation: The site is a piñon pine (Pinus edulis, Engelm.) and juniper (Juniperus monosperma (Engelm.) Sarg.) woodland, with several other commonly observed woody shrub species present, notably; Mahonia spp. (algerita), Falugia paradoxa (Apache plume), Quercus turbinella (shrub live oak), and Rhus spp. (sumac). Multiple species of cacti and agave (Cylindropuntia spp., Opuntia spp., and Yucca spp.) are present, along with numerous species of perennial grasses and forbs; including an extensive inter-canopy coverage by grasses of the genus Bouteloua. , Climate: Climate records (20-yr) from a nearby Sevilleta LTER meteorological station (Cerro Montoso #42; http://sev.lternet.edu/) indicate a mean annual precipitation total of 362.7 mm/yr. The region is strongly influenced by the North American Monsoon, with a large fraction of annual precipitation occurring in July, August, and September when monsoon circulation is active. Mean annual temperature (20-yr) at this nearby LTER site was 12.78 C, with a mean July maximum of 31.08 C and a mean December minimum of 3.38 C.
Pockman, William; McDowell, Nathan (2014): Ecosystem-scale rainfall manipulation in a Pinon-Juniper woodland: Tree Sapwood and Leaf Area Data (2011). Long Term Ecological Research Network. http://dx.doi.org/10.6073/pasta/48afd3abe50dceba70f6f616cf54fd7b