Soil Water Content in Experimental Precipitation Manipulation Plots on the Jornada Basin, 2007-2009

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A 2-year experiment with ambient, reduced, and enhanced precipitation was designed to compare the performance of the  encroaching C3 shrub (honey mesquite Prosopis glandulosa) to the dominant C4 grass (black grama Bouteloua eriopoda) in terms of photosynthetic rates and leaf water status. Precipitation manipulations dramatically enhanced natural variability and generated a range of rainfall scenarios that could have only been studied only after a multi-decade effort using natural conditions. Responses were highly asymmetric, with precipitation (PPT) additions generally influencing volumetric water content (vwc) to a greater extent than PPT reductions. Desert soils are usually close to their minimum water content  and thus when soils were dry, the effects of reducing PPT were relatively minor compared to the effects of adding PPT. Volumetric soil water content was, on average, lower and more variable at the shallower (0–5 cm) depth (mean 9.3 ± 0.14%; range 5.7–14.3%) than at the deeper (30–50 cm) depth (mean 14.4 ± 0.12%; range 10.8–18.1%. This study is complete. For further information and results, see:

Throop, H., L. G. Reichmann, Os. Sala, and S. Archer. 2012.  Response of dominant grass and shrub species to water manipulation: an ecophysical basis for shrub invasion in a Chihuahuan desert grassland. Oecologia 169: 373-383.




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field data sheets

Manipulations during the 2007–2008 growing seasons consisted of five levels of PPT relative to ambient: −80, −50%, control, +50, +80%, with 12 replicates per treatment. Plots were 2.5 × 2.5 m, and each was centered around a single mature mesquite shrub that was similar in size to other mesquite on this ecological site (mean height 0.5 m, canopy diameter 1.1 m). Shrubs were surrounded by black grama . Reductions in PPT were achieved via passive rainout shelters; supplementation was achieved via a sprinkler irrigation system. Desired levels of rainfall reduction (−80 and −50%) relative to ambient were achieved by modifying the number of transparent V-shaped acrylic strips per shelter [Acrylite FF; CYRO Industries, Parsippany, NJ; see Yahdjian and Sala 2002 for a detailed description of shelter design and their minimal effect on the microenvironment]. A PVC-pipe irrigation system was used at the enhanced PPT plots, which were irrigated the day after PPT events >2 mm, with an amount of water equal to 50 or 80% of the event. Supplementation occurred during the spring and fall of 2007 and 2008 using PPT that was captured nearby and stored. Control plots received ambient PPT and had neither shelters nor irrigation systems.

Soil volumetric water content (VWC) was quantified simultaneously with ecophysiological measurements. VWC was measured in six plots per treatment at depths of 0–5 and 30–50 cm using EC-5 and EC-20 soil moisture sensors (Decagon Devices, Pullman, WA), respectively. Probes were calibrated for soils at the site following the protocol developed by the manufacturer (r 2 of 0.98 and 0.96 for EC-5 and EC-20 probes, respectively).

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Visual verification of data.


Additional information: 

 Exclosures 1, 2, and 3 within Pasture 13 (Study Area West) of USDA-ARS Jornada
 Experimental Range.



This study is complete.

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