Plant Cover in Precipitation and Nitrogen Manipulation Plots on the Jornada Basin, 2007-2009

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A 2-year experiment with ambient, reduced, and enhanced precipitation as well as nitrogen additions 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.We use vegetation cover as a proxy for plant biomass to avoid confounding spatial and temporal variability or confounding the impact of harvesting.  Individual cover of plant species was measured  using three parallel transects per plot. This study is complete.

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field data sheets and digital voice recorder

Manipulations during the 2007–2008 growing seasons consisted of five levels of PPT relative to ambient: −80, −50%, control, +50, +80%, with 24 precipitation replicates per each precipitation reduction or addition plots and 36 replicates for the control plots. Half of the replicates in each treatment received nitrogen additions, and half  did not. 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. Fertilized plots received 10 grams nitrogen per square meter per year. 

We measure vegetation cover in the fall (maximum biomass) in 132 2.5m x 2.5m plots. There are three parallel lines per plot, each of 250 cm length, and 60 cm apart, which beginning and end are marked with rebar. There, we record green interception per species for all plants present in the plot, and use existing regressions to obtain an estimate of biomass by species.

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QA/QC: Visual verification of data. Preliminary treatment of data: Calculations performed in Microsoft Excel: Individual cover of plant species measured as cm of green biomass that intercepts 3 250cm long lines, then averaged for 3 lines/plot.

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


yearly every fall beginning 2006

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