Photosynthetic Rates for Leaves from Treatments of Enhanced and Reduced Precipitation on the Jornada Basin, 2007-2008

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Increases in woody vegetation and declines in grasses in arid and semi-arid ecosystems have occurred globally since the 1800s, but the mechanisms driving this major land-cover change remain uncertain and controversial. Working in a shrub-encroached grassland in the northern Chihuahuan Desert where grasses and shrubs typically differ in leaf-level nitrogen allocation, photosynthetic pathway, and root distribution, we asked if differences in leaf level ecophysiology could help explain shrub proliferation. We predicted that the relative performance of grasses and shrubs would vary with soil moisture due to the different morphological and physiological characteristics of the two life-forms. In a 2-year experiment with ambient, reduced,and enhanced precipitation during the monsoon season, respectively, the encroaching C3 shrub (honey mesquite Prosopis glandulosa) consistently and substantially outperformed the historically dominant C4 grass (black grama Bouteloua eriopoda) in terms of photosynthetic rates while also maintaining a more favorable leaf water status. This study is complete.


Please refer to:

Throop, H., Archer, S. R. , and L. G. Reichmann. 2011. Response of dominant grass and shrub species to water manipulation: an ecophysiological basis for shrub invasion in a Chihuahuan Desert Grassland. Oecologia 169: 373-383.

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-Measurements were made on a LiCor 6400 with optimal conditions for photosynthesis (PAR = 1500-2000 umol m-2 sec-1, leaf temperature 25-32 C, RH 30-70%).


Manipulations during the 2007–2008 growing seasons consisted of water levels of PPT relative to ambient: -80%, -50%, control, +50%, +80%, with 12 replicates per treatment.Time constraints to perform ecophysiological measurements on both black grama and mesquite simultaneously led us to measure these variables in four replicate plots for each of the PPT treatments. Plots were 2.5 X 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 (mean basal area 11.48 cm2); black grama individuals selected for measurements were >0.75 m from plot edges. 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. A PVC-pipe irrigation system was used at the enhanced PPT plots, which were irrigated the day after PPT events, 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.Leaf ecophysiology was quantified during the 2007 and 2008 growing seasons on dates with contrasting PPT patterns in the days preceding the sampling date. One mesquite and one black grama leaf from each plot (n = 20 plots; 5 PPT treatments X 4 replicate plots/treatment) were measured during the morning on two dates in 2007 (25 July and 18 August) and three dates in 2008 (21 July, 25 July, 13 August).

Quality Assurance: 

QA/QC: Date Entry by QA/QC Description ---------- ---------------------------- ---------------------------------------------- 09/04/2009 Heather Throop Data checked upon instrument download Preliminary treatment of data: Photosynthetic rates recalculated based on cuvette leaf area

Additional information: 

Block 2 of Sala's Global Change experiment (LTER Study 278) in Pasture 13. Refer to methods in metadata for package 210278002 for more information about this study.


Measurements were taken approximately 3 times per growing season, contingent upon precipitation. This data collection is complete.

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