woody encroachment

woody encroachment

SRM01 ShRaMPs (Shrub Rainout Manipulation Plots): interactive effects of drought and fire on grass and shrub physiology and productivity at Konza Prairie


ShRaMPs (Shrub Rainout Manipulation Plots) is a drought x fire experiment aimed at understanding the interactive effects of drought and fire frequency on tallgrass prairie communities experiencing varying degrees of shrub encroachment. Passive rainout shelters were constructed over existing, mature shrub islands and co-existing herbaceous communities on neighboring 1-year and 4-year burn watersheds (K1B and K4A). Shelters were either 'control' (ambient precipitation) or 'drought' (~50% precipitation reduction). A variety of leaf-level (gas exchange, water potential, turgor loss point, leaf δ13C) and whole-plant level (herbaceous and shrub biomass, plant water uptake) variables were measured during the 2019-2022 growing seasons to determine the physiological responses of a dominant C4 grass (Andropogon gerardii; big bluestem) and an encroaching clonal shrub (Cornus drummondii; rough-leaf dogwood). Soil volumetric water content was measured on-site for the duration of the experiment.

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Research location: Konza Prairie Biological Station, boundary between watersheds K1B and K4A.

Leaf gas exchange: Measurements were conducted every 3-4 weeks throughout each growing season (2019-2022) using a LI-COR 6400XT or 6800 infrared gas analyzer. Two individual plants per species were measured in each shelter during each sampling round. For all measurements, reference CO2 concentration was set to 400 umol mol-1, PAR (photosynthetically-active radiation) was set to 2000 umol m-2 s-1, and relative humidity was maintained between 50-65%. Measurements were taken between the hours of 10:00 and 13:00 on clear, sunny days.

Leaf water potential: Predawn and midday leaf water potential measurements were conducted every 3-4 weeks during each growing season (2019-2022) on the same dates as leaf gas exchange measurements. Midday measurements were conducted between the hours of 12:00 and 13:00, and predawn measurements were taken 1-2 hours before dawn, either on the same day as midday measurements or the next morning. Three replicates per species, per shelter were collected during each sampling round. For C. drummondii, the youngest, most distal leaves at the top of a ramet were colelcted, and for Andropogon gerardii, the youngest fully expanded leaf from each individual was collected. Measurements were conducted using a Scholander pressure chamber.

Turgor loss point: In 2022 only, leaves from C. drummondii and A. gerardii were collected at five time points during the growing season. At each time point, one sample per species was colelcted from each shelter. Measurements of osmotic potential at full turgor were conducted using a VAPRO Vapor Pressure Osmometer (Model 5600).

Plant/soil water isotopes: Non-photosynthetic tissue from C. drummondii (stem tissue) and A. gerardii (root crown tissue) was collected every 3-4 weeks throughout each growing season (2019-2022). Tissue was collected from 3-5 individuals and composited into one sample per specise per shelter. Samples were immediately sealed in exetainer vials and refrigerated. Surface soil (0, 10, 20 cm depth) was collected at the same time as plant tissue collection. Under each shelter, root-free soil was collected from each depth and immediately sealed in exetainer vials and refrigerated. Deep soil cores (1 m) were collected from each watershed using a hydraulic push-corer (540MT Geoprobe Systems) in the early (June / July) and late (August / September) growing season each year. Water from each sample was extracted using cryogenic vacuum distillation and analyzed for d18O and d2H on a Picarro WS-CRDS isotopic water analyzer at K-State.

Plant biomass: Herbaceous vegetation was collected at the end of each growing season (late August / early September; 2019-2022) to determine annual net primary productivity (ANPP). Biomass was collected from 0.1 m2 frames in two subplots per shelter. Samples were sorted into graminoid, forb, and woody biomass. Previous year's dead biomass was separated for samples from the 4-year burn shelters. For shrub aboveground biomass, C. drummondii stems were counted and stem basal diameters were measured in one subplot per shelter during the winter following each growing season (2019-2022). The same subplot was measured each year. Stem density was recorded for each shelter and aboveground biomass was estimated from stem counts and diameters using allometric equations.

Leaf carbon isotopes: 3-4 leaves from C. drummondii and A. gerardii were collected for stable isotope analysis on the same sampling dates as water potential and gas exchange. Leaves were dried at 60 °C for at least 72 hours and then ground up until each sample was homogenized. Samples were then analyzed for δ13C, δ15N, percent carbon, and percent nitrogen at K-State.

Soil moisture: Each shelter at ShRaMPs was equipped with 30 cm time domain reflectrometry (TDR) probes at 10, 15, and 30 cm depths. Voluetric water content data was recorded at 30-minute intervals from 2019-2022.

Gas exchange, water potential, and leaf δ13C will continue to be collected and the dataset will be updated annually.

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