Experimental design
The experimental design for the Cross-Scale Interactions study
(CSIs) consists of 15 - 1 ha blocks spatially distributed across
the study area selected to have similar total percentage
vegetation cover within a block, but to vary in the proportion of
perennial grass and mesquite cover among blocks. At each block
location, four 15 x 25 meter experimental plots were identified
with similar percentage grass and shrub cover. Within an
experimental plot was a buffer area, an 8 x 8 meter treatment or
control subplot (the "focal patch"), and 8 x 8 meter
upwind and downwind subplots. Each experimental plot was arrayed
perpendicular lengthwise to the dominant southwest to northeast
wind direction, and was placed > 15 meters away from the other
experimental plots to minimize between-plot treatment effects.
Excluding the buffer zones, the distance between focal patches and
subplots in different experimental plots ranges from > 20 to
~40 meters.
Each experimental plot within a block was randomly assigned to one
of four connectivity treatments: (1) plant scale where all
mesquite plants within and surrounding the focal patch were killed
in place with herbicide to modify competitive interactions between
woody plants and recovery of perennial grasses and other
herbaceous plants with no direct effects on horizontal transport
by wind and water, (2) patch scale where Connectivity Modifiers
(ConMods, Okin et al. 2015) were located in bare soil interspaces
between plants in the focal patch to reduce gap size and to modify
transport of water, soil, nutrients, litter, and herbaceous seeds,
(3) both patch- and plant-scale manipulations were conducted in
each focal patch, and (4) no manipulations [controls]. Block and
plot selection were completed in June 2012 followed by the
characterization of initial vegetation cover in all plots in June
2013 when treatments were initiated.
Assignment of treatments to experimental plots (by block) is
described in the attached files "csis_treatment_key.csv"
and "Study413_CSIS_Plot_Treatments.pdf".
Overhead photo collection and analysis
In each plot (4 per block), repeat overhead photos of ten randomly
selected microplots were photographed annually. Photos were taken
from directly above the microplot (overhead, or downward-looking
orientation) from a height of approximately 85cm above ground
level using a 12.1 megapixel digital camera, resulting in image
dimensions of 4,000 x 3,000 pixels. In ConMod-only and ConMod +
Herbicide treatments, photos were centered over the center post of
the ConMod structure, and in Control and Herbicide-only plots
photos were centered over a marker nail affixed to the soil
surface. Annual photos were taken in winter (Jan-Feb) from
2013-2016 to try to better distinguish perennial grass from annual
plant biomass. In 2017 the method changed and photos were taken
near peak biomass during the growing season (July-Sept) in order
to better capture and identify living biomass. Images were
archived for later analysis of plant growth and litter
accumulation within the microplots.
These images were standardized and then analyzed for percent cover
by approximately 27 plant, litter, and other cover categories
using USDA SamplePoint software(Booth et al. 2006, Peters et
al. 2020). Before cover analysis, photos were standardized by
rotating and then cropping to 50 cm x 50 cm with the image
centered over the footprint of the microplot. A grid of 100 points
was sampled in each image and assigned to one of the 27 cover type
categories. Percent cover by each category was calculated by first
subtracting cover points coming from ConMod structures and outside
mesquite branches from the 100 point grid to produce an
"actual" total cover value, then dividing the cover
points from each desired cover category by this number. Several of
the cover types in the resulting data file overlap and can be
summed to obtain functional group covers such as annual grasses or
forbs. Data here include grid size (100), actual cover
(100-(ConMods & outside branches)), grid counts by type, and
percent cover by type.
References
Okin, Gregory S., Mariano Moreno-de las Heras, Patricia M. Saco,
Heather L. Throop, Enrique R. Vivoni, Anthony J. Parsons, John
Wainwright, and Debra P. C. Peters. 2015. "Connectivity in
Dryland Landscapes: Shifting Concepts of Spatial
Interactions." Frontiers in Ecology and the Environment 13
(1): 20–27. https://doi.org/10.1890/140163.
Booth, D. Terrance, Samuel E. Cox, and Robert D. Berryman.
"Point sampling digital imagery with 'SamplePoint'."
Environmental Monitoring and Assessment 123 (2006): 97-108.
Peters, Debra P. C., Gregory S. Okin, Jeffrey E. Herrick, Heather
M. Savoy, John P. Anderson, Stacey L. P. Scroggs, and Junzhe
Zhang. 2020. "Modifying Connectivity to Promote State Change
Reversal: The Importance of Geomorphic Context and Plant–Soil
Feedbacks." Ecology 101 (9): e03069.
https://doi.org/10.1002/ecy.3069.
This method step describes provenance-based metadata as specified in the LTER EML Best Practices.
This provenance metadata does not contain entity specific information.