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 upwind and
downwind suplots. 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 other
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 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 file "csis_treatment_key.csv".
Lateral photo collection
In each ConMod plot (ConMod-only and ConMod + Herbicide treatments
only), repeat lateral photos of ten randomly selected microplots
were photographed annually from 2013-2017. Photos were taken
looking laterally across the microplot (parallel to ground, or
side-looking orientation) in four cardinal directions (N, E, S, W)
from a height of approximately 10cm above ground level and 50-cm
away from the ConMod center axis. Photos were taken using a 12.1
megapixel digital camera, resulting in image dimensions of 4,000 x
3,000 pixels. 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. Lateral photos were
discontinued after 2017 when density and cover of herbaceous
plants made the analysis of litter and soil accumulation difficult
and prone to error.
Before area analysis, photos were standardized by cropping, and in
some cases by rotating, for use in SigmaScan software (methods
described in Peters et al 2020; SigmaScan pro 5.0: Systat
Software, Inc. San Jose, CA USA). Photos in this dataset are the
ones used in these later analyses and are archived by year in zip
archives. A directory to the photos is provided in the file
"jrn413006_photo_archive_dir.csv".
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.