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Abstract
The spatial organization of an ecosystem speaks to the environmental conditions and intrinsic qualities of the organisms present. This is because extrinsic factors, such as climate, apply stressors on organisms that respond by spatially organizing in a way that best suits their needs for survival. At Chico Basin Ranch, Colorado, the blue grama grasses visually show spatial patterning, with stripes of vegetation alternating with stripes of bare ground oriented horizontally along hillslope. This pattern is likely driven by short range facilitation, where plants facilitate their own growth by improving their growing conditions via increased surface water infiltration, and long-range inhibition, where growing conditions away from the densely vegetated patch are made inhabitable by the population inside the vegetation patch. These patterns likely formed as a response to increasing aridity in Colorado, which is limiting water. Limited water encourages plants to reorganize in order to persist under more challenging conditions. However, periodic patterning can be an early warning sign that a vegetated landscape may succumb to desertification. Understanding if a landscape is patterned periodically is important in informing conservation efforts to prevent desertification.
In order to determine if the patterning visible at Chico Basin Ranch is periodic and statistically significant, we used a 2-Dimensional Fast Fourier Transform (FFT) and the Autocorrelation Function (ACF). We applied FFT and ACF to datasets derived from aerial imaging from Google Earth for the years 2019, 2017, 2005, and 2003 of our AOI at three different scales using a custom-build code in R and the software PaSSAGE 2. Through this analysis, we were able to determine the presence and details of spatial patterning for the years examined. FFT and ACF analysis for the year 2019 showed distinct periodic patterning with waves ranging from 15-20 meters in the NW-SE direction. The analysis of 2017 showed congruence between ACF and FFT with similar results to 2019, meaning it detected periodicity with the same directionality and frequency. In contrast, the results from 2005 and 2003 both show incongruencies between methods of analysis, and our methods did not detect any significant periodic patterning. We interpreted these years as displaying some patchy spatial organization, however there is little to no periodicity, unlike the years 2017 and 2019. This leads us to conclude that there has been an emergence of periodic patterning in the population of blue grama at Chico Basin Ranch between the time periods of 2003-2005 and 2017-2019. These results are congruent with the significant decrease in blue grama at the Chemical Depot in Pueblo, 30 km south of our field site, which additionally confirms co-occurring changes in climate with increasing aridity and decreasing precipitation over the course of the study (Rondeau, 2016).