Examining multi-scale dynamics of drylands amid multi-year wet and dry periods using imagery

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Lucas J. Heintzman, ORISE/SCINet, USDA-ARS, Las Cruces, NM, Debra Peters, USDA ARS Jornada Experimental Range and Jornada Basin LTER Program, Las Cruces, NM, Heather M. Savoy, USDA-ARS Jornada Experimental Range & Jornada Basin LTER, NM and Geovany Ramirez, New Mexico State University, Las Cruces, NM

Background/Question/Methods
Our objective was to examine changes in patch- and landscape-scale vegetation dynamics for ecosystems representing the Chihuahuan Desert (upland grasslands, creosotebush shrublands, mesquite shrublands, and tarbush shrublands) during multi-year wet and dry periods. Twelve locations were selected that were distributed across the Jornada Long Term Ecological Research site in southern New Mexico, USA. Since 1989, these locations have been field-surveyed annually (via 49 1m2 quadrats per site) and used for validation of the imagery. We categorized vegetation into four functional vegetative groups (shrubs, grasses, forbs/subshrubs, and barren). We assessed landscape patterns derived from aerial imagery in 2005, 2009, 2011, 2014, 2016, and 2018; using unsupervised classification for each of the four categories. Additionally, a secondary objective was to identify regions of inferred high erosional potential. These years were chosen as they encompassed periods of regional shifts in hydrological regime (wet year: 2005; dry years: 2009, 2011; average years: 2014, 2016; 2018) and featured data availability that captured our study extent.
Results/Conclusions
At the patch scale: our preliminary results indicated levels of vegetative dynamism, with highly variable changes in landscape metrics for composition, configuration, and connectivity through time. When compared with quadrat-based information, our imagery based classification results were congruent with respect to functional group dominance at a ~60% rate. This rate increased to ~70% when presence/absence was assessed. However vegetative changes were often subtle, and frequently did not correspond to field-based estimates (due to multiple species assemblages inducing spectral mixing, and soil background masking effects). Identification of areas of high erosional potential are on-going. Yet, our patch-based observations emphasize the influence of wet-dry periods on dryland ecosystem structural properties and their potential corresponding vegetative lag effects across scales. Moreover, our observations at both the patch- and landscape-scale also highlight the importance of expanding landscape connectivity theory beyond traditional two-dimensional applications.