Status and trends of terrestrial vegetation and soils at Casa Grande Ruins National Monument, 2008–2016

McIntyre, Cheryl; Hubbard, J.; Studd, Sarah

This report summarizes results of the Sonoran Desert Network’s second round of terrestrial vegetation and soils monitoring in upland areas of Casa Grande Ruins National Monument (NM), in Arizona. We resampled six permanent field-monitoring sites (3 originally estab-lished in 2008 and 3 established in 2011) from 2013 to 2016. Our objectives were to deter-mine status and detect trends in cover of vegetation, soil, and biological soil crusts; stabil-ity of surface soils; and frequency of vegetation. We also assessed the degree of ecological impact from ongoing stabilization efforts on key archeological sites within the monument. Groundwater depletion has led to tremendous change in the monument flora over the past century. However, terrestrial vegetation, biological soil crusts, and other dynamic soil-function parameters have remained consistent since monitoring efforts began in 2008. The sparse native shrublands of the monument are very stable and appear to be highly resistant to drought, frost, and other perturbations. Perennial plant diversity is remarkably low at the monument—a near monoculture of creo-sotebush (Larrea tridentata), perhaps due to extensive soil modification during the period of intensive occupation of the Casa Grande complex (AD 950–1450). We suggest additional research to evaluate these conditions and the potential for recruitment of other native plants—including saguaro cacti, the iconic Sonoran Desert plant that is common on similar sites elsewhere. In contrast to the low plant diversity, biological soil crusts are an extensive and important component of terrestrial ecosystems at Casa Grande Ruins NM. During the most recent sampling, biological soil crusts, primarily light cyanobacteria, occupied more than half of the available habitat on the monument. Surface soil aggregate stability was similarly high, while the cover of exposed bare soil was relatively low. In combination, soil conditions throughout most of the monument should limit the degree of soil erosion. This is particularly important because nearly the entire unit contains near-surface archeological resources. Surface soil ag-gregate stability and cover of vegetation and biological crusts were substantially lower—and bare soil was markedly higher—on plots where recent archeological stabilization efforts have occurred. We recommend continued observation of these sites to detect and address wind and water erosion of soils. Although we have observed 35 exotic plant species at Casa Grande Ruins NM, they ap-pear to be largely restricted to roadsides and other high-disturbance areas. None has been detected on monitoring or stabilization sites since monitoring began in 2008. Likely due to the paucity of exotic and native grasses and forbs, the potential for altered fire regimes (i.e., wildfire in low-elevation desert ecosystems, driven by fine fuels) is currently low at Casa Grande Ruins NM. As with localized erosion potential, we will continue to monitor exotic-plant occurrence and fine-fuel accumulation, and recommend vigilance in the face of these potentially system-changing management issues. At Casa Grande Ruins National Monument, a suite of management assessment points have been proposed relative to issues of vegetation management concern. Management assess-ment points are intended to serve as a potential early warning system, giving scientists and managers the opportunity to pause, review the available information in detail, and consider options. Management assessment points for monitoring described in this report identified several issues of potential concern (see figure, next page). Erosion hazard due to the preva-lence of exposed bare soil was identified as a potential issue of management concern on the recently manipulated stabilization plots. The cover of mature biological soil crusts could not be differentiated from the assessment point on either monitoring or stabilization plots.

Status and trends of terrestrial vegetation and soils at Casa Grande Ruins National Monument, 2008–2016

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