COS 218-6 - Invasive annual grasses are changing fire regimes for pinyon-juniper ecosystems on the Colorado Plateau

Abstract: Fire regimes have been changing worldwide and many have been linked to the introduction and success of invasive annual grasses (IAG), particularly in dryland ecosystems of the western United States. IAGs pose a severe threat to drylands of the United States by increasing the occurrence and severity of wildfires and out-competing native species, resulting in overall ecosystem degradation. Our goal was to analyze the dynamics of IAG presence on the landscape, how populations change through time, and how it is related to fire dynamics across two fire-prone land types (ecological site groups [ESGs]). These ESGs are dominantly Pinyon-Juniper woodland and mixed shrublands, which have not been as well characterized and modeled for fire as forested systems. However, these land types account for nearly 43% of fires in the Colorado Plateau drylands. Based on landscape federal monitoring data, these sites also have widespread IAG presence, but little is understood about how wildfire at these ESGs may be affected by IAGs.

Here, we use LANDSAT satellite imagery to map and understand IAGs annually across the Colorado Plateau from 1985-2021, primarily cheatgrass (Bromus tectorum), with the Detection of Early Season Invasives (DESI) methodology. DESI uses the difference of the Normalized Difference Vegetation Index (dNDVI) from two-time points, early spring and mid-summer, capitalizing on the difference in phenology between IAGs and surrounding native vegetation.

We analyzed the DESI maps of IAG establishment using Space-Time Pattern Mining which can identify eight spatiotemporal patterns for aggregated points (annual IAG maps): new, consecutive, intensifying, persistent, diminishing, sporadic, oscillating, and historical. We then used those categories and conducted discriminant analyses to identify relationships between IAG hot spots, fire, topography, soils, antecedent weather, and other biophysical variables. We found that IAGs are changing the dynamics of fire regimes on the Colorado Plateau, with intensifying and persistent IAG hot spots strongly linked to fire occurrence and higher fire intensities. Sporadic and oscillating hotspots are also linked with fire occurrence, though not as strongly as persistent IAG hot spots. Even though historical hot spots were not strong indicators for fire occurrence, the correlation with other biophysical variables suggests IAGs drive these ESG communities into less desirable conditions with strong management implications. Final maps of the IAG hot spots, fire analysis, and interpretations will be shared with land and fire managers for integration into targeted invasive species management programs, helping decision-makers determine where to allocate resources toward conservation of these landscapes.