Vegetation Production and Phenology at Fossil Butte National Monument

View across Fossil Butte National Monument from a high point on the butte, showing the sagebrush steppe, scattered aspen stands, and exposed sedimentary formations that define the area's ecological and geological character.

NPS / Amy Washuta

What We Wanted to Know

At a Glance

• Growing season production increased in all vegetation groups between 2000 and 2019.
• Disturbed and Wet Meadow were the most drought-tolerant alliance groups.
• Mesic Sagebrush was the least drought-tolerant alliance group.
• The timing of peak growth occurred an average of 6.4 days later in all alliance groups.

Scientists with the Northern Colorado Plateau Network (NCPN) used two decades of satellite imagery and climate data to examine how vegetation at Fossil Butte National Monument (NM) responds to weather and climate. The goal was to understand which vegetation types are most likely to change, where in the park those changes occur, and why, recognizing that not all vegetation responds to climate in the same way. In a semiarid region where precipitation is variable, vegetation response to long-term moisture patterns reveals which plant communities are resilient—and which are more likely to shift as the climate changes. The results offer a foundation for anticipating future change and preparing for shifts in vegetation composition and productivity.

What We Did

Researchers analyzed MODIS satellite imagery alongside daily climate data to evaluate long-term trends in vegetation productivity and the timing of growth (phenology) from 2000 to 2019. Using polygons mapped to 10 vegetation alliance groups—such as Mesic Sagebrush, Dry Sagebrush, and Quaking Aspen—they assessed how vegetation production and phenology changed over time and how sensitive each group was to different aspects of climate (Figure 1).

Figure 1. NCPN alliance groups analyzed for trends in production, phenology, and climate relations. Areas not colored within the study boundary are alliance polygons smaller than the minimum target area of 8.3 hectares.

NPS / David Thoma

The study calculated “pivot points” (the value of a climate variable where vegetation teeters between below- and above-average condition) and “responses” (amount vegetation production changes in response to changes in a climate variable). These two measures of plant traits help characterize each vegetation group’s drought tolerance and climate sensitivity. And since climate can be evaluated in different ways, the study tested vegetation response to multiple variables, including precipitation, soil moisture, actual evapotranspiration, and water deficit.

What We Learned

Although the study began during a dry period, multiple indicators suggest vegetation production increased in and near Fossil Butte NM between 2000 and 2019.Three years of actual evapotranspiration was the best indicator of growing season production at the monument scale (Figure 2). Quaking Aspen, Mesic Sagebrush, and Mixed Montane Shrublands were the alliance groups most sensitive to variation in annual actual evapotranspiration.

Figure 2. Plot showing pivot points and responses in polygons relative to water-year AET. Each line represents a unique polygon of the 10 types evaluated in the study. Steeper lines represent vegetation more sensitive to AET. Lines shifted left represent more drought tolerant vegetation.

NPS / David Thoma

Drought tolerance varied across the monument. Based on water deficit pivot points, Mesic Sagebrush alliance group was the least drought tolerant, while Disturbed and Wet Meadow were most drought tolerant.Phenology metrics revealed subtle but widespread changes in the timing of growth. Most vegetation groups experienced a slightly earlier start and later end to the season, resulting in a modestly extended growing period overall. The timing of peak growth also shifted earlier—likely reflecting responses to changing snowmelt timing, temperature increases, and precipitation variability.

What We Recommend

Park managers can use the results of this study—and the tools developed from it—to interpret vegetation response in real time, anticipate future conditions, and inform both near-term decisions and long-term conservation goals:

Use pivot points to interpret current conditions: Real-time climate data—such as soil moisture, precipitation, or evapotranspiration—can be compared to vegetation-specific pivot points. This “now-cast” approach allows managers to estimate whether current-year vegetation production is likely to be above or below average (without the complexity of analyzing satellite imagery), helping guide timely decisions during drought or high-heat events.

Monitor high-sensitivity and high-response areas: Alliance groups like Quaking Aspen, Mesic Sagebrush, and Dry Shrubland were among the most responsive to climate variables, meaning they may serve as early indicators of ecological shifts. Monitoring these areas can help managers track change and prioritize adaptive actions.

Incorporate legacy effects in planning: Because vegetation production often reflects conditions from previous years, restoration planning and invasive-species strategies could factor in multi-year precipitation and soil moisture trends.

Prioritize high-value or sensitive areas: High value vegetation that is important as wildlife habitat, forage or even visitor enjoyment, such as the visitor center and waypoints, may warrant higher management concern if it is very climate sensitive.

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Information in this article was summarized from Landscape phenology, vegetation condition, and relations with climate at Fossil Butte National Monument, 2000–2019 by D. Thoma (2025). Content was edited and formatted for the web by E. Rendleman.