Vegetation Production and Phenology at Cedar Breaks National Monument, 2000–2019

View from a rim at Cedar Breaks National Monument, showing vivid red and orange cliffs descending into forested canyons. Mist clings to distant slopes, and cloud layers drift across the horizon, adding contrast to the monument’s high-elevation landscape.

NPS/Amy Washuta

What We Wanted to Know

Scientists with the Northern Colorado Plateau Network (NCPN) used two decades of satellite imagery and climate data to examine how vegetation at Cedar Breaks 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. Vegetation response to long-term climate 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 (i.e., plant growth from photosynthesis).

At-a-Glance

• Vegetation growth is positively influenced by temperature and negatively influenced by snow.
• Growing season production increased in 8 of 12 vegetation alliance groups between 2000 and 2019.
• Ponderosa Pine required the most heat to maintain production, whereas Spruce-Fir required the least.
• Alpine Dry Shrubland and Spruce-Fir alliance groups were most sensitive to heat, whereas Bristlecone Pine was least sensitive.
• The growing season was starting 8 days later on average across all alliance groups and ending 8 days later.

What We Did

Researchers analyzed Moderate Resolution Imaging Spectroradiometer (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 12 vegetation alliance groups, 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 at Cedar Breaks National Monument. Areas not colored within the study boundary are alliance polygons smaller than the minimum target area of 8.3 hectares.

NPS/David Thoma

Extended description of map showing alliance groups at Cedar Breaks NM

A shaded relief map of Cedar Breaks National Monument displays vegetation alliance groups analyzed for trends in production, phenology, and climate relations. The park boundary is outlined, with alpine shrubland shown in dark green, bristlecone pine in light green, and quaking aspen in bright green. Ponderosa pine appears in orange, spruce–fir forest in dark blue, and white fir in light gray. Wet meadows are highlighted in purple, riparian areas in light blue, and subalpine grasslands in pink. Disturbed areas appear in black, while geology polygons are shown in tan. Uncolored areas within the study boundary represent polygons smaller than 8.3 hectares, while gray areas outside the boundary were not analyzed. The legend on the right identifies alliance groups by color. A north arrow and scale bar in kilometers are shown at the bottom.

The study calculated “pivot points” (climate variable value 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, temperature, soil moisture, actual evapotranspiration, and water deficit.

What We Learned

Although the study began during a dry period on the Colorado Plateau, both precipitation and temperature increased during the study, making it more challenging to determine which caused the increase in growing season production observed in eight of 12 alliance groups at Cedar Breaks NM between 2000 and 2019. An explanation for the change became apparent when scientists found production was positively related to temperature and negatively related to snow. This is consistent with vegetation response in cold or high elevation environments were plant growth increases with warmer temperature and decreases with longer periods of snow cover. However, there is variation in which vegetation types are more sensitive to temperature than others and which are sensitive to moisture too.

At Cedar Breaks NM, Dry Alpine Shrubland and Spruce-Fir alliance groups at higher elevations were most responsive to variation in annual growing degree days a measure of heat needed for plant growth. Bristlecone Pine, Mixed Montane Shrubland, and Geology were the least sensitive to changes in growing degree days. Larger increases in production rates were found at the highest elevations that include non-forested alliance groups and subalpine fir and quaking aspen alliance groups. Alliances that did not increase in production were mostly slow growing forests (Figure 2).

Figure 2. Plot of pivot points and responses for polygon types relative to water-year growing degree days. Steeper lines indicate greater sensitivity; lines shifted left indicate greater drought tolerance.

NPS/David Thoma

Extended description of graph showing growing-season responses at Cedar Breaks NM

A line graph plots growing-season change (y-axis) against water-year growing degree days in degrees Celsius (x-axis) for vegetation types at Cedar Breaks National Monument. Each colored line represents a vegetation polygon. Steeper slopes indicate greater sensitivity to accumulated heat, while farther-left pivot points indicate lower heat tolerance. A horizontal black line marks zero change for reference.

Vegetation response is a formal component of vegetation vulnerability assessments. The importance of response is related to the need to understand which vegetation groups may respond sooner or more strongly to a given amount of climate exposure, and response indicates where change is likely to happen first and fastest as the climate shifts.

Analysis of the timing of start of spring growth and late summer senescence revealed notable changes in phenology. By the end of the study, across all alliance groups, the growing season was starting 8.3 days later and ending 8.4 days later. These findings suggest that spring temperatures may have cooled or snow cover have lasted longer in the spring as the study unfolded, but warmer temperatures later in the growing season extended the period of growth. A longer growing season is consistent with rising temperatures and increasing annual growing degree days that can extend the growing season when moisture is available.

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: Growing degree days tracked in real-time 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 the growing season.

Monitor high-sensitivity and high-response areas: Of the alliance groups in this study, Dry Alpine Shrubland and Spruce-Fir were among the most responsive to growing degree days, meaning they may serve as early indicators of ecological shifts. Monitoring these areas can help managers anticipate 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 temperature and precipitation trends.

Prioritize high-value or sensitive areas: High value vegetation that is important as wildlife habitat, forage or even visitor enjoyment, such as shade trees near campgrounds and picnic areas, may warrant higher management concern if it is very climate sensitive. Although climate sensitivity varied among alliance groups at Cedar Breaks NM, continued monitoring remains important to detect early signs of change in high-value or vulnerable vegetation types due to secondary effects from climate or weather that promote forest disease and drought conducive to wildfire.

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